Document Detail

Trimethoprim interferes with serum methotrexate assay by the competitive protein binding technique.
MedLine Citation:
PMID:  6774838     Owner:  NLM     Status:  MEDLINE    
Administration of Bactrim (a combination of trimethoprim and sulfamethoxazole) to a patient who also was receiving methotrexate caused a significant increase in apparent plasma methotrexate concentrations as determined by competitive protein binding assay with use of dihydrofolate reductase (EC from Lactobacillus casei as the binding protein. This spurious increase was caused by trimethoprim in the patient's plasma. A plasma trimethoprim concentration of 0.1 mg/L inhibited binding of radiolabeled methotrexate to dihydrofolate reductase by 50%. In contrast, radioimmunoassay for methotrexate was not affected by concomitant administration of trimethoprim. The competitive protein binding assay for methotrexate should not be used in patients being treated with Bactrim or Septra (a similar combination). However, the L. casei competitive protein binding assay technique can be used to assay plasma trimethoprim concentrations with sensitivity to 0.02 mg of trimethoprim per liter.
K Hande; J Gober; R Fletcher
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Publication Detail:
Type:  Case Reports; Journal Article; Research Support, Non-U.S. Gov't; Research Support, U.S. Gov't, Non-P.H.S.; Research Support, U.S. Gov't, P.H.S.    
Journal Detail:
Title:  Clinical chemistry     Volume:  26     ISSN:  0009-9147     ISO Abbreviation:  Clin. Chem.     Publication Date:  1980 Oct 
Date Detail:
Created Date:  1980-12-18     Completed Date:  1980-12-18     Revised Date:  2008-11-21    
Medline Journal Info:
Nlm Unique ID:  9421549     Medline TA:  Clin Chem     Country:  UNITED STATES    
Other Details:
Languages:  eng     Pagination:  1617-9     Citation Subset:  IM    
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MeSH Terms
Binding, Competitive / drug effects
Chemical Phenomena
Drug Combinations
False Positive Reactions
Lactobacillus casei / enzymology
Leukemia, Lymphoid / drug therapy*
Methotrexate / blood*
Sulfamethoxazole / therapeutic use
Tetrahydrofolate Dehydrogenase / metabolism
Trimethoprim / blood*,  therapeutic use
Reg. No./Substance:
0/Drug Combinations; 59-05-2/Methotrexate; 723-46-6/Sulfamethoxazole; 738-70-5/Trimethoprim; EC Dehydrogenase

From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine

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