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ELEVATED BILIRUBIN MAY CAUSE FALSE POSITIVE ACETOMINOPHEN LEVELS USING AN ENZYMATIC COLORIMETRIC ASSAY
JL Shin1, PC Chan2, DN Juurlink1, L Cohen1
1Departments of Medicine, and 2Biochemistry, Sunnybrook Hospital, University of Toronto, Toronto, Ontario
BACKGROUND: The workup of hepatitis and hepatic failure frequently includes serum acetaminophen levels, especially where the history may be unclear. We present a case where a persistently positive acetominophen assay was found to be falsely associated with high bilirubin levels, at levels lower those typically suggested by methodological interference from the manufacturer of the assay.
Case: A 50 year old man presented with jaundice. He had marked elevations of both bilirubin and transaminases (bilirubin peak 696 µmol/L, AST 1182 IU/L, ALT 1388 IU/L, ALP 142 IU/L). He denied any acetominophen or alcohol use. Negative investigations include viral hepatitis serology, Wilson’s disease, autoimmune disease, and obstruction on ultrasound. The etiology of his liver disease was suspected to be multifactorial from iron overload, and acetominophen toxicity given his ferritin of 4069 (normal 20-400) and iron saturation of 100%, and multiple positive acetominophen assays. His acetominophen concentrations measured using an enzymatic assay ranged from 39-196 µmol/L, and correlated with bilirubin levels ranging from 210 to 696 umol/L. Analysis by the more labour-intensive high pressure liquid chromatography confirmed the absence of any acetominophen in 10 samples.
Discussion: Enzymatic colorimetric assays measure the absorbance of a colour complex formed as a result of the reaction between a chromogen and the breakdown product of acetominophen. The level of absorbance can be chemically and/or spectrally interfered by bilirubin. Assay monographs report less than 10% change in acetominophen concentration in the presence of bilirubin at 478.8 µmol/L (Roche Diagnostics, Indianapolis, US). However in our case we found falsely positive acetominophen levels at bilirubin levels as low as 210 µmol/L.
CONCLUSION: Colorimetric assays can record positive acetominophen levels in the presence of high bilirubin. The value at which this interference is seen may be much lower than diagnostic manufacturer’s specifications. A false diagnosis of acetominophen hepatotoxicity may obscure another etiology that would be contributory. Therefore, alternate testing strategy such as serial dilution or gas chromatography would be helpful, and in the interim, further investigations should continue to find the true cause of hepatotoxicty. Relevant literature is also reviewed.