Direct bilirubin is the fraction that the liver has already conjugated with glucuronic acid and prepared for excretion into bile, measured as the portion that reacts without an added accelerator. Because conjugation happens inside the liver cell and excretion happens through the bile ducts, this fraction reports on the stretch of the pathway that comes after uptake. Subtracting it from the total gives the unconjugated fraction, and the ratio between them carries most of the interpretive weight.
No readings for this marker yet.
A raised direct fraction most often points toward impaired excretion: obstruction anywhere along the bile ducts, cholestasis within the liver, or hepatocellular disease severe enough to disturb bile secretion. When the total is raised but the direct fraction is not, attention shifts instead to red cell breakdown or to inherited conjugation variants. Low values are unremarkable and are not interpreted on their own. Any raised result is followed up with repeat testing and a doctor's assessment rather than read as a diagnosis.
The figure is measured directly rather than calculated from the total, but haemolysed or light-exposed samples make it unreliable; assay methods differ enough that results from different laboratories are not always interchangeable. It is read together with total bilirubin, from which the unconjugated fraction is calculated, and with alkaline phosphatase and GGT, which mark cholestasis, while ALT and AST indicate hepatocyte injury.
Reference ranges are general adult values and depend on lab, assay, age and sex. For orientation, not diagnosis — discuss results with your doctor.