Factors contributing to intra-individual variation of serum constituents: 5. Short-term day-to-day and within-hour variation of serum constituents in healthy subjects.

We evaluated the variations in some serum constituents in a group of healthy young men for two selected time intervals: short-term day-to-day changes and withinhour changes. In the first case, we used a two-way ANOVA model to compute the main-day effect and the subject-day interaction terms, which were combined to yield the total day-to-day variation. A main-day effect was seen to be statistically significant only for acid phosphatase, while all of the 18 serum constituents except for sodium, calcium, and albumin demonstrated a statistically significant subject-day interaction. For the within-hour biologic variation, a three-way ANOVA model was used to analyze results of duplicate serum samples drawn at 1100 h and 1130 h on two different days. Although a significant main effect of hour was found only for total lipids and alkaline phosphatase, pooling the main effect of hour, subject-hour interaction, and subject-day-hour interaction terms resulted in a chemically significant variation for potassium, total protein, albumin, iron, total lipids, cholesterol, and bilirubin. The relationship of these biological fluctuations is compared to the expected analytical variation in all