False-high blood salicylate levels in neonates with hyperbilirubinemia

Drug assays may yield false-positive results caused by cross-reacting compounds. After finding a serum salicylate concentration of 81 μg/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 intake, the authors aimed to compare this situation with the phenomenon involving endogenous digoxin-like substances, which cross-react with the routine assay of digoxin. None of the participants in the study had been exposed to salicylate. Salicylate concentration was measured in all patients using Trinder's colorimetric method and in the second stage of the study also by AxSYM salicylate assay. Salicylate concentration using Trinder's method was 18 ± 25 (4-81) μg/mL among nine seriously ill children in the pediatric intensive care unit, of whom two children with extensive burns had salicylate levels of 30 and 81 μg/mL, respectively. Salicylate concentrations were 107 ± 24 (45-143) μg/mL and 60 ± 25 (28-92) μg/mL, among 18 premature newborns and 18 term newborns, with hyperbilirubinemia, respectively. In the second stage, which involved 22 jaundiced term newborns and cord blood from 21 pregnant women, Trinder's method yielded elevated salicylate blood levels among the hyperbilirubinemic infants: 82 ± 5 (73-89) μg/mL; however, the AxSYM assay yielded significantly lower blood levels: 2.5 ± 3.4 (0-10.9) μg/mL (P < 0.0001). Among the pregnant women, salicylate cord blood levels were found to be low - within the limit error of the assay with both assay methods. In conclusion, when salicylate intoxication is suspected, particularly during the neonatal period, it is advisable to measure salicylate levels by immunoassay technology.