EDTA contamination - a cause of spurious hyperkalemia that can go undetected

The anticoagulant potassium ethylenediaminetetra-acetic acid (K+ EDTA) is present in sample tubes used to collect samples for full blood count (FBC) and some other tests. However, the anticoagulant has the effect of markedly increasing plasma potassium concentration and reducing plasma calcium, magnesium and zinc concentrations and so the EDTA-anticoagulated sample is unsuited for these analyses. 

K+ EDTA contamination, arising during sample collection when blood is wrongfully decanted from a full blood count bottle to the correct sample tube for potassium analysis, is a well-documented cause of spuriously raised potassium (pseudohyperkalemia). 

Such frank contamination is easily detected because the resulting hyperkalemia is so severe that it is actually incompatible with life and accompanied by the tell-tale signal of marked hypocalcemia. According to the results of a recent study, less severe EDTA contamination occurs and goes undetected because the resulting falsely raised potassium is not sufficiently high to alert laboratory staff. 

During a 1-month period, the UK laboratory where this study was conducted processed 28,471 samples for potassium analysis. Of these, 117 had serum potassium ≥6.0 mmol/L. All 117 hyperkalemic samples were submitted for EDTA estimation. This revealed that 28 of the 117 (25 %) samples were contaminated with EDTA. A repeat sample was obtained from 27 of the 28 cases. 

In all of these cases plasma potassium was within the reference range on repeat testing, demonstrating the spurious nature of hyperkalemia. In 18 of these 28 cases, laboratory staff had identified EDTA contamination as the cause of hyperkalemia before EDTA estimation (for these 18 cases potassium ranged from 6.8 to 45.6 mmol/L) but in 10 cases (in which potassium ranged from 6.1 to 8.9 mmol/L) EDTA contamination went unrecognized and spurious results were reported. 

As a result of this study the authors have adopted the cautionary measure of submitting all samples with potassium >6.0 mmol/L for EDTA estimation.