Centrifugation, storage and sample contamination can significantly affect potassium levels and lead to falsely elevated results

Correct answers: a, b and d

Explanation of the answers

a) Correct.
A tube containing separator gel should never be centrifuged a second time, as repeat centrifugation may affect laboratory results (source: CSCQ – Swiss Centre for Quality Control).

For potassium testing, an increase of 1 to 2 mmol/l may be observed when a separator gel tube is centrifuged twice.

If a separator gel tube has not been centrifuged sufficiently, the serum should first be transferred into a clean, empty container before being centrifuged again.

b) Correct.
When uncentrifuged whole blood is stored in the refrigerator, the potassium concentration in the serum increases.

This is because refrigeration inhibits the pump on the surface of red blood cells that continuously exchanges sodium and potassium ions. The same phenomenon can also occur outside the refrigerator once the available glucose, which provides energy to the cells, has been depleted.

Serum potassium concentration increases by approximately 6% after 3 to 12 hours and by approximately 15% after 24 hours (source: Daves et al., Riv. Ital. Med. Lab., 2017).

After a maximum of 48 hours, a reliable potassium result can no longer be obtained.

For this analysis, whole blood should therefore be centrifuged as soon as the required clotting time has elapsed. If no separator gel tube is used, the serum should be pipetted into another clean, empty container.

c) Incorrect.
In an EDTA tube, EDTA binds calcium. Potassium present in certain EDTA tubes can therefore result in a characteristic pattern indicating sample contamination:

• Potassium > 10 mmol/l

• Calcium < 0.2 mmol/l

d) Correct.
The body regulates electrolyte balance very tightly. If pre-analytical requirements have been correctly followed and the result is considered reliable, the cause of severe hyperkalaemia must be investigated urgently.

Conclusion

Pre-analytical requirements are essential when measuring potassium. If they are not followed correctly, serum potassium levels may increase and produce falsely elevated results.

Centrifugation should be carried out as soon as possible after blood collection, once clotting is complete. When a tube with separator gel is used, it should only be centrifuged once.

For further information, Dr Dany Mercan, FAMH specialist in clinical chemistry, is available on 022 716 20 75.