Conjugation with glucuronic acid is essential for efficient excretion of bilirubin. Bilirubin glucuronidation is mediated by a microsomal enzyme, the bilirubin uridine diphosphate glucuronosyltransferase (UGT1A1; EC 18.104.22.168) . Gilbert's syndrome (syn. icterus juvenilis Meulengracht) is an inherited benign disorder characterized by unconjugated hyperbilirubinemia in the absence of structural liver disease or haemolysis . The elevated unconjugated bilirubin (UCB) concentration is usually noted first in adolescence, either as an incidental finding or because of a slight yellow discoloration of the sclera. The serum bilirubin concentration characteristically fluctuates daily, occasionally falling within the normal range. During fasting, physical exercise, stress, intercurrent illness, or menstruation bilirubin exceeds the upper normal limit, sometimes by as much as four times. Gilbert's syndrome only occasionally causes critical symptoms, if treatment with cytostatic drugs as irinotecan is necessary. In these circumstances, live threatening complications as grade 4 neutropenia and diarrhoea may occur [3, 4]. Furthermore, patients with a UGT1A1 promoter polymorphism are more likely to develop hyperbilirubinemia after treatment with the anti-retroviral protease inhibitor atazanavir .
GS prevalence ranges from 2% to 12% in different populations [6, 7]. The inheritance of GS is either autosomal dominant with an incomplete penetrance or recessive. A promotor polymorphism of the TATAA box within the UGT1A1 gene is associated with GS in more than 50% of patients .
The clinical diagnosis of Gilbert's syndrome is made by exclusion of liver disease or overt haemolysis. Several provocation tests have been established to discriminate GS patients from patients with acute or chronic liver disease, haemolytic anemia and healthy volunteers.
The "caloric restriction" or "starving" test has been inaugurated by Augustine Gilbert itself . It involves restricting the caloric intake to 400 kcal per 24 h for two days. This should double UCB from the baseline level and clearly distinct GS patients from healthy volunteers . The discrimination from non-GS patients was insufficient, however, if patients with liver cirrhosis or acute hepatitis have been compared with GS patients [11, 12]. Moreover, this test is time-consuming and requires hospital admission to control blood glucose levels and the exact caloric intake throughout the test period. Moreover, an evaluation for the presence of GS fails the "Appropriateness Evaluation Protocol" (AEP) criteria of admission to hospitals and is not reimbursed by the public health insurances in Germany.
The nicotinic acid test involves overnight fasting and intravenous (50 mg) or oral (170 – 300 mg) administration of nicotinic acid (NA) [13, 14]. A significant increase of UCB is seen in GS patients. The most frequent adverse event of nicotinic acid is vasodilatation. It is frequently accompanied with tachycardia, headache, nausea and vomiting. Another adverse event is haemolysis, which is probably caused by a higher susceptibility of erythrocytes to splenic haemolysis, since the NA triggered UCB rise is abolished after splenectomy [15, 16]. In a small series, indomethacin pre-treatment was able to reduce NA side effects without changing the hyperbilirubinemic effect of nicotinic acid . All studies, which directly compared the starving test and the intravenous nicotinic acid test found a higher bilirubin increase in the latter [10, 17].
Rifampin induces cytochrome P-450 isoenzymes and competes for the excretory pathways in liver cells. The "rifampin test" with 900 mg orally administered rifampin had the ability to distinguish GS patients from controls in one pilot study . Actually, Hallal and colleagues confirmed the usefulness of the rifampin test in a study with 89 GS patients and pointed to the better diagnostic value of the relative rather than the absolute bilirubin increase after rifampin administration . In both studies, conflicting findings regarding rifampin induced haemolysis were reported. Furthermore, both studies did not report on the frequency of adverse events after rifampin administration, such as heartburn, vomiting or even hepatitis. As well, severe complications such as disseminated intravascular coagulation or serious hypersensitivity reactions including the Stevens-Johnson Syndrome and toxic epidermal necrolysis may rarely occur after rifampin administration.
The aim of this study was to evaluate a simple diagnostic test for GS, which avoids hospitalization and exposure to toxic test substrates. Single case observations revealed a rapid decline of UCB in overnight fasting GS patients, if they have ingested a standard European lunch. This approach is inexpensive, non-toxic and practicable in an out-patients department. Deriving from these observations, we conducted this prospective study to evaluate the diagnostic accuracy of this "inverse starving test" in patients with GS. The test was pre-assigned to be clinical useful, if an UCB-decline of at least 50% could be achieved. So far, no gold standard for the diagnosis of GS has been established. As a reasonable control investigation, we used the nicotinic acid test, which should increase UCB by 50%.