Pseudonormokalemia in Patients with Thrombocytosis.

Introduction: Pseudohyperkalemia represents an artificial elevation in serum potassium concentration. It is well described that patients with thrombocytosis may have elevated serum but normal plasma potassium. The difference between serum and plasma potassium is felt to be due to potassium release from platelets during clotting. We propose to prove that a similar mechanism will lead to a "pseudonormokalemia," where serum potassium appears to be in the normal range (3.5–5.0 MEq/L) despite below-normal levels in the plasma(<3.5 MEq/L).

Method: This is an interim analysis of a prospective, IRB-approved planned comparison of 146 patients. We compared 36 thrombocytosis patients (platelets>500,000/uL) to 36 control patients (platelets<500,000/uL). Patients were identified from a list of lab results generated by a computer search and serum and plasma potassium and CBC were then drawn concomitantly. The two groups were compared using either the t-test or the Mann-Whitney U test depending on the distribution of the variables. A p <.05 was considered a priori to indicate statistical significance.

Results: There was no statistically significant difference in sex distribution or age between the two groups. The average platelet count was 643,190/uL in the thrombocytosis group (SD 134,426 uL) and 280,220/uL (SD 106,217 uL) in the control group with a p value <0.001. While the serum potassium was noted to be significantly different between the two groups, the plasma potassium was not (see table). This was reflected in the difference between the serum and plasma potassium in the two groups. The thrombocytosis group was noted to have a difference between serum and plasma potassium of .52 MEq/L (SD .32 MEq/L) while the control group had a difference of .18 MEq/L (SD .23 MEq/L). The thrombocytosis group had 14 cases in which the difference between serum and plasma potassium was over 0.5 MEq/L whereas the control group had none.

Conclusion: In this study, patients with thrombocytosis had higher mean serum potassium levels than the control group but similar mean plasma potassium levels. The mean difference between serum and plasma potassium (Delta) exceeded 0.5 MEq/L in the thrombocytosis arm and there was a statistically significant difference in the Delta values between the two groups. The mean platelet volume (MPV) of the thrombocytosis group was smaller than that of the control group. This indicates that the elevation in serum potassium in the thrombocytosis group cannot be attributed to the actual size of the platelets but rather to the number of platelets involved. Patients with thrombocytosis and normal serum potassium may actually be hypokalemic as this study demonstrates. As we continue to evaluate patients, we believe this difference will become more demonstratable. On interim analysis, our study suggests that in patients with thrombocytosis and normal serum potassium, plasma potassium should be considered along with routine labs.