False positive acetaminophen concentrations in patients with liver injury

Abstract

Background: Acetaminophen toxicity is the most common form of acute liver failure in the U.S. After acetaminophen overdoses, quantitation of plasma acetaminophen can aid in predicting severity of injury. However, recent case reports have suggested that acetaminophen concentrations may be falsely increased in the presence of hyperbilirubinemia.

Methods: We tested sera obtained from 43 patients with acute liver failure, mostly unrelated to acetaminophen, utilizing 6 different acetaminophen quantitation systems to determine the significance of this effect. In 36 of the 43 samples with bilirubin concentrations ranging from 1.0-61.5 mg/dl no acetaminophen was detectable by gas chromatography-mass spectroscopy. These 36 samples were then utilized to test the performance characteristics of 2 immunoassay and 4 enzymatic-colorimetric methods.

Results: Three of four colorimetric methods demonstrated 'detectable' values for acetaminophen in from 4 to 27 of the 36 negative samples, low concentration positive values being observed when serum bilirubin concentrations exceeded 10 mg/dl. By contrast, the 2 immunoassay methods (EMIT, FPIA) were virtually unaffected. The false positive values obtained were, in general, proportional to the quantity of bilirubin in the sample. However, prepared samples of normal human serum with added bilirubin showed a dose-response curve for only one of the 4 colorimetric assays.

Conclusions: False positive acetaminophen tests may result when enzymatic-colorimetric assays are used, most commonly with bilirubin concentrations >10 mg/dl, leading to potential clinical errors in this setting. Bilirubin (or possibly other substances in acute liver failure sera) appears to affect the reliable measurement of acetaminophen, particularly with enzymatic-colorimetric assays.

Fig. 1

Apparent effect of total bilirubin concentration on acetaminophen measurements in serum samples from 36 patients with acute liver failure assayed using various methods/instruments: EMIT/AU400 (A); AAA-INDP/Dimension (B); AAA-THQ/Vitros 250 (C); FPIA/AxSym (D); AAA-INDP/Hitachi (E); Stanbio/AU400 (F). Abbreviations are the same as in Table 1. Horizontal dotted line represents LLD for each acetaminophen assay; vertical dotted line represents total bilirubin concentration cited by manufacturer as providing <10% error in measured acetaminophen concentration; and, solid line in panel C is the linear least-squares regression line (y=0.2707x+1.7125; r=0.9566). Note differences in scale of y-axis between panels.

Fig. 2

Extent of increased acetaminophen measurements in serum samples (n=36) from patients with acute liver failure. Abbreviations are the same as in Table 1. The lower and upper lines in the central box represent the 25th and 75th percentile values of the data set, while the middle line represents the median value. The vertical line extends from the lowest to the highest value in the data set, excluding outliers. Outliers are displayed as separate points (empty squares). The asterisk* denotes statistically significant effect.