Components of variance of some plasma constituents in patients with myocardial infarction.

Analytical, intra-individual, and inter-individual components of variance were estimated for eight plasma analytes in 20 patients who had myocardial infarction. Replicate analyses were performed on an average of 19.5 specimens obtained from each patient over a four-day period. Analytical variance was greater than 25% of the total variance for sodium, chloride, and bicarbonate levels but less than or equal to 12% for potassium, urea, creatinine, calcium, and albumin. Sodium, bicarbonate, urea, and creatinine concentrations showed strong individuality while those of potassium chloride, calcium, and albumin did not. Estimates of average intra-individual variation were larger than those previously documented; this is probably due to variation in sample collection, transport, and handling. Analytical goals were derived from intra-individual variation but it is not recommended that these be adopted in hospital laboratories.