Is Fasting Before Blood Tests Necessary? New Study Suggests Not for Most Tests
Børge Grønne Nordestgaard and his colleagues at Copenhagen University Hospital point out that demonstrating how fasting adds little to no value when interpreting lab results for the majority of blood draws delivers a crucial message to patients, medical staff, and diagnostic facilities.
Ten Analytes Showed Fasting-Related Differences
A retrospective cross-sectional study examining lab data gathered between 2021 and 2023 from a single 670-bed university hospital was carried out by researchers under the direction of Sunghwan Shin, a laboratory medicine physician at the University of Ulsan’s Asan Medical Center in Seoul. The study subjects were adult outpatients with documented pre-bloodwork fasting durations. The research team divided these durations into three categories: less than eight hours, eight to 12 hours, and more than 12 hours. Sollip Kim of the University of Ulsan College of Medicine and colleagues reported that the final dataset included over 9.7 million test results from more than 101,000 patients. The authors scoured the data for signs of fasting-related differences that exceeded established analytical performance specifications.
After performing between-group comparisons and within-person analyses, the researchers found a link between fasting duration and performance-exceeding differences for just 10 of the 121 analytes targeted by the blood tests. Fasting did prove helpful for those few analytes. Glucose, triglycerides, gamma-glutamyl transferase, and lipase were higher in patients who fasted for less than eight hours.healio.com reported that 27 of 121 routine analytes exceeded specifications overall, but only 10 were confirmed in paired within-person analyses.
Glucose showed differences of +13.0 mg/dL and +10.8 mg/dL. Triglycerides showed increases of +17.1 mg/dL and +20.1 mg/dL. Gamma-glutamyl transferase registered differences of +21.7 U/L and +14.8 U/L. Lipase recorded a difference of +36.4 U/L versus fasts exceeding 12 hours.
Fasting Durations Beyond Twelve Hours Confer No Advantage
Prolonged fasting beyond 12 hours conferred no measurable advantage for any analyte examined in the study.healio.com noted that fasting beyond 12 hours had little effect on results, except for total bilirubin and 25-hydroxyvitamin D. For individuals who fasted more than 12 hours, total bilirubin was 17 percent higher, and 25-hydroxyvitamin D was 9.9 percent higher. No analyte exceeded analytical performance specifications when comparing the 8- to 12-hour group against the group fasting for more than 12 hours.
Conversely, individuals who fasted less than 8 hours before a test showed a linear trend in which glucose and lactate were significantly lower per additional hour of fasting. Healio.com noted that lactate and glucose were 27.9 percent and 15.4 percent lower, respectively, from 0.5 to 4 hours of fasting. Meanwhile, lipid results showed no significant short-term trend within that window.
Rigid Fasting Rules Originated From Lipid and Glucose Testing
Rigid fasting rules grew originally from concerns that patient diets might jeopardize clinical accuracy. Results of blood tests often provide the basis for important medical decisions, and some measurements are genuinely sensitive to pre-test variables like food intake. Medical professionals started instructing patients to complete a fast lasting eight to 12 hours prior to giving blood in order to cut down on this variability. The study authors explained that this practice originated with lipid and glucose testing but is now applied by default to much broader test panels.
Despite the prevalence of this practice, recommendations for optimal fasting duration remain inconsistent across laboratories.
Routine Blood Tests Require Flexible Fasting Requirements
The study authors wrote that their comprehensive analysis suggests most routine blood tests are minimally affected by short-term fasting, and fasting beyond 12 hours is not associated with meaningful additional differences. They suggested that traditional inflexible fasting protocols might benefit from being reevaluated in favor of a more selective, analyte-specific strategy for pretest preparation. Adequate fasting of eight or more hours should be maintained when glucose or precise lipid measurement is the goal, while greater flexibility may be reasonable for many other routine tests.
In an accompanying commentary, clinical biochemist Liv Tybjærg Nordestgaard of Copenhagen University Hospital and her colleagues noted the study provides strong support for accumulating evidence that fasting blood sampling is not necessary. The research findings suggest that patients could eat and drink in the hours before many routine tests without throwing off results, though the study authors caution that these results alone do not warrant immediate changes to pre-test fasting standards or mean patients should disregard medical instructions to fast.