Establishing a practical blood platelet threshold to avoid reporting spurious potassium results due to thrombocytosis.

BACKGROUND

Thrombocytosis is one of several pre-analytical factors which contribute to spuriously high serum potassium concentrations, yet there is little published data to guide analysts in the selection of a specific platelet count threshold above which serum potassium results become unreliable. We have studied the sensitivity and specificity of blood platelet count as a predictor of false elevations in potassium.

METHODS

Paired serum and plasma potassium measurements together with full blood count were performed for 300 patients. All samples were stored at room temperature and analysed within 4 h of collection. The difference between serum and plasma potassium was plotted against blood platelet count.

RESULTS

When the difference (serum-plasma) in potassium concentration was plotted against platelet count, there was a direct linear relationship. Blood platelet counts of >500 x 10(9)/L will detect elevations in serum relative to plasma potassium of >0.5 mmol/L, with a sensitivity of 71% and a specificity of 89%.

CONCLUSION

It is recommended that where blood platelet count is above 500 x 10(9)/L, potassium measurements should be repeated using lithium heparin plasma. When serum potassium results of >5.4 mmol/L are obtained, it is our policy to check the platelet count if a sample is available before reporting results. If available and above 500 x 10(9)/L, potassium results are withheld and plasma requested.