Which ratio is used to test for patency of the liver?

• A. Direct bilirubin
• B. Indirect bilirubin
• C. Ratio of direct to total bilirubin
• D. Albumin

Answer: (C)

Bile flow status is best assessed by looking at how much bilirubin is in the conjugated form relative to the total bilirubin. The liver conjugates bilirubin to direct (conjugated) bilirubin, which is excreted into bile. When the biliary ducts are patent, conjugated bilirubin can reach the gut efficiently, so the direct portion in blood stays relatively proportionate to total bilirubin. If bile flow is blocked, conjugated bilirubin cannot leave the liver and backs up into the bloodstream, increasing the direct bilirubin fraction and hence the direct-to-total bilirubin ratio. Therefore, the ratio of direct to total bilirubin reflects the degree of cholestasis or obstruction of bile flow better than either value alone. Albumin indicates synthetic liver function, not patency of the biliary tract, so it isn’t used for this purpose.