Bilirubin terminology may be challenging for some learners. A helpful tip may be to remember English grammar:
VOWELS for B1 (indirect, unconjugated, insoluble)
CONSONANTS for B2 (direct, conjugated, water-soluble)
While some studies reported direct and indirect terminology is outdated, the truth of the matter is that this terminology is still commonly used in the clinical laboratory today.
Bishop, M. L. (2022). Clinical Chemistry: Principles, Techniques, and Correlations (9th ed.). Jones & Bartlett Learning.