Factors contributing to intra-individual variation of serum constituents. 1. Within-day variation of serum constituents in healthy subjects.

We evaluated the within-day variation of serum constituents in a group of 11 healthy young men. Twenty-two constituents were assayed at the same time, 19 of them on the “AutoChemist” multi-channel analyzer. The statistical model of analysis-of-variance was used to separate certain factors-subject, time of day, and subject-time interactionfrom the analytical variation. The estimate of analytical variation was based on data for duplicate samples of blood which were taken from all subjects at 0800 h, 1100 h, and 1400 h. The following serum constituents varied significantly (P <0.05) as a function of time of day: sodium, potassium, chloride, urea, iron, bilirubin, total lipids, and acid phosphatase.