Glucose-6-phosphate dehydrogenase (G6PD) deficiency, a recessive X-linked trait, is the most common enzyme deficiency in the world. The most devastating clinical consequence of this deficit is severe neonatal jaundice, which results in sensorineural deficit, and severe haemolytic anemia. However, patients may be asymptomatic. The most common clinical sign is hyperbilirubinemia (h↑), that is also related to Gilbert's syndrome, a condition associated with the promoter polymorphism of the UDP-glucuronosyltransferase 1 (UGT1A1) gene. The aim of this study was to underline (as is usually done by DNA molecular analysis) to detect and to clarify the genetic deficiency that is the reason of the disorder in question. In this study, different techniques were applied to analyse a family of four individuals presenting with hyperbilirubinemia: bilirubinic dosage, electrophoresis and enzymatic activity dosage of G6PD; molecular analysis of the UGT1A promoter to detect a thymine-adenine (TA) insertion, that causes the [A(TA)7TAA] mutation. The results showed that in certain cases, the presence of hyperbilirubinemia is not only associated with G6PD deficiency, but may be caused by the co-presence of a mutation in the UGTA1 promoter related to Gilbert's syndrome. As being affected by these two conditions predisposes to adverse effects towards certain drug treatments, it is advisable to study the UGTA1 gene before prescribing drugs for specific antineoplastic or retroviral tratment. We emphasize that investigating both the UGT1A gene and G6PD activity is the most reliable way to make a correct differential diagnosis.

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