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An observational study of more than 100,000 patients in Korea found that fewer than 8 hours of fasting before routine blood tests did not meaningfully change the vast majority of 372 laboratory test items. Only a small number of tests, including glucose, triglycerides, and gamma-glutamyl transferase, rose measurably with shorter fasts.
Fasting less than 8 hours before routine blood tests generally did not change results, with a small number of exceptions, according to an observational study of more than 100,000 patients published in JAMA Internal Medicine. In paired within-person analyses covering 372 test items, the vast majority of tests did not exceed established analytical performance specifications when patients fasted less than 8 hours, reported Sollip Kim, MD, PhD, of the University of Ulsan College of Medicine in Seoul, Korea, and colleagues.
The retrospective single-center study drew on adult outpatients in Korea whose fasting duration was documented and whose blood was drawn by the institution’s central phlebotomy unit. The final dataset comprised 9,755,547 test results from 101,148 patients across 372 test items. The median patient age was 61 years, and 43% were women. To judge whether fasting-related differences were clinically meaningful, the researchers used the European Federation of Clinical Chemistry and Laboratory Medicine Biological Variation Database as a reference for total allowable error — in effect, the largest difference in a lab result that remains acceptable without materially affecting clinical interpretation. The team also used paired within-person analyses to minimize confounding between patients.
While most tests were unaffected, within-person differences were somewhat higher for certain analytes when comparing a fast of less than 8 hours against both 8-12 hours and more than 12 hours. Glucose was higher by 13.0 mg/dL and 10.8 mg/dL, respectively; triglycerides by 17.1 mg/dL and 20.1 mg/dL; and gamma-glutamyl transferase by 21.7 U/L and 14.8 U/L. For lipase, a comparison versus 8-12 hours was not available, but the difference versus more than 12 hours was 36.4 U/L.
Participants who fasted less than 8 hours showed a linear trend in which glucose and lactate were significantly lower with each additional hour of fasting, the authors reported. In contrast, lipid results were not any better with more hours of fasting. The researchers also found that prolonged fasting beyond 12 hours conferred no measurable advantage for any analyte.
What This Means for Blood Test Patients
The study challenges a longstanding practice in laboratory medicine: requiring patients to fast — typically 8 hours or more — before blood draws in an effort to standardize preanalytical variation in test results. If most tests are minimally affected by fasting duration, as this study suggests, patients could avoid early-morning appointments, long stretches without food, and repeat visits when they arrive at a lab unprepared.
In an accompanying editorial, Børge Grønne Nordestgaard, MD, DMSc, of Copenhagen University Hospital in Denmark, and colleagues wrote that the study “provides strong support for accumulating evidence that fasting blood sampling is not necessary.” Eliminating fasting requirements, they argued, should reduce the number of patients who present to laboratories unprepared, cut administrative tasks tied to fasting samples, and decompress crowded morning schedules, freeing time and resources for other work. For patients, they added, lighter preparation and fewer repeated visits “should improve adherence with testing, follow-up visits, and prescribed therapies, and may lead to better satisfaction with the healthcare system.”
Existing Guidelines Already Ease Fasting
Current clinical practice has already moved away from universal fasting in some areas. Diabetes diagnosis and management now relies heavily on HbA1c, a measure of average blood sugar over the previous 2-3 months that is less sensitive to day-to-day variability. Fasting plasma glucose remains an important test, however, particularly when HbA1c is not reliable.
On cholesterol, U.S. guidelines already strongly endorse either a nonfasting or fasting lipid profile for adults and children in general. A fasting lipid profile is still favored for LDL cholesterol assessment in certain groups: people with a family history of dyslipidemia or premature heart disease, those with a known or suspected disorder of triglyceride metabolism, and those whose nonfasting triglyceride levels exceed 400 mg/dL.
“This cross-sectional study found that fasting duration was associated with differences exceeding analytical performance specifications for only a few analytes. These findings support analyte-specific rather than uniform fasting requirements.”
— Sollip Kim, MD, PhD, and colleagues, University of Ulsan College of Medicine
Study Limitations and Open Questions
The study’s generalizability is limited in several ways that the authors and editorialists acknowledged. It was conducted at a single center’s outpatient phlebotomy unit in Korea, and the study population skewed older, with a median age of 61. Its applicability to younger people is uncertain. The researchers did not adjust for the macronutrient composition of food patients ate before testing, which could influence postprandial changes in some analytes.
Nordestgaard and colleagues additionally noted the limited applicability of the results to ethnicities other than East Asian, though they added: “we believe that the results in this study are generally relevant, including for all ethnicities and countries.” As an observational, cross-sectional analysis, the study identifies associations rather than proving cause and effect. It is also not yet clear whether laboratories and professional guideline bodies will revise fasting recommendations on the basis of these findings.
Toward Analyte-Specific Fasting Rules
The authors recommend moving from uniform fasting requirements to analyte-specific rules, in which only tests genuinely sensitive to recent food intake — such as glucose, triglycerides, gamma-glutamyl transferase, and lipase — would require a defined fasting window. Whether hospitals, reference laboratories, and guideline-setting bodies adopt that approach remains to be seen. In the meantime, the editorialists suggest laboratories and clinicians can already act on the evidence by simplifying patient instructions and expanding same-day nonfasting testing, while continuing careful interpretation of the small set of postprandial-sensitive analytes. Patients with specific conditions — such as suspected triglyceride metabolism disorders or known dyslipidemia risk — should continue to follow their clinicians’ fasting instructions until guidance changes.
Key Questions
Do I still need to fast before a blood test?
That depends on the test. The study found most of 372 test items were unaffected by fasting less than 8 hours, but glucose, triglycerides, gamma-glutamyl transferase, and lipase showed higher results with shorter fasts. Follow your clinician’s instructions until guidance changes.
Which blood tests were most affected by not fasting?
Glucose was higher by 13.0 mg/dL with less than 8 hours of fasting compared with 8-12 hours, triglycerides by 17.1 mg/dL, and gamma-glutamyl transferase by 21.7 U/L. Lipase was higher by 36.4 U/L compared with fasting beyond 12 hours.
Is fasting longer than 12 hours better for test results?
No. The researchers reported that prolonged fasting beyond 12 hours conferred no measurable advantage for any analyte, and lipid results were not better with more hours of fasting.
How reliable is this study?
It analyzed over 9.7 million test results from more than 100,000 patients, but it was a retrospective, single-center study in Korea with mostly older patients and no adjustment for the composition of pre-test meals. Its applicability to younger people and non-East Asian populations is uncertain.
Will fasting requirements for lab tests change?
That is not yet clear. The authors support analyte-specific fasting rules, and an accompanying editorial argues fasting sampling is unnecessary for most tests, but any change depends on laboratories and guideline bodies adopting new recommendations.
Source: rss
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