Pseudohyperkalemia

Potassium is one of the blood analytes most frequently measured in the clinical laboratory and at the point of care. Compared with most laboratory measurements, potassium is unusually prone to preanalytical error so that pseudohyperkalemia, defined as raised serum or plasma potassium concentration despite normal in vivo potassium concentration, is a relatively common occurrence that must be considered in any patient with unexplained hyperkalemia. 

 There are many ways in which poor sample collection and handling can give rise to spuriously raised serum/plasma potassium. A recently published UK study focuses on one of these - contamination of the sample at the time of collection with K+-EDTA, the anticoagulant additive used to preserve samples destined for hematological investigation. 

Gross K⁺-EDTA contamination of biochemical samples arises from the wholly unacceptable practice of decanting blood from the hematology sample tube to the biochemistry sample tube. The gross contamination that results from this practice is easily recognized because it causes spuriously raised potassium (and reduced calcium due to EDTA chelation) of such magnitude that it is almost, if not actually, incompatible with life. 

This study focused on the more subtle contamination that can occur as a result of backflow of K⁺-EDTA- contaminated blood-to-blood collection system (e.g. vacutainer) with transfer of minute amounts of K⁺-EDTA to subsequently collected samples. 

Those responsible for this study measured EDTA in all 665 samples identified during a 1-month study period that had either a raised serum potassium, reduced serum calcium, reduced serum magnesium or reduced serum zinc. EDTA contamination was found in 31 samples. Twenty-five of the 31 patients were available for immediate re-testing and in all but two, biochemistry was entirely normal on re-testing. 

In the other two cases results were closer to normal than they had previously been. In discussion of their findings the authors remind that pseudohyperkalemia due to subtle K⁺-EDTA contamination is avoided by the simple expedient of collecting blood for biochemistry testing before collecting blood into K⁺-EDTA- containing tubes. 

They suggest that, since education on correct order of blood draw is unlikely to be entirely successful in eliminating the problem, laboratories should consider measuring EDTA in all samples with raised potassium or reduced calcium, magnesium or zinc to avoid reporting spurious results, a policy that they have already in part adopted at their laboratory.