Influence of diet and sample collection time on 77 laboratory tests on healthy adults

We studied the effect of a standardized breakfast or lunch before blood sampling on 77 analytes.

The mean difference between assays from 20 healthy adults was calculated on blood samples taken before and after food intake. Significant differences were tested using two-tailed Student t-test and compared to the acceptable limits derived from analytical and intraindividual biological variation.

Most of the analytes investigated were not significantly affected by food intake. Six of them were influenced by breakfast or lunch: triglycerides, glucose, creatinine, C-peptide and insulin were significantly upregulated, whereas testosterone was downregulated. Fourteen parameters were more influenced by time of sampling than by meals: nine decreased during the day (total bilirubin, BNP, myoglobin, cortisol, TSH, C-telopeptide, prolactin, ACTH, uric acid) and two increased (white blood cells, neutrophils). Three parameters showed levels that were similar at 9:00 am and 5:00 pm but their lowest level at 12:30 pm (inorganic phosphorus, osteocalcin, PTH).

Fasting is necessary for some laboratory tests. Clinicians should be aware of variations due to sampling time before ordering non-fasting tests, and in the subsequent interpretation of results.