Investigations and diagnosis
There is a commercial test, the "Invader assay" which can identify mutations in the UGT 1A1 gene. However, this test is not available in Australia. Rifampicin has been used as an alternative test to confirm the diagnosis of Gilbert's syndrome.
Rifampicin induces cytochrome P450 enzymes and competes with bilirubin for excretory pathways in the liver. In individuals with Gilbert's syndrome who have reduced levels of UGT, addition of rifampicin causes an exaggerated elevation in total serum bilirubin levels.
Method: A single dose of rifampicin 900 mg is given and the total serum bilirubin is measured at baseline and 2, 4 to 6 hours after the dose has been administered.
A rise in total serum bilirubin to > 32 micromol/L (in a fasting state) or to > 25 micromol/L (in a non-fasting state) distinguishes patients with Gilbert's syndrome from those without.
Note: This test should be used with caution in patients with liver disorders.