Different methods of expressing the results of the aminopyrine COz breath test (ABT) were compared to determine the method that would be most sensitive for evaluating liver function. The patient population included healthy controls (n = 22); patients with mild (4), moderate (9), and severe (7) alcoholic liver disease; and patients with chronic persistent hepatitis (7), chronic active hepatitis (18), chronic active hepatitis with bridging (18), and chronic active hepatitis with cirrhosis (17). The ABT was performed with 2 pCi ['4C]aminopyrine or 2 mg per kg [13C]aminopyrine, and the results were expressed as the instantaneous labeled COz excretion rates at 30, 60.90, and 120 min after the dose, the maximum excretion rate, and the 120-min cumulative excretion. The 30-min parameter had the highest sensitivity toward moderate alcoholic hepatitis, severe alcoholic hepatitis, chronic active hepatitis with bridging, and chronic active hepatitis with cirrhosis (96%); however, when the ABT was repeated in six normal controls with 225 ml of orange juice, the 30-min rate was significantly reduced (7.8 .+. 1.3 vs. 5.6 f 1.2% dose per hr), and the rate of false-positive test results was increased (0 of 6 vs. 2 of 6) indicating that gastric delay influenced the 30-min parameter. The 60-min excretion rate, maximum rate, and 120-min cumulative excretion were the next most sensitive expressions of the ABT, and were not altered by the small gastric load. Of these three expressions, the 60-min parameter was the most convenient because it required fewer samples and a shorter test period.The aminopyrine CO2 breath test (ABT) measures the excretion of labeled C02 following the oral administration of N-methyl-labeled aminopyrine, a drug that is demethylated almost exclusively by the hepatic microsomal mixed-function oxidase system (1). The test is noninvasive, easy to perform using either stable or radioactive isotopes (21, and has demonstrated clinical utility for the evaluation and staging of hepatic disorders including alcoholic cirrhosis, chronic hepatitis, and liver injury resulting from acetaminophen overdose (3-6). The ABT has also been utilized to measure hepatic induction in patients receiving anticonvulsant drugs (7).The manner of expressing the results of the breath test has been chosen somewhat arbitrarily and has, therefore, varied between investigators. For example, Hepner and Vesell (3) developed a test based on a sample of breath Received December 11, 1981; accepted March 31, 1982. ?. COS at 2 hr after aminopyrine administration, while Bircher et al. (5) used multiple breath samples through 8 or 10 hr and calculated the disappearance rate of labeled COz from the breath. Other investigators have used the cumulative per cent dose excretion through 2 hr (8) or the peak excretion rate (4) calculated from breath samples collected every half hour. Henry et al. (8) recently compared the three former methods and concluded that the cumulative per cent dose excretion through 2 hr and the labeled C02 disappearance rate were mor...