Perchlorate (Irenat®) may falsely lower measured ionised calcium

Discrepancies in ionised calcium concentrations between results from several point-of-care devices derived from intensive care unit (ICU) patients were postulated to be caused by perchlorate, a thyroid blocking agent. The deviations were serious concerning the diagnosis of hyper- or hypocalcaemia and administration of calcium infusions.

The problem was studied from three perspectives. First: quantification of ionised calcium in heparinised blood samples spiked with sodium perchlorate from healthy volunteers measured using five blood gas analysers (BGAs from IL, Radiometer, Roche and Siemens). Second: verification of clinical concentrations of perchlorate in blood samples after routine use for blood gas analysis from ICU patients. Third: retrospective analysis of data stored from patients during their stay in general ICU of the Departments of Anesthesiology and General Surgery by a patient data management system.

Category 1: three of the point-of-care testing devices measure clinically relevant falsely low ionised calcium concentrations when exposed to concentrations perchlorate >0.1 mmol/L. Two were not able to identify hypercalcaemia of up to 4 mmol/L ionised calcium when specific perchlorate concentrations are exceeded. Category 2: measured clinical concentrations of perchlorate ranged between the lower limit of quantification [LOQ=0.03 mmol/L and 2.75 mmol/L (median=0.29 mmol/L; mean=0.585 mmol/L)]. A concentration above 0.1 mmol/L perchlorate was found in 74.3% of all samples that tested positive. Category 3: between February 2006 and July 2009, 42 patients per year (2.2%) received sodium perchlorate with a median length of treatment of 25 days.

Perchlorate causes clinically relevant lowering of measured ionised calcium in BGAs from two providers at concentrations obtained in samples from ICU patients (affecting about 1.1% of all ICU patients).