Diagnostic accuracy of icteric index to detect abnormal total bilirubin values

Total bilirubin tests are individually inexpensive but can result in huge costs to healthcare systems worldwide, because they are very commonly requested. The main objective of this study was to evaluate the diagnostic accuracy of the icteric index, a test with zero reagent cost, as a preliminary biomarker to decide when total bilirubin measurement is necessary.

We retrospectively reviewed all the requested total bilirubin values and their corresponding icteric index results in a 2-year period (2010-11). Receiver operating characteristic (ROC) curve analysis was used to determine the optimal icteric index cut-off value to discriminate between normal and abnormal bilirubin values (>20.5 μmol/l; >1.2 mg/dl).

The study included total bilirubin and icteric index results of 100,207 patients. The ROC curve analysis suggested 34.2 μmol/l (2 mg/dl) as the optimal icteric index threshold to identify abnormal bilirubin values. The area under the curve was 0.981. Regardless of the patient population studied (primary care, hospital and different individual hospital departments), sensitivity was greater than 94% and specificity was over 98%. Likelihood ratio values provided strong evidence that total bilirubin is less than 20.5 μmol/l (<1.2 mg/dl) when the icteric index is less than 34.2 μmol/l (<2 mg/dl).

This study supports the use of the icteric index to identify patients with abnormal total bilirubin values. Considerable economic savings can be achieved if this strategy is applied on a clinical daily basis. Its implementation and report should be discussed and planned by laboratory professionals together with requesting physicians.