Background: Pseudohyperkalaemia is when the in vitro blood potassium concentration is artefactually raised while the in vivo concentration is normal. With unexplained hyperkalaemia, pseudohyperkalaemia needs to be excluded to avoid unnecessary and potentially detrimental therapy. There are numerous causes, but no systematic approach for the investigation of outpatients with potential pseudohyperkalaemia exists in the literature.

Aims: To evaluate the in-house protocol.

Methods: Patients referred for investigation of potential pseudohyperkalaemia underwent an outpatient based protocol which is designed to determine whether the cause was due to delayed blood separation, clotting, centrifugation or a haematological abnormality.

Results: 32 patients with serum potassium of 5.5–7.1 mmol/l were referred. All patients had pseudohyperkalaemia; the most frequent causes were full blood count (FBC) abnormalities (28%), time >4 hours from sampling to centrifugation (28%) and sample clotting (25%). Anaemia was more likely to be found in male patients.

Conclusion: Before a problem can be treated, it must be confirmed and its aetiology identified. A systematic approach to investigate potential pseudohyperkalaemia has been presented. This confirmed the clinician’s suspicion of pseudohyperkalaemia and in the majority of patients the aetiology was also identified. The use of serum and plasma potassium with an FBC in the initial investigation will identify whether clotting or a haematological abnormality is the cause in about half of the cases. Assay of whole-blood potassium is less important as centrifugation is a rare cause. Time to centrifugation is likely to play a major part in the majority of the remaining cases.