Pseudohyperkalemia in patients with increased cellular components of blood

OBJECTIVE: We performed a study to investigate the difference between serum and plasma potassium concentration in patients with increase in one or more of the cellular components of blood. DESIGN AND METHODS: This study was performed in two phases. During the first phase, we performed a cross-sectional comparison of the difference between serum and plasma potassium concentration (Dk) in 341 patients with the various clinical conditions where pseudohyperkalemia has been described, as well as with secondary or spurious erythrocytosis and in 30 normal controls. A cut-off value of Dk discriminating polycythemia vera from other erythrocytoses was estimated. In the second phase we studied the significance of this cut-off value as predictor of polycythemia vera in 90 naive patients who were referred with an elevated hematocrit. RESULTS: Dk was significantly increased in the groups with platelet, erythrocyte or with a mixed type disorder compared to the controls (P < 0.01). Among these groups, Dk was significantly increased in the groups with thrombocytosis and mixed type disorder, compared to the group with erythrocytosis (both P < 0.01). A cut-off value of Dk discriminating polycythemia vera from other erythrocytoses was estimated (0.70 mmol/L). Dk (> or = 0.70 mmol/L), platelet and white blood cell count were identified as significant independent predictors of polycythemia vera. CONCLUSIONS: The Dk is increased in patients with erythrocytoses, thrombocytoses or both. This phenomenon is more profound in patients with a mixed type disorder, such as polycythemia vera patients, compared to those with erythrocytoses alone.