Correlation of selected serum constituents: 1. Inter-individual variation and analytical error

The intra-subject correlations of three clinically meaningful combinations of serum constituents--(a) potassium, calcium, and albumin; (b) urea, creatinine, and uric acid; and (c) aspartate aminotransferase, lactate dehydrogenase, and alkaline phosphatase--were studied in 11 healthy men. Duplicate serum samples were obtained at 800 h, 1100 h, and 1400 h on five different days. All assays were performed on the AutoChemist Multichannel Analyzer. Correlation coefficients differed significantly among the subjects for the following six pairs of serum constituents: urea and creatinine, urea and uric acid, creatinine and uric acid, aspartate aminotransferase and lactate dehydrogenase, aspartate aminotransferase and alkaline phosphatase, and lactate dehydrogenase and alkaline phosphatase. Nonbiological positive correlation between analytical errors (i.e., errors of two different assays performed on the same specimen) was demonstrated for two of the pairs: potassium and calcium, and aspartate aminotransferase and lactate dehydrogenase. The error correlations of these two pairs of constituents comprised a significant component of the observed intra-subject correlations. Probable reasons for these analytical error correlations are discussed.