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.