- Background: Hyperbilirubinemia ofthe newborn is common. Rarely is an underlying disease other than physiologic hyperbilirubinemia considered the cause of high bilirubin levels. Some of the laboratory tests recommended by the American Academy of Pediatrics are expensive and do not always lead to diagnosis. Objective: To evaluate the efficacy of standard laboratory tests performed on newborn infants requiring phototherapy for hyperbilirubinaemia. Methods: We conducted a retrospective chart review that included neonates born during a 6 month period with birth weight > 2500 g treated with phototherapy for hyperbilirubinemia (n=282) according to published guidelines. The main outcome measures were primary and maximal bilirubin values (mg/dl), time to jaundice (in days), the number of bilirubin tests undertaken and whether the patient showed abnormal functioning, and the number of days in follow-up. Results: Thirty-three neonates (11.7%) were positive in at least one laboratory test (defined as "Abnormal" in our study), 45.5% of whom met the criteria for phototherapy during the first 48 hours of life. Among the newborns who were negative for all laboratory tests (defined as "Normal"), only 6.8% met phototherapy criteria within their first 48 hours of life (P < 0.001). In the Normal group there was a consistent decrease in total serum bilirubin values shortly after phototherapy was begun, while the Abnormal group presented an increase in serum bilirubin values during the first 12 hours of phototherapy. None of the infants had conjugated (direct) hyperbilirubinemia during the study period. Conclusions: Most neonates presenting with a laboratory identifiable etiology for hyperbilirubinemia (i.e., hemolysis) can be distinguished from those who test negative, mainly based on the timing of presentation and response to phototherapy. A more meticulous selection of patients and reduction in the magnitude of routine laboratory testing can safely reduce discomfort to infants with hyperbilirubinemia as well as costs.