Abstract
Pre-analytical errors account for the majority of laboratory-associated errors. In a 5 months old infant hospitalised with lung dysfunction due to prematurity, a routine measurement of zinc revealed an unexpected elevated concentration of 20.2 µmol/L (reference interval 10.0 - 19.0 µmol/L) compared to 11.6 µmol/L five days earlier. Zinc measurement was repeated two days later and had further increased to 42.4 µmol/L. Of note, there were no clinical signs of the increased zinc concentrations. Performance data for the zinc analysis (performed by inductively coupled plasma mass spectrometry) was found satisfactory. A thorough review of the patient´s medication and nutrition supplements revealed no relevant zinc content. The blood was obtained through capillary blood sampling, and anything at the skin puncture site containing zinc could therefore potentially contribute to the elevated zinc results. It was investigated if any ointment containing zinc had been applied at the puncture site, which revealed that the mother had applied vitamin E ointment containing zinc-oxide at the infant's heel. A capillary sample obtained from the opposite heel, where no vitamin E ointment had been applied, revealed a zinc concentration of 14.3 µmol/L. In conclusion, pre-analytical contamination with ointments must be considered in case of unexpected measurements from capillary blood.