Effect of platelet count on serum and plasma potassium: evaluation using database information from two hospitals.

The availability of retrospective data from potassium (K+) analyses from two hospitals, one using serum and the other plasma for electrolyte measurements, offered us the possibility to investigate the effect of blood platelet count on serum and plasma K+ concentrations. A weak correlation between plasma K+ and platelet count was observed. The in vitro increase of serum K+ in proportion to the platelet count has clinical significance in conditions, where it may impede the detection of an underlying true K+ disorder. Nomograms and correction factors, based on the correlation between platelet count and serum K+, have been suggested also in some recent reports. In the present study unselected routine patient data was used as source data. The effect of platelet count on the concentration of K+ in serum was lower than reported in previous studies, as indicated by the regression analysis. An increase of 1000 x 10(9)/l in the blood platelet count would cause an increase of about 0.7 mmol/l in the serum K+ concentration (p < 0.0001, r = 0.155). The weak correlation between platelet count and serum K+ does not support the application of platelet-count-based correction of serum K+ level in thrombocytosis. The laboratory should notify the clinician of the significance of the in vitro increase of K+ caused by increased platelet count. K+ should be measured from plasma in such cases.