EDTA sample contamination is common and often undetected, putting patients at unnecessary risk of harm.

BACKGROUND: Potassium ethylenediaminetetraacetic acid (EDTA) is a sample tube anticoagulant used for many laboratory analyses. Gross potassium EDTA contamination of blood samples is easily recognised by marked hyperkalaemia and hypocalcaemia. However, subtle contamination is a relatively common, often unrecognised erroneous cause of spurious hyperkalaemia. Potassium EDTA contamination may also cause hypomagnesaemia and hypozincaemia. There are, however, no data on the prevalence of EDTA contamination as a cause of hypocalcaemia, hypomagnesaemia and hypozincaemia.
METHODS: Following a recent service evaluation, we measure EDTA in serum samples from patients with unexplained hyperkalaemia (serum potassium > 6.0 mmol/l). In addition, over a 1-month period EDTA concentrations were measured in hypocalcaemic (serum adjusted calcium < 2.0 mmol/l), hypomagnesaemic (serum magnesium < 0.7 mmol/l) and hypozincaemic (serum zinc < 11 micromol/l) serum samples.
RESULTS: Ethylenediaminetetraacetic acid contamination was detected in 31 samples, nine of which were detected by our routine screening programme. The remaining 22 samples represented 14.3% (19/133) of hypocalcaemic samples, 4.8% (5/104) of hypomagnesaemic samples and 1.4% (2/139) of hypozincaemic samples. A total of 25/31 (80.6%) of patients were re-bled, of which 23/25 (92%) results normalised.
CONCLUSIONS: Factitious hyperkalaemia, hypocalcaemia and hypomagnesaemia caused by potassium EDTA contamination in our studies are relatively common, and if unrecognised may adversely affect patient care and waste scarce healthcare resources. Correct order of draw of blood samples, improved education and routine laboratory screening of EDTA are necessary to prevent and identify EDTA contamination.