Excess EDTA interferes with cortisol measurement using a solid-phase, chemiluminescent enzyme immunoassay.

Hormone assays that use a solid-phase, automated, chemiluminescent enzyme immunoassay (CEIA) with an alkaline phosphatase-tagged hormone or antibody as a reporter are performed on serum or EDTA plasma in our laboratory. CEIA cortisol results appeared to increase in the presence of excess EDTA. We investigated the effect of the addition of different amounts of EDTA on cortisol concentrations in pooled canine serum samples. The recommended EDTA plasma concentration of 4.1 mmol/L (1.8 mg/mL) did not alter cortisol concentrations when added to serum pools; however, the addition of 5.1 mmol/L (2.25 mg/mL) of EDTA increased apparent concentrations of cortisol. Supplementation of serum samples with MgCl2 to 5 mmol/L reversed the effect of EDTA up to a concentration of similar to 8.1 mmol/L (3.6 mg/mL). Our findings show that CEIA cortisol results on EDTA plasma can be artificially increased if the EDTA concentration exceeds 5.1 mmol/L.