When to collect blood specimens: midmorning vs fasting samples

This study was carried out to define the consequences of collecting blood specimens during the forenoon instead of using fasting specimens collected early in the morning. Extensive laboratory data were obtained from specimens collected from fasting participants at 0800, after breakfast at 0930, and again at 1100. The subjects were inpatients in medical and surgical wards (n = 51; 13 women and 38 men; ages, 32-87 years) and subjectively healthy volunteers corresponding to outpatients (n = 51; 31 women and 20 men; ages, 18-63 years). The coefficient of variation (CV, %) of the patient results was compared with the analytical CV. The observed CVs of the subjects' results far exceeded the analytical CV (%), the average being 3.5-fold and up to 14-fold for some analytes. In individual results the observed change often exceeded the medically derived clinical critical difference. Laboratory data should always be interpreted in the context in which they were obtained. Clinical decisions should be based on objective data (observations) more than on experience and educated guesses. Different medical specialities and different clinical situations may require different kinds of procedures.