The effect of thrombocytosis on serum potassium and phosphorus concentrations.

Thrombocytosis is a cause of falsely elevated serum potassium concentrations, and phosphorus concentrations may be similarly distorted. Because plasma concentrations are not affected, the difference between the serum and plasma concentrations detects spurious elevations. The authors, in this study, sought to determine the degree of correlation between thrombocytosis and false elevations in serum potassium and phosphorus concentrations. Ninety-one general, medical/surgical patients with elevated platelet counts were identified by laboratory reports. Subjects were stratified into blocks by platelet count. Samples were obtained simultaneously for serum and plasma potassium and phosphorus concentrations and complete blood counts. The serum minus plasma concentrations for potassium (Kdiff) and phosphorus (Pdiff) were calculated and analyzed against each other and the platelet count by linear regression. A control group of 20 subjects with normal platelet counts was used to verify laboratory results with literature values. The Kdiff and Pdiff values in the control group very closely approximated literature values of 0.4 mmol/L and 0.08 mmol/L, respectively. Platelet count was a moderate predictor of Kdiff, r2 = 0.55 (p = 0.00001). Kdiff exceeded the upper limit of control at a platelet count of approximately 600 x 10(9)/L. Platelet count also correlated with Pdiff, r2 = 0.31 (p = 0.00001). Additionally, Kdiff correlated with Pdiff, r2 = 0.39 (p = 0.00001). Thrombocytosis is associated with false elevations in measured serum potassium and phosphorus concentrations. Additionally, the magnitude of elevations in potassium and phosphorus concentrations appear to be related.