Transport and temperature effects on measurement of serum and plasma potassium

Transport of blood samples from general practice to a central laboratory ca n result in spuriously high or low potassium concentrations, The importance of this phenomenon was studied in a general practice serving a population of 15 000 patients, 27 km from the pathology laboratory that routinely meas ured serum potassium. The design involved comparison of potassium levels be tween control serum (plain gel-separation serum tubes centrifuged in the su rgery), routine serum (plain gel-separation tubes centrifuged in the labora tory) and routine plasma samples (lithium-heparin tubes centrifuged in the laboratory). Complete triple sets of data were obtained for 371 samples. Altman and Blan d plots for the control serum vs routine serum samples showed a mean differ ence of +0.1 mmol/L with limits of agreement (+ 2SD) +0.6 mmol/L, - 0.4 mmo l/L and for control serum vs routine plasma a mean difference of +0.2 mmol/ L with limits of agreement +0.8 mmol/L, - 0.4 mmol/L. There was a negative association between mean weekly routine plasma potassium levels with mean w eekly temperatures achieved, Regression analysis indicated that both maximu m temperature achieved and time to centrifugation significantly contributed to differences observed in the routine plasma samples, but not with the ro utine serum samples, For plasma samples exposed to high temperatures a clin ically significant lowering of potassium concentrations can arise. These results confirm that spurious lowering of potassium concentrations oc curs in plasma samples collected in a primary care setting. The preferred m ethod is to centrifuge samples soon after venepuncture, Where this is not p ossible, collection into plain gel-separation tubes (serum) ensures less va riation due to temperature and time to centrifugation than does collection into lithium-heparin tubes (plasma).