A pitfall of measuring lithium levels in neonates.

If lithium therapy is required during pregnancy or lactation, serum lithium monitoring may be indicated in the newborns. In the neonatal population, however, blood samples are often obtained with a sampling device containing lithium heparin. Given the infrequent nature of lithium measurement in the neonatal population, a risk of oversight on the use of these lithium-containing devices for lithium measurements exists. Two such neonatal cases are reported, which may have been mismanaged if the measured levels of lithium were not suspected to be spurious. Patient 1 was a 3-day-old infant with a congenital heart disease born at 31 weeks' gestation to a mother on lithium. After a surgical repair, because of a potential need to monitor lithium in the infant's serum during breastfeeding, a remaining sample of apparently serum from a previous blood testing on day 1 of life was measured for lithium as a baseline value. Despite the lack of toxic signs, the result showed a toxic lithium level of 4.19 mmol/L. Lithium levels in follow-up samples were 0.11 mmol/L (day 4) and undetectable (day 6). Patient 2 was a full-term infant exposed to maternal lithium throughout the fetal life and breastfeeding. Serum lithium at day three of life in a hospital was undetectable, but after discharge, the lithium concentrations increased to 1.1 mmol/L on day 18, with no sign of lithium toxicity or renal dysfunction. This raised a possibility of significant exposure through breastfeeding, but later they were found to be spurious due to the use of lithium-containing devices. Separate investigation of effects of sampling devices on lithium levels indicated that levels of >3 mmol/L were possible if a sampling volume of blood was substantially small compared with the capacity of a tube containing lithium heparin [corrected]