The diagnostic value of the nicotinic acid (NA)-induced hyperbilirubinaemia was compared with that resulting from caloric restriction in 40 patients with Gilbert's syndrome (GS) and 20 controls. Both tests resulted in a significant higher level of serum bilirubin in GS than in controls (P less than 0.001). When the serum bilirubin level 240 min after NA administration (5.9 mumol/kg i.v.) was higher than 18 mumoles/l, this test had a specificity and sensitivity of 100%, both in males and females with the syndrome. The discriminatory value of the test was lower when either the area under the time concentration curve or the maximal increment of serum unconjugated bilirubin were used. Reduction in caloric intake (400 calories/day) showed a lower specificity and sensitivity than the NA test, particularly in females. An increment of bilirubin at 24 h greater than 15 mumoles/l was more diagnostic than an increase by 100% or more over the pre-diet value. The efficacy was not improved by prolonging the test for additional 24 h. From these data we conclude that NA-induced hyperbilirubinaemia and, in particular the concentration of the pigment 240 min after drug administration, is more efficient than fasting-induced hyperbilirubinaemia in the diagnosis of the Gilbert's syndrome both in males and in females.
