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Objective: We compared the ascitic fluid to serum bilirubin ratio with three other ways of classifying ascitic fluid to the categories of exudate or transudate: the serum-ascites albumin gradient, the total protein concentration of the fluid, and the adaptation of Light's criteria for the detection of pleural fluid exudate, i.e., fluid to serum protein or LDH ratio or fluid LDH concentration. (Recently it has been reported that the pleural fluid to serum bilirubin ratio is statistically equivalent to Light's criteria.) Also, we evaluated whether the addition of the bilirubin ratio to the other criteria increases their diagnostic accuracy., Methods: Eighty-one specimens of ascitic fluid from 81 different patients were obtained. They were analyzed prospectively by SMA12, whereas the category of the fluid was determined according to the clinical diagnosis. The diagnostic accuracy of each criterion alone and in combination with the bilirubin ratio, with reference to the contended etiology, were evaluated., Results: The best criterion is the albumin gradient (overall accuracy = 0.84). The bilirubin and LDH ratio criteria had equivalent overall accuracy (0.815 and 0.802, respectively). The addition of the bilirubin ratio to any criterion did not improve its predictive or overall accuracy., Conclusions: Ascitic fluid to serum bilirubin ratio is an additional marker for the distinction of transudate from exudate. A ratio > 0.6 has a statistically significant association with exudate.

Whether fluid accumulating in the pleural space is a transudate or an exudate is determined by the widely used criteria of the pleural fluid to serum LDH and protein concentration ratios. Such a distinction is important for limiting the extent of the differential diagnosis of possible causes for this condition. We have found that a pleural fluid to serum total bilirubin ratio can serve the same purpose. The correlation of a bilirubin concentration ratio of 0.6 or more with the presence of an exudate as determined by established criteria is statistically highly significant; and its sensitivity, specificity, positive predictive accuracy, and overall accuracy in relation to etiology and LDH or protein criteria (Light's criteria) are about 90 percent. Hence, the bilirubin criterion is statistically equivalent to the widely accepted LDH and protein criteria.

An in vitro study demonstrated inerference with the total serum bilirubin assay by toxic rifampin levels. This interference was not observed with therapeutic rifampin levels.