False-high blood salicylate levels in neonates with hyperbilirubinemia

Drug assays may yield false-positive results caused by cross-reacting compo unds. After finding a serum salicylate concentration of 81 mug/mL by using Trinder's colorimetric method, in a comatose child admitted to the authors' pediatric intensive care unit, in the absence of reported salicylate intak e, the authors aimed to compare this situation with the phenomenon involvin g endogenous digoxin-like substances, which cross-react with the routine as say of digoxin. None of the participants in the study had been exposed to s alicylate. Salicylate concentration was measured in all patients using Trin der's colorimetric method and in the second stage of the study also by AxSY M salicylate assay. Salicylate concentration using Trinder's method was 18 +/- 25 (4-81) mug/mL among nine seriously ill children in the pediatric int ensive care unit, of whom two children with extensive burns had salicylate levels of 30 and 81 mug/mL, respectively. Salicylate concentrations were 10 7 +/- 24 (45-143) mug/mL and 60 +/- 25 (28-92) mug/mL, among 18 premature n ewborns and 18 term newborns, with hyperbilirubinemia, respectively. In the second stage, which involved 22 jaundiced term newborns and cord blood fro m 21 pregnant women, Trinder's method yielded elevated salicylate blood lev els among the hyperbilirubinemic infants: 82 +/- 5 (73-89) mug/mL; however, the AxSYM assay yielded significantly lower blood levels: 2.5 +/- 3.4 (0-1 0.9) mug/mL (P < 0.0001). Among the pregnant women, salicylate cord blood l evels were found to be low-within the limit error of the assay with both as say methods. In conclusion, when salicylate intoxication is suspected, part icularly during the neonatal period, it is advisable to measure salicylate levels by immunoassay technology.