kEDTA Sample Contamination: A Reappraisal

Potassium EDTA (kEDTA) contamination of serum samples is common, causing spurious hyperkalemia, hypozincemia, and hypocalcemia that if unrecognized may adversely affect patient care. Gross kEDTA contamination is easy to detect, but identification of spurious electrolytes due to small amounts of contamination requires measurement of serum EDTA. We validated an EDTA assay on the Abbott Architect and reassessed its value in identifying kEDTA contamination and in studying mechanisms for contamination.

Within- and between-batch imprecision, linearity, recovery, interference, and carryover were assessed. Serum supplemented with k₂EDTA plasma, to mimic sample contamination, was used to study its effect on potassium, calcium, zinc, magnesium, and alkaline phosphatase. Our current laboratory protocol for identification of kEDTA contamination, based on measurement of serum calcium, was compared to that of EDTA measurement.

The EDTA assay displayed acceptable performance characteristics. Hemoglobin was a positive interferent. EDTA was detectable in serum contaminated with 1% (v:v) k₂EDTA plasma. An increase in serum potassium of 0.54 mmol/L (11.9%) was observed at a measured EDTA concentration of 0.19 mmol/L, equivalent to 3.2% (v:v) contamination. At this EDTA concentration reductions were also observed in zinc (71%), calcium (1%), alkaline phosphatase (ALP) (4%), and magnesium (2.4%). The serum EDTA assay detected contamination in 31/106 patient samples with hyperkalemia (potassium ≥6.0mmol/L), 20 of which were undetected by the current laboratory protocol.

The EDTA assay displayed acceptable performance, with the ability to reliably measure EDTA at low concentrations. Only a small amount of kEDTA causes significant spurious hyperkalemia and is only reliably detected with EDTA measurement.