Correction factors for estimating potassium concentrations in samples with in vitro hemolysis: a detriment to patient safety.
Abstract: * Context.--Correction factors have been proposed for estimating true potassium concentrations in blood samples with evidence of in vitro hemolysis.

Objective.--We used 2 different models of true (ie, non-simulated) in vitro hemolysis to evaluate the clinical utility of correction factors for estimating potassium concentrations in samples with evidence of in vitro hemolysis.

Design.--Potassium correction factors were derived using 2 different models. In model 1, potassium and plasma hemoglobin were measured with the Hitachi 747 analyzer in 132 paired blood samples, with each pair consisting of 1 sample with evidence of hemolysis and 1 without, collected during the same phlebotomy procedure. The change in measured potassium concentration was plotted versus the change in plasma hemoglobin concentration for each pair of samples. In model 2, the potassium levels of 142 784 blood samples and the corresponding hemolytic index values were measured with the Beckman LX20 analyzer. Potassium concentrations at the 10th, 25th, 50th, 75th, and 90th percentiles were calculated for each hemolysis index category.

Results.--From our 2 models, we derived correction factors expressing an increase in potassium concentration of 0.51 and 0.40 mEq/L for every increase in plasma hemoglobin concentration of 0.1 g/dL. These correction factors are much higher than those reported in studies that simulated in vitro hemolysis by freeze-thaw lysis or osmotic disruption of whole blood.

Conclusions.--Use of correction factors for estimating the true potassium concentration in samples with evidence of in vitro hemolysis is not recommended. Derivation of correction factors by using samples with nonsimulated in vitro hemolysis suggests that the actual increase in potassium in hemolyzed samples is much higher than that reported in previous studies that produced hemolysis with artificial means.

(Arch Pathol Lab Med. 2009;133:960-966)
Article Type: Report
Subject: Hemolysis and hemolysins (Care and treatment)
Hemolysis and hemolysins (Diagnosis)
Potassium in the body (Research)
Potassium in the body (Physiological aspects)
Phlebotomy (Usage)
Patients (Care and treatment)
Patients (Safety and security measures)
Veins (Puncture)
Veins (Usage)
Authors: Mansour, Mai M.H.
Azzazy, Hassan M.E.
Kazmierczak, Steven C.
Pub Date: 06/01/2009
Publication: Name: Archives of Pathology & Laboratory Medicine Publisher: College of American Pathologists Audience: Academic; Professional Format: Magazine/Journal Subject: Health Copyright: COPYRIGHT 2009 College of American Pathologists ISSN: 1543-2165
Issue: Date: June, 2009 Source Volume: 133 Source Issue: 6
Topic: Event Code: 310 Science & research; 260 General services
Product: Product Code: 2834792 Potassium Supplements NAICS Code: 325412 Pharmaceutical Preparation Manufacturing SIC Code: 2834 Pharmaceutical preparations
Accession Number: 230152026
Full Text: The preanalytic phase of laboratory medicine is the phase that can most adversely affect the accuracy of test results. (1) Poor specimen quality is recognized as the most frequent source of error in this phase of the testing process, with in vitro hemolysis accounting for the approximately 40% to 70% of samples received in the clinical chemistry laboratory that are unsuitable for analysis. (2-4)

Although many analytes are subject to interference effects from in vitro hemolysis, the influence of hemolysis on measured potassium concentrations is probably most widely recognized. The frequency of potassium measurements, along with the serious consequences of misdiagnosis of hypokalemia or hyperkalemia, has led to the development of various options for handling hemolyzed samples. (4,5) These options include outright rejection of hemolyzed samples, analysis of hemolyzed samples and the reporting of potassium levels with a disclaimer stating that the result may be incorrect, or analysis of hemolyzed samples and adjustment of the measured potassium result with a correction factor based on the magnitude of hemolysis.

Several studies have advocated using correction factors to account for the effect of in vitro hemolysis on measured potassium concentrations. (6-9) While correction factors may help provide a better assessment of the true potassium concentration in patients, the variability in the correction factors that have been proposed is alarming. A variety of issues may account for this difference, including the mechanism used to simulate in vitro hemolysis, interindividual variability in erythrocyte hemoglobin concentrations, and the effect of erythrocyte age on intracellular potassium concentrations. (10,11) The variations in proposed correction factors suggest that adopting them to estimate potassium concentrations may be inappropriate and not in the best interest of patient safety. To better assess the clinical utility of correction factors, we investigated the effects of in vitro hemolysis by using 2 different models of true (ie, nonsimulated) in vitro hemolysis. In addition, we compared the results of our investigations with those of other studies that have derived correction factors by using a variety of different mechanisms to simulate in vitro hemolysis.

MATERIALS AND METHODS

We used 2 different models, 2 different instrument platforms, and 2 different sample types to evaluate the effects of in vitro hemolysis on measured potassium concentrations. Data were analyzed with Microsoft Excel 2007 (Microsoft, Redmond, Wash).

Model 1

The effects of in vitro hemolysis were investigated by using serum obtained from paired blood samples collected from patients during the same phlebotomy procedure. It is not uncommon to obtain 2 or more blood collection tubes from one patient during the same phlebotomy procedure that differ in the degree of hemolysis. If one of the samples so collected showed visual evidence of hemolysis, while another sample did not, the pair was analyzed for serum potassium concentrations. For statistical analysis, samples were considered hemolyzed if their hemoglobin concentration was greater than 0.090 g/dL, while the "control", or nonhemolyzed sample of each pair had a hemoglobin concentration of 0.090 g/dL or less. We performed regression analysis by plotting the difference in potassium concentrations versus the difference in the hemolysis (H) index values measured in the hemolyzed and nonhemolyzed samples. All samples obtained for this phase of the investigation were collected by using serum-separator collection tubes (Becton, Dickinson and Company, Franklin Lakes, NJ).

Potassium was measured with the Hitachi 747 analyzer (Roche Diagnostics, Indianapolis, Ind) by using an indirect ion selective electrode method. Plasma hemoglobin concentrations were estimated quantitatively with the H index measured with the Hitachi 747. The H index calculates plasma hemoglobin levels by using direct spectrophotometry and applying an algorithm to the resultant data. For each pair of samples, the H index value in the sample not showing visual evidence of hemolysis was subtracted from the H index value in the sample with evidence of hemolysis.

Deming linear regression was used to analyze the difference between the potassium concentration in the hemolyzed and the nonhemolyzed samples versus the difference in the H index value measured in both. We also constructed a Bland-Altman difference plot to assess the ability of the regression equation to correct for the effects of hemolysis in samples. We plotted potassium concentrations in the sample with hemolysis against the difference between potassium concentrations in the sample without evidence of hemolysis and the potassium concentration estimated from the regression equation.

Model 2

We evaluated patient results for potassium that were obtained from 142 784 blood samples measured during a 1-year period with the Beckman LX20 analyzer (Beckman Instruments, Brea, Calif). Analyses were performed using the manufacturer's reagents. Hemolysis was assessed by using direct spectrophotometry to calculate a semiquantitative hemolysis index graded on a scale of 0 through 10. The hemolysis index levels and the hemoglobin range associated with each are shown in Table 1.

Patient blood samples were assigned to 1 of 11 categories based on the hemolysis index value. We calculated the 10th, 25th, 50th, 75th, and 90th percentile values for potassium concentrations within each hemolysis index category. The potassium concentration for samples with no evidence of hemolysis (ie, hemolysis index = 0) was considered to be the baseline or "true" potassium concentration. Potassium concentrations measured in samples with evidence of hemolysis (ie, hemolysis index of 1-10) were compared to potassium concentrations in samples with no evidence of hemolysis. Clinically significant differences between results of samples with and without evidence of hemolysis were determined by using the analytic precision of the assay and the normal intraindividual variation for potassium, according to the method of Fraser et al. (12) Differences in test results that were more than 95% likely to indicate a true change were calculated as follows:

Significant Change (%) = 2.8 x [square root of ([Cv.sup.2.sub.a]) + ([Cv.sup.2.sub.i])/100]

where [Cv.sub.a] is the long-term analytic coefficient of variation for potassium measured with the Beckman LX20 analyzer and [Cv.sub.i] is the normal intraindividual variation of potassium in healthy individuals.

In our laboratory, long-term analytic imprecision for potassium is 2.5% when measured with the Beckman LX20 at a potassium concentration of 4.1 mEq/L. The within-subject biological coefficient of variation for potassium has been reported to be approximately 4.7%. (13,14) Using these criteria for analytic imprecision and intraindividual biological variation, we defined a clinically significant change in potassium concentrations as a percent difference of greater than 15.0 between samples with hemolysis index values of 0 and those with index values of 1 through 10.

If the difference between potassium concentrations in samples with a hemolysis index of 0 and those with a hemolysis index of 1 through 10 was not significant, as assessed by using the above formula, then hemolysis was considered not to be clinically significant.

Literature Review

We also reviewed the literature for articles in which investigators estimated the effect of hemolysis on measured potassium concentrations. We used Ovid MEDLINE to search the literature from 1960 through February 2008. We limited our search to those articles containing an abstract, written in the English language, and evaluating human subjects. We searched for peer-reviewed manuscripts by using the following search terms: potassium AND hemolysis, potassium AND haemolysis, hemolysis AND interference, and haemolysis AND interference. Information such as method used to simulate in vitro hemolysis, instrument platform used to measure potassium, and derived correction factor were recorded, where available.

RESULTS

Model 1

We collected a total of 132 paired blood samples from 129 hospitalized adult patients; 3 patients provided 2 pairs of samples each. Median age was 60 years (range, 18-95 years), with 69 women and 60 men. Samples with evidence of in vitro hemolysis had a median hemoglobin concentration of 0.210 g/dL (range, 0.098-1.903 g/dL), while the median for the nonhemolyzed sample of each pair was 0.011 g/dL (range, 0.000-0.078 g/dL).

Figure 1 shows the relationship between plasma hemoglobin concentration, as measured with the H index, and increment in potassium concentration. The regression equation obtained with all 132 samples (y = 5.101x - 0.0062; [r.sup.2] = 0.87) corresponds to a correction factor of approximately 0.51; ie, potassium concentration increased by 0.51 mEq/L for every increase in plasma hemoglobin concentration of 0.100 g/dL. Since plasma hemoglobin concentrations greater than 0.500 g/dL represent gross hemolysis, and samples with this degree of hemolysis are often not analyzed in many laboratories, we also plotted data for samples containing plasma hemoglobin concentrations less than 0.500 g/dL. The regression equation for this subset of 117 samples (y = 5.1000 x - 0.0223) corresponded to a correction factor of 0.50, not significantly different from the correction factor obtained with all 132 samples.

[FIGURE 1 OMITTED]

We used Bland-Altman analysis to assess the ability of the regression equation to evaluate the agreement between potassium concentrations estimated with our derived correction factor and measured potassium concentrations in each of the 132 paired samples. The difference between measured potassium concentrations in the nonhemolyzed samples and estimated potassium concentrations in the hemolyzed samples is shown in Figure 2. Bland-Altman analysis showed that the applied correction factor performed similarly at all measured concentrations of potassium. The 95th percentile limits for estimated potassium concentrations ranged from -0.67 mEq/L to 0.54 mEq/L of the potassium concentration measured in the nonhemolyzed sample.

[FIGURE 2 OMITTED]

Model 2

We collected data from 142784 patient blood samples and measured potassium levels and the hemolytic index value with the Beckman LX20 analyzer. Samples evaluated were from individuals 18 years and older. We used the long-term analytic precision of the Beckman LX20 method for measurement of potassium, and the normal intraindividual variation of potassium as the basis for determining clinically significant change in measured potassium levels due to in vitro hemolysis. At a potassium concentration of 4.0 mEq/L, a percent difference of 15 corresponds to a change of 0.6 mEq/L.

We found clinically significant interference at a hemolysis index value of 3 or greater. This index value corresponds to a serum hemoglobin concentration of approximately 0.100 g/dL to 0.150 g/dL. Plotting the median potassium concentration for each of the 11 different hemolysis index groups showed that, on average, potassium concentrations increased by 0.20 mEq/L with each increment in the hemolysis index value. Since an increment of 1 in the Beckman LX20 hemolysis index value corresponds to an increase in plasma hemoglobin concentration of approximately 0.050 g/dL, potassium concentrations increased by approximately 0.40 mEq/L (95% confidence interval, 0.38-0.43) for every increase in plasma hemoglobin concentration of 0.100 g/dL. Table 2 shows the number of patients with hemolysis index values of 0 thru 10, along with the 10th, 25th, 50th, 75th, and 90th percentile values for potassium concentrations in each of the 11 different hemolysis index groups. Figure 3 shows the same data displayed as a box-and-whisker plot. Clinically significant differences in measured potassium concentrations due to in vitro hemolysis did not occur consistently in all percentile ranks. Results for potassium levels in the 75th and 90th percentile ranks showed clinically significant interference at hemolysis index values of 3 and greater, while results in the 10th percentile did not show clinically significant interference until the hemolysis index was 6 or greater (see Table 2).

[FIGURE 3 OMITTED]

Review of Studies Reporting Correction Factors for Potassium Associated With In Vitro Hemolysis

We reviewed the literature and summarized the results of studies that evaluated the effect of in vitro hemolysis on potassium concentrations. Using the search terms potassium AND hemolysis, potassium AND haemolysis, hemolysis AND interference, and haemolysis AND interference, we identified 744 articles for consideration. Of these, 43 were identified for systematic review. Twelve of the 43 articles reported the increment in potassium as a function of plasma hemoglobin concentration (Table 3). The increase in potassium concentration varied as much as 2.5-fold, from an increase of 0.21 mEq/L to an increase of 0.50 mEq/L per plasma hemoglobin concentration of 0.100 g/dL. The median value reported for potassium concentration was 0.31 mEq/L per plasma hemoglobin concentration of 0.100 g/dL.

COMMENT

The most common preanalytic factor affecting the clinical utility of specimens received in the clinical laboratory is in vitro hemolysis, which can occur by a variety of mechanisms including exposure of samples to extremes of heat and cold, collection of blood using small-bore needles, centrifugation of blood before clotting is completed, and agitation of collection tubes. (4,5) Intracellular potassium concentrations are reportedly 22-fold greater than those found in the extracellular fluid. (16) Thus, in vitro hemolysis can release large quantities of potassium. Guidelines have been established for the assessment of interference effects due to in vitro hemolysis, (17) and various experimental designs have been advocated. (16,18,19) Unfortunately, these methods often ignore the appropriateness of artificially prepared samples as representative of true in vitro hemolysis. Interference from in vitro hemolysis is most often evaluated by the addition of whole blood lysate prepared by freezing and thawing of whole blood, or osmotic lysis of cells after the addition of distilled water to packed cells. These techniques result in lysis of all cells, including reticulocytes, mature erythrocytes, platelets, and leukocytes present in the sample.

In vitro hemolysis due to mechanical disruption of cells during blood collection typically does not result in lysis of all cells. In addition, aged erythrocytes are more prone to shear stresses that occur during sample collection than are younger cells. (20) Shear-induced deformation of erythrocytes results in the loss of cellular potassium in proportion to shear forces, and loss of erythrocyte potassium occurs at shear stresses far below the threshold needed to cause cell lysis. (21) Erythrocyte shear stress that occurs during blood collection can result in the formation of erythrocyte membrane pores that allow the egress of small ions such as potassium, but block passage of large molecules such as hemoglobin. (22) Thus, the use of plasma hemoglobin as a marker for in vitro hemolysis will not account for erythrocyte potassium loss due to shear stress during blood collection that does not result in complete cell lysis.

Variability in membrane permeability as a result of disease may also cause significant differences in potassium loss from erythrocytes. (23) Older erythrocytes have greater permeability to cations than do younger cells (24) and contain at most one half the potassium content of younger cells. (25) Because of the relationship between erythrocyte potassium concentration and cell age and between permeability to cations and cell age, hemolysis due to mechanical trauma may cause much different changes in serum potassium concentration when compared with hemolysis induced by freezing and thawing of whole blood or osmotic disruption of cells. The former process is more selective for the efflux of potassium from predominantly older cells; the latter process results in nonselective disruption of all blood cells. Because of the greater tendency for older cells to hemolyze or lose potassium without undergoing complete cellular lysis, correction factors derived from interference studies that prepare hemolysate with techniques that lyse all cells will not account for this tendency.

Another factor that may affect increases in potassium levels after in vitro hemolysis is erythrocyte concentration. Dimenski et al (8) found that decreased erythrocyte concentrations were associated with an increase in potassium release following in vitro hemolysis. They hypothesized that decreased numbers of erythrocytes may lead to faster flow through the needle during phlebotomy, thus resulting in increased shear forces and cell membrane rupture. In addition to the effects of erythrocyte concentration on potassium, increases in leukocyte concentration cause increases in potassium levels. In certain conditions where leukocytes are more mechanically fragile, such as in individuals with chronic lymphocytic leukemia, the degree of potassium increase is even more significant. (26)

The ability of automated analyzers to measure plasma hemoglobin in patient blood samples has proved to be an efficient and reliable means for identifying in vitro hemolysis. We evaluated the effects of in vitro hemolysis by using 2 models that represent a more realistic approach for investigating these effects. Hemolyzed samples obtained from actual patients are representative of the population for which in vitro hemolysis of blood samples is seen in a hospital-based clinical laboratory setting. In addition, we obtained samples from many patients with a variety of disease states. Studies that evaluate the effects of in vitro hemolysis with limited number of samples collected from healthy volunteers, or pooled samples, may not give results that are representative of the interference effects seen in hospitalized patients. We defined clinically significant interference by using the analytic precision of the assay and the normal intraindividual biological variation of potassium.

As seen in Table 2, our results suggest that in vitro hemolysis causes much greater increases in measured potassium concentrations than have been reported in other studies that have attempted to address this issue. It should be noted that the correction factors obtained with our 2 models, 0.51 and 0.40 mEq of potassium per liter for every 0.100 g of plasma hemoglobin per deciliter, are considerably different. The reason for this difference is unclear, although samples used in model 1 were sera collected using gel-separator tubes, while samples used in model 2 were from heparinized plasma, collected without separator gel. We are not aware of any studies that have attempted to evaluate the effect of sample collection tubes on in vitro hemolysis and induced increases in potassium levels.

A wide variety of mechanisms have been used to simulate in vitro hemolysis. As shown in Table 2, these include osmotic lysis, (6,10,27,28) freeze-thaw cycles, (29,30) physical disruption by forcing blood through a small-bore needle, (7,8) physical disruption of clotted blood with wooden applicator sticks, (31) or homogenization of whole blood in a blender. (23) In addition, some studies removed platelets and leukocytes before inducing hemolysis, (23) whereas others did not. (6-8,10,27-30) The use of these different methods to hemolyze erythrocytes may explain the wide variability in correction factors reported by studies that have attempted to quantify the effects of in vitro hemolysis on potassium concentrations. Interestingly, one study that used physical disruption of cells to produce hemolysis derived a correction factor of 0.50, similar to that derived by our first model. (31) Results obtained with model 2 showed that clinically significant hemolysis is present when the hemolysis index, measured with the Beckman LX20, is 3 or greater. This index value corresponds to a plasma hemoglobin concentration of approximately 0.100 to 0.150 g/dL. The same observation has also been made independently by 3 other groups of investigators, (32-34) who found that samples with hemolysis index values of 3, measured with the Beckman LX20, have an increase in potassium concentration of approximately 0.4 mEq/L or more, an increase that was considered to be clinically significant.

Several studies have been published that advocate the use of correction factors to estimate potassium concentrations in samples with evidence of in vitro hemolysis. (6-9) Dimenski et al (8) recommended reporting a qualifying comment along with a corrected potassium concentration. They considered samples with plasma hemoglobin concentrations greater than 0.600 g/dL to be grossly hemolyzed and not amenable to correction. Jay and Provasek (6) developed a computer algorithm to estimate the magnitude of increase in potassium on the basis of plasma hemoglobin concentration. A comment alerting clinicians to the magnitude of the false increase in potassium due to in vitro hemolysis is appended to potassium results. Owens et al (7) derived correction factors after physically disrupting whole blood by repeatedly forcing samples, collected from 20 healthy adults, through a 27-gauge needle. The derived correction factor was used to estimate the magnitude of false increase in hemolyzed pediatric blood samples. They suggest that corrected potassium results falling into the normal reference interval obviate the need for repeated venipuncture. Hawkins et al (10) derived correction factors after evaluating in vitro hemolysis on 100 hospitalized patients. They simulated in vitro hemolysis by osmotic disruption, followed by freezing of whole blood. They reported a wide range of correction factors expressing an increase in potassium concentration of 0.20 to 0.35 mEq/L per plasma hemoglobin concentration of 0.100 g/dL. These authors suggest a more practical approach in that they advocate reporting a range of corrected potassium results based on the magnitude of in vitro hemolysis. For example, samples with plasma hemoglobin concentrations between 0.100 and 0.200 g/dL would have corrected potassium concentrations in the range of 0.2 mEq/L to 0.7 mEq/L lower than those measured in the hemolyzed sample. This approach has been suggested as being more appropriate since it takes into account the uncertainty of the derived correction factor. (9)

Our results suggest that it is inappropriate to apply correction factors to estimate potassium concentrations in samples with evidence of in vitro hemolysis. The multitude of factors that can affect erythrocyte potassium loss suggests that the use of correction factors based on a semi-quantitative measurement of plasma hemoglobin is an overly simplistic solution to a complex process. In addition, legal ramifications concerning the release of test results on samples are known to be inaccurate. Appending a disclaimer to a laboratory test result known to be adversely affected by in vitro hemolysis does not absolve the laboratory of any adverse medical event that may occur as a result of this test result. (35) Laboratories should adopt practices that are legally defensible for reporting of hemolyzed samples. These approaches include reassessment of what constitutes an unacceptable level of hemolysis and integrating this definition into laboratories' policies and procedures. If however, laboratories do use correction factors for reporting potassium concentrations of samples with evidence of in vitro hemolysis, results should probably be reported as a range, thereby indicating the degree of uncertainty.

One of the shortcomings of our study was that none of the samples used were obtained from neonates or from specimens collected via heel stick or finger stick. Hemolysis in blood samples collected via heel stick or finger stick is often attributed to excessive squeezing of the puncture site to obtain a sufficient volume of blood. (36) Thus, the correction factors obtained in our studies may not be appropriate for use with hemolyzed specimens obtained via heel stick or finger stick. In addition, all samples were obtained from individuals 18 years or older. Further studies need to be performed on younger patient populations.

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Mai M. H. Mansour, MS; Hassan M. E. Azzazy, PhD, DABCC; Steven C. Kazmierczak, PhD, DABCC

Accepted for publication September 15, 2008.

From the Department of Chemistry, American University in Cairo, Cairo, Egypt (Drs Mansour and Azzazy); and the Department of Pathology, Oregon Health & Science University, Portland, Ore (Dr Kazmierczak).

The authors have no relevant financial interest in the products or companies described in this article.

Reprints: Steven C. Kazmierczak, PhD, DABCC, Department of Pathology, Oregon Health & Science University, 3181 SW Sam Jackson Park Rd, L471, Portland, OR 97239 (e-mail: kazmierc@ohsu.edu).
Table 1. Relationship Between Hemolysis Index Level
and Hemoglobin Concentration Measured With the
Beckman LX20 Analyzer

Hemolysis Index    Range of Approximate Hemoglobin
     Level               Concentrations, g/dL

0                           None detected
1                               <0.050
2                           0.050 to 0.099
3                           0.100 to 0.149
4                           0.150 to 0.199
5                           0.200 to 0.249
6                           0.250 to 0.299
7                           0.300 to 0.349
8                           0.350 to 0.399
9                           0.400 to 0.449
10                          0.450 to 0.499

Table 2. Potassium Concentrations at 5 Percentile Ranks for Samples
in Each Hemolysis Index Category *

              Potassium Concentration at Different Percentile Ranks
 Hemolysis
Index Value      n      10th, mEq/L (%)   25th, mEq/L (%)

0             89 492      3.4               3.7
1             41 704      3.4 (0.0)         3.7 (0.0)
2              6 036      3.5 (2.9)         3.9 (5.4)
3              2 291      3.7 (8.8)         4.1 (10.8)
4              1 159      3.9 (14.7)        4.3 (16.2)#
5                726      3.9 (14.7)        4.4 (18.9)#
6                503      4.2 (23.5)#       4.7 (27.0)#
7                391      4.5 (32.4)#       4.9 (32.4)#
8                178      4.6 (35.6)#       5.1 (37.8)#
9                164      4.9 (44.1)#       5.4 (46.0)#
10               140      5.1 (50.0)#       5.4 (46.0)#

              Potassium Concentration at Different Percentile Ranks
 Hemolysis
Index Value   50th, mEq/L (%)   75th, mEq/L (%)   90th, mEq/L (%)

0               4.0               4.3               4.7
1               4.1 (2.5)         4.4 (2.3)         4.9 (4.3)
2               4.2 (5.0)         4.7 (9.3)         5.3 (12.8)
3               4.5 (12.5)        5.0 (16.3)#       5.6 (19.2)#
4               4.7 (17.5)#       5.2 (20.9)#       5.8 (23.4)#
5               4.9 (22.5)#       5.5 (27.9)#       6.2 (31.9)#
6               5.2 (30.0)#       5.7 (32.6)#       6.2 (31.9)#
7               5.3 (32.5)#       5.8 (34.9)#       6.5 (39.2)#
8               5.5 (37.5)#       6.1 (41.9)#       6.4 (36.2)#
9               5.9 (47.8)#       6.5 (51.2)#       7.1 (51.1)#
10              5.9 (47.5)#       6.6 (53.5)#       7.3 (55.3)#

* Values in parentheses are percent differences in potassium
concentrations for samples in each group relative to the potassium
concentration of samples with a hemolysis index value of 0. Bolded
values indicate clinically significant change (ie, >15.0%) in
potassium concentrations due to in vitro hemolysis.

Note: Values indicate clinically significant change (ie, >15.0%)
in potassium concentrations due to in vitro hemolysis are indicated
with #.

Table 3. Summary of Studies Evaluating Interference Effects
of In Vitro Hemolysis on Measured Potassium Concentrations *

                         Method Used to
Correction              Simulate In Vitro
Factor ([dagger])           Hemolysis                 Source, y

0.51                Model 1: Analysis of        Current study
                      paired samples by
                      indirect potentiometry
                      with the Hitachi 747
                      analyzer (Roche
                      Diagnostics,
                      Indianapolis, Ind).
0.40                Model 2: Increase in        Current study
                      potassium concentration
                      of 0.20 mEq/L with each
                      increment in the
                      hemolysis index value
                      as measured by indirect
                      potentiometry with the
                      Beckman LX analyzer
                      (Beckman Instruments,
                      Brea, Calif).
0.30                Osmotic lysis followed      Frank et al, (27) 1978
                      by addition of saponin.
                      Indirect potentiometer
                      with Technician Stat
                      Ion analyzer (Technicon
                      Instruments, Tarrytown,
                      NY) and atomic
                      emission spectroscopy.
0.21                Lysis by freezing and       Lippi et al, (30) 2006
                      thawing of whole blood
                      from 12 healthy
                      volunteers. Indirect
                      potentiometry with
                      Roche Modular Analyzer
                      (Roche Diagnostics).
0.30                Osmotic lysis followed      Jay & Provasek, (6)
                      by freezing and thawing   1993
                      of whole blood.
                      Indirect
                      potentiometry with the
                      Hitachi 717 analyzer
                      (Roche Diagnostics).
0.33                Osmotic lysis followed      Mather & Mackie, (28)
                      by freezing and thawing   1960
                      of whole blood. Atomic
                      emission spectroscopy.
0.28                Osmotic lysis followed      Hawkins, (10) 2002
(range,               by freezing and thawing
0.20-0.35)            of whole blood. Blood
                      from 100 hospitalized
                      patients studied.
                      Indirect potentiometry
                      with the Hitachi 917
                      analyzer (Roche
                      Diagnostics).
0.30                Blood collected from 3      Brydon & Roberts, (23)
                      healthy volunteers and    1972
                      homogenized in a
                      blender. WBCs removed
                      before homogenization.
                      Atomic emission
                      spectroscopy.
0.36                Physical disruption by      Dimeski et al, (8)
                      forcing whole blood       2005
                      from 41 volunteers
                      through a 21-gauge
                      needle. Indirect
                      potentiometry with
                      Roche Modular analyzer.
0.32                Physical disruption by      Owens et al, (7) 2005
(95% CI,              repeatedly forcing
0.29-0.35)            blood from 20 healthy
                      volunteers through a
                      27-gauge needle.
                      Indirect potentiometry
                      with Dade Behring
                      Dimension RXL (Siemens
                      Healthcare Diagnostics,
                      Deerfield, Ill).
0.50                Physical disruption         Pai & Cyr-Manthey,
                      (clot lysis with wooden   (31) 1991
0.24-0.26             applicator stick).        Sonntag, (29) 1986
                      Lysis by freezing and
                      thawing of whole blood
                      from 1 healthy
                      volunteer. Potassium
                      measured by atomic
                      emission spectroscopy
                      (0.25), indirect
                      potentiometry (0.26),
                      and direct
                      potentiometry (0.24).
0.32                Potassium measurement       Shepard et al, (33)
                      repeated within 24 h      2006
                      after receipt of
                      hemolyzed specimen.
                      Thirty-five patients
                      studied. Difference in
                      potassium between 2
                      samples correlated to
                      serum index. Potassium
                      concentration of 0.16
                      mEq/L for each
                      increment in hemolysis
                      index value as measured
                      by indirect
                      potentiometry with
                      Beckman LX (Beckman
                      Instruments).
0.28                Potassium concentration     Vermeer et al, (32)
                      of 0.14 mEq/L for each    2007
                      increment in the
                      hemolysis index value
                      as measured by indirect
                      potentiometry with
                      Beckman LX (Beckman
                      Instruments).
                      Evaluation of 307
                      patient samples with in
                      vitro hemolysis.

* CI indicates confidence interval; WBC, white blood cells.

([dagger]) Increase in potassium concentration in mEq/L per plasma
hemoglobin concentration of 0.100 g/dL.
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