A Methodfor SimultaneouslyEstimatingPlasma,Erythrocyte,and LeukocyteSodium, Potassium,and Magnesium:ReferenceValuesand Considerationsfrom BiologicalVariation Data

We describe a method for measuring plasma, erythrocyte, and Ieukocyte sodium (Na), potassium(K), and magnesium(Mg2) concentrationsin 10-mLbloodspecimens.After separatingcells with Ficoll-Hypaque and washingwith isotoniccholinechloride,erythrocytesand leukocytesare counted, so that resultscan be expressedas amountof substance per cell and also to monitor cell integrity and possible contamination.Plasma and cell lysates were analyzed (CV 7.O%) with flame photometryand atomicabsorptionspectrometry. Reference intervals for an elderly population with values for plasma electrolytes withinreferenceintervalsare similarto those for youngerhealthy subjects.From data on biologicalvariation, referencevalues for erythrocytecations are notof muchuse, and analyticalgoalsforprecisionare not met, but the results might be usefulfor monitoringdisease progression in individual patients. In contrast, reference values for leukocyte cations are theoreticallyof use and goals are achieved, but large changes are requiredbefore consecutiveresultscan confidentlybe saidto be significantly different.