Paracetamol, or acetaminophen, overdose is a common means of self-poisoning worldwide due its wide availability and accessibility. It has been reported as the most common drug overdose either accidentally or unintentionally in the United Kingdom (UK), Europe, United States (US), and Australasia [^1-3]. Paracetamol overdose is recognised to cause a range of hepatic damage from mild to severe hepatotoxicity, leading to acute liver failure (ALF) and death, despite the availability of antidote therapy. ALF resulting from paracetamol overdose has been extensively reported in the UK, US, France, Canada and Australia [⁴,⁵]. ALF due to paracetamol overdose has been reported to be most common in the U.K. (60-75% of ALF aetiology), but less frequent in the U.S. (approximately 20% of ALF aetiology) and even lower in certain parts of Europe (2% of ALF aetiology in France) [⁴,⁶]. However, in recent years, the incidence of paracetamol-induced ALF cases in the US has increased exponentially [⁷].

In a recent review article on the differences in aetiopathogenesis of ALF between Western and Eastern populations, it was reported that viral hepatitis remained the commonest cause of ALF in Asians. Paracetamol overdose resulting in ALF was believed to be infrequent in Asia due to differences in healthcare cultural practices and lack of availability of over-the-counter drugs [⁸]. Whilst the latter fact may be true in lesser developed countries, many rapidly developing Asian nations have a wide availability of over-the-counter drugs and cultural practices which are similar to the West. To date, two reports from small sample-sized studies in Hong Kong and Penang, Malaysia, have only demonstrated low rates of paracetamol-induced hepatotoxicity ranging from 2 - 6% [⁹,¹⁰]. Details on predictive factors for paracetamol-induced hepatotoxicity among Asians remain uncertain. This study aimed to examine the prevalence of hepatotoxicity in a large sample of adults with paracetamol overdose in a multi-ethnic Asian population, and determine predictive factors for hepatotoxicity in these patients.

Between January 2005 to December 2009, a total of 1,410 cases were identified as paracetamol overdose according to ICD 9 or ICD 10 coding systems in both UMMC and HKL. 386 cases were excluded due to a combination of missing medical records, coding misclassification and age < 18 years. 1024 cases of medical records were finally available for analysis. The incidence of paracetamol overdoses from January 2005 to December 2009 in UMMC and KLGH are highlighted in Figure 1. In general, there had been an increase in incidence in both institutions from 2005 to 2006, with a steady rate of paracetamol overdose cases in the last 4 years. More cases of paracetamol overdose had been admitted to UMMC (n = 583) compared to KLGH (n = 441) during the period of study.

This retrospective study represents one of the largest case series of paracetamol overdoses that has been reported to date. The duration of 5 years demonstrated little variation in admission patterns (apart from the first year) and there had been no major changes in the medical management of paracetamol overdose during this period. As all cases of poisoning in this country are solely managed in public institutions, the data from this study is fairly representative of the population from the largest city in this country. Furthermore, the general Malaysian urban population consists of 3 major ethnic groups, i.e. Malays, Chinese and Indians [¹³]. Hence, our study has relevance to other populations in Asia as well.

In this study, we have demonstrated that despite significant numbers of paracetamol overdose over a 5-year duration, only 7.3% of this multi-ethnic Asian adult population developed hepatotoxicity and no (i.e. n = 0) patients developed ALF following paracetamol overdose. Our results are in concurrence with previous reviews that paracetamol overdose is a rare cause of ALF in Asians. It is noteworthy that > 50% of patients who ingested > 10 g of paracetamol and had a delayed (> 24 hours after paracetamol ingestion) administration of NAC did not develop hepatotoxicity. Whilst the usual characteristics of paracetamol pharmacology such as cumulative dose and delayed timing of NAC administration predicted hepatotoxicity, this study identified that ethnicity, particularly those of Malay (OR 2.22) and Chinese (OR 3.26), were independent determinants of hepatotoxicity. The latter fact may suggest, but does not confirm, that ethnic differences in paracetamol metabolism may contribute towards paracetamol-induced hepatotoxicity.

We have compared our case series with previous publications and summarised the salient features in Table 3. Paracetamol-induced hepatotoxicity rates, using a similar definition as ours, in Western patients have been reported to range from 15% to 36% [⁵,⁹,¹⁰,^14-22]. The only exception was a study from Edinburgh, UK, which reported a 4% rate of hepatotoxicity among 987 patients with paracetamol overdose, but patients who presented > 15 hours post paracetamol ingestion had been excluded [²¹]. Paracetamol overdose had a mortality rate of up to 7% despite NAC administration, although mortality rates have declined over time due to successful liver transplantation in many centres. In contrast to Caucasian patients in Western studies, two publications from Asia and a single study from the Caribbean have reported hepatotoxicity rates ranging from 2% to 7% [⁹,¹⁰,¹⁹]. Acute liver failure was not a feature and no mortality had been reported in the latter case series. The data from our large case series of 1024 patients appears to mirror the findings from these earlier smaller sample-sized studies.

A possible explanation for the lower hepatotoxicity rates in non-Western studies may be explained by the cumulative dose of paracetamol ingested by individuals. The minimal amount of paracetamol known to cause toxicity in adults is approximately 7.5 g and liver toxicity is usually associated with paracetamol doses of > 10 g [²³]. In our study, we estimated that 52% of patients had ingested > 10 g of paracetamol and the median dose in our population was 10 g. In several of the studies that have been reported from Western centres, the mean or median doses of paracetamol ingested have been reported to be higher, although not in all [¹⁷,²¹,²²]. The mean/ median doses of paracetamol ingested in studies from the U.K., U.S. and Denmark have been reported as 15 g, 18 g and 25 g respectively (Table 3). However, other studies on Caucasian populations with similar median paracetamol doses (to ours) still reported higher hepatotoxicity rates of 14 - 31% (Table 3). An additional point is that NAC administration in our patients was based on normalised dosing. Asian patients may have inadvertently received higher NAC doses compared to Western patients of a higher averaged body weight.

An alternative explanation for an increased paracetamol-induced hepatotoxicity among Western patients may be related to alcohol co-ingestion. Chronic alcohol exposure is recognised to increase short term toxicity from paracetamol overdose by 2 to 3 fold increase in hepatic content of cytochrome P4502E1, the major isoform responsible for the generation of the toxic metabolite from paracetamol [²⁴]. About 25% of Western patients with paracetamol overdose were documented to have a regular alcohol consumption and excessive alcohol co-ingestion with paracetamol overdose was reported in 20-40% of cases [¹⁴,¹⁵,¹⁸]. In contrast, the rate of alcohol co-ingestion was only 4.2% in our study and 10 - 17% in other Asian studies [¹⁰,¹⁹].

Variation in the timing of NAC administration may be another explanation for differences in hepatotoxicity rates between studies. Delayed administration of NAC to patients with potentially toxic doses of paracetamol is a recognised risk factor for hepatotoxicity, an observation that was also demonstrated in our study. Patients with paracetamol overdose who had received NAC > 24 hours after paracetamol ingestion in our study were 10.4 times more likely to develop hepatotoxicity compared to those who received NAC earlier. Among our study cases, 92% of patients received NAC within 24 hours. Unfortunately, data on the timing of NAC administration has not been reported widely in the literature, and further comparisons have not been possible. However, it is notable that despite 80% of patients receiving NAC within 24 hours of paracetamol overdose a 14% hepatotoxicity rate (i.e. double the rate in our study) was reported in a recent Australian study [⁵].

Pharmacogenetic variation in the metabolism of paracetamol between Caucasians and Orientals has previously been studied [²⁵]. In a study comparing urinary excretion of paracetamol metabolites in 125 Caucasians and 33 ethnic Chinese, heterogeneity in the conversion of paracetamol cysteine conjugates (toxic paracetamol metabolites) to mercapturate via N-acetylation had been demonstrated. Adults with Chinese ethnicity demonstrated relatively extensive glucuronidation but lower sulfation in paracetamol metabolism, when compared to Caucasians. Whilst clinical relevance of this variation in metabolic pathways remains uncertain, it is possible that intrinsic differences in the pharmacogenetics of paracetamol metabolism may be a major reason for differences in hepatotoxicity between Asians and Caucasians.

This study has obvious limitations in the light of its' retrospective design. Nevertheless, its' large sample size, representative study population and accurate capture of computerised laboratory data are its' major strengths. We have demonstrated that the rates of hepatotoxicity among 1024 Asian patients with paracetamol overdose is low at 7.3%. Although differences in the clinical characteritics of paracetamol overdose between Western and Asian patients are recognised, it is possible that intrinsic differences in paracetamol metabolism may be a contributory factor as well. Our data supports the findings from a recent study demonstrating that N-acetyl cysteine therapy is not cost-effective in the management of Asian patients with paracetamol overdose [²⁶], and treatment algorithms developed in the West may not be appropriate in the East.

The authors declare that they have no competing interests.

ARM collected the data, performed initial data analysis and drafted the manuscript. NYY provided administrative support. SM conceived and designed the study, performed final data analysis and helped to draft the manuscript. All authors read and approved the final manuscript.

The pre-publication history for this paper can be accessed here:

http://www.biomedcentral.com/2050-6511/13/8/prepub

We would like to thank the University of Malaya Short Term Research Grant for their partial funding in the conduct of this study.