Direct bilirubin is a sub-component of total bilirubin. Total bilirubin assays measure both direct (conjugated and delta) and indirect (unconjugated) bilirubin. When total bilirubin is low or undetectable there is no value in measuring the direct bilirubin level. Limiting direct bilirubin testing to individuals with elevated total bilirubin has been demonstrated to decrease unnecessary testing. Additionally, implementation of a laboratory reflexive testing algorithm for infants, where direct bilirubin is automatically tested when total bilirubin is elevated, has been proposed to accelerate the identification of biliary atresia while also reducing the need for additional blood collections.
Sources:
Katzman BM and Karon BS. Test Utilization Proposal for Reflex Bilirubin Testing: Why Order Two Tests When One Will Do? J App Lab Med, July 2021;6(4):980-984. PMID: 33454760.
Lam L, Musaad S, Kyle C, and Mouat S. Utilization of Reflex Testing for Direct Bilirubin in the Early Recognition of Biliary Atresia. Clinical Chemistry, May 2017;63(5): 973–979. PMID: 28283556.
Zhang G-M and Hu Z-D. Conjugated bilirubin as a reflex test for increased total bilirubin in apparently healthy population. J Clin Lab Anal. Feb 2018;32(2):e22233. PMID: 28523701.