Fasting Duration and Differences Across Routine Laboratory Tests

Importance Fasting before blood collection is widely practiced to reduce preanalytical variability, yet whether fasting-related differences are clinically meaningful across a broad panel of routine analytes has not been systematically evaluated using clinical data. Objective To evaluate whether fasting duration is associated with routine laboratory test results and whether observed differences exceed established analytical performance specifications (APS). Design, Setting, and Participants Retrospective single-center cross-sectional study using laboratory information system data collected between February 2021 and December 2023 from a single 670-bed secondary-care university hospital in the Seoul metropolitan area, Republic of Korea. All samples were drawn from outpatients at the institution’s central phlebotomy unit. Participants were adult outpatients (≥18 years) with documented fasting duration. Data were analyzed from January 1, 2024, to July 24, 2026. Exposure Fasting duration derived from the time elapsed between the patient’s last meal and blood collection, categorized as less than 8 hours, 8 to 12 hours, and more than 12 hours. Main Outcomes and Measures Adjusted means across fasting groups were estimated using linear mixed-effects models with patient-level random intercepts, adjusting for age, sex, and department. Paired within-person analyses among repeatedly tested patients were performed to minimize between-patient confounding. Differences were compared with the European Federation of Clinical Chemistry and Laboratory Medicine biological variation–based total allowable error. Postprandial trends within the subgroup fasting less than 8 hours were assessed at 30-minute intervals. Results The final dataset comprised 9 755 547 test results from 101 148 patients across 372 test items. Between-group comparisons identified APS-exceeding differences for 27 analytes. However, only 10 were confirmed in paired within-person analyses, suggesting confounding. Glucose (10.8%), triglycerides (20.1%), γ-glutamyl transferase (14.8%), and lipase (36.4%) were higher with less than 8 hours fasting. No analyte exceeded APS between the 8- to 12-hour and more than 12-hour groups. Within the less than 8-hour subgroup, glucose and lactate decreased significantly with increasing fasting duration, whereas lipid concentrations showed no significant linear trend. Conclusions and Relevance This cross-sectional study found that fasting duration was associated with differences exceeding analytical performance specifications for only a few analytes, mainly glucose, triglycerides, γ-glutamyl transferase, and lipase. No estimable within-person difference between the 8- to 12-hour and more than 12-hour groups exceeded an available APS. These findings support analyte-specific rather than uniform fasting requirements.