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An observational study of more than 100,000 patients in Korea found that fasting less than 8 hours before routine blood tests did not change results for the vast majority of 372 test items, with glucose, triglycerides, and a few other markers as exceptions. The authors support analyte-specific fasting requirements rather than a uniform 8-hour rule. Generalizability beyond older East Asian outpatients at one center remains uncertain.
Fasting less than 8 hours before routine blood tests generally did not change laboratory results, with only a few exceptions such as glucose and triglycerides, according to an observational study of more than 100,000 patients published in JAMA Internal Medicine. The findings, reported by Sollip Kim, MD, PhD, of the University of Ulsan College of Medicine in Seoul, Korea, and colleagues, support replacing the traditional uniform 8-hour fasting requirement with analyte-specific fasting rules for individual tests.
The retrospective single-center study drew on data from 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 to define the largest difference in a result that remains acceptable without materially affecting clinical interpretation.
In paired within-person analyses comparing tests taken after less than 8 hours of fasting against tests taken after 8-12 hours and more than 12 hours, the vast majority of the 372 test items did not exceed those allowable differences. However, within-person results were somewhat higher with shorter fasting for a small set of tests. Compared with 8-12 hours of fasting, glucose was 13.0 mg/dL higher and triglycerides were 17.1 mg/dL higher with less than 8 hours; compared with more than 12 hours, glucose was 10.8 mg/dL higher, triglycerides were 20.1 mg/dL higher, gamma-glutamyl transferase was 21.7 U/L higher, and lipase was 36.4 U/L higher.
The study also found a linear trend in which glucose and lactate were significantly lower with each additional hour of fasting, while lipid results were no better with more hours of fasting. Prolonged fasting beyond 12 hours conferred no measurable advantage for any analyte, the authors reported. “This cross-sectional study found that fasting duration was associated with differences exceeding analytical performance specifications for only a few analytes,” Kim and colleagues wrote. “These findings support analyte-specific rather than uniform fasting requirements.”
Impact on Patients, Labs, and Clinics
The findings matter because the standard 8-hour fast is a longstanding practice believed to standardize preanalytical variation in lab results, but it imposes real costs on patients and health systems. In an accompanying editorial, Børge Grønne Nordestgaard, MD, DMSc, of Copenhagen University Hospital in Denmark, and colleagues wrote that eliminating routine fasting requirements “should reduce the number of patients who present to the laboratory unprepared for sampling, decrease unnecessary administrative tasks related to fasting blood samples, and decompress morning schedules.”
For clinicians, the editorialists argued, same-day nonfasting testing can simplify patient instructions and lead to faster diagnosis and treatment. For patients, they wrote, “lessening the burden of preparation before blood sampling and repeated visits should improve adherence with testing, follow-up visits, and prescribed therapies.” The editorial noted the study “provides strong support for accumulating evidence that fasting blood sampling is not necessary” for most tests.
The study also carries implications for diabetes care, where fasting plasma glucose remains an important diagnostic tool, particularly when HbA1c — the standard measure of average blood sugar over 2-3 months — is not reliable. A 13 mg/dL average difference in glucose depending on fasting time could matter for interpretation in those cases.
Existing Guidelines on Fasting Tests
: “Current practice already reflects some movement away from uniform fasting. In cholesterol testing, U.S. guidelines 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 specific situations: patients with a family history of dyslipidemia or premature heart disease, a known or suspected disorder of triglyceride metabolism, or nonfasting triglyceride levels exceeding 400 mg/dL.
Diabetes management has likewise shifted toward HbA1c, which is less sensitive to day-to-day variability than fasting plasma glucose. The new study adds within-person evidence across a broad menu of 372 tests, extending the debate beyond glucose and lipids to the routine chemistry panel as a whole.
“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. It was conducted at a single center’s outpatient phlebotomy unit in Korea, and the cohort was mostly older patients, with a median age of 61 years, so applicability to younger people is uncertain. The analysis did not adjust for the macronutrient composition of food patients consumed before testing, which could affect postprandial changes in glucose, triglycerides, and other analytes.
The accompanying editorial, by Nordestgaard and colleagues, also pointed to the limited applicability of the results to ethnicities other than East Asian. “That said, we believe that the results in this study are generally relevant, including for all ethnicities and countries,” they wrote — an opinion, not a demonstrated finding. As an observational, cross-sectional analysis, the study also cannot establish causation, and the authors did not report which specific tests beyond the highlighted exceptions came closest to exceeding allowable differences.
Toward Analyte-Specific Fasting Rules
The study’s authors and the editorialists both suggest that laboratories and guideline bodies could move toward test-specific fasting requirements, keeping fasting instructions only for postprandial-sensitive analytes such as glucose and triglycerides while dropping them for the majority of routine tests. Confirmation in more diverse populations — younger patients, multiple centers, and non-East Asian ethnic groups — would strengthen the case for changing practice. In the meantime, patients should follow the fasting instructions they are given for scheduled blood draws, since individual labs and clinicians may still require fasting for specific tests, and diabetes diagnostic workups in particular still rely on fasting plasma glucose in some cases.
Key Questions
Do I still need to fast before a blood test?
Follow the instructions given by your clinician or laboratory. The study suggests fasting is unnecessary for most routine tests, but glucose, triglycerides, and a few other markers can still be affected by recent meals, and current practice has not yet changed.
Which tests were most affected by not fasting 8 hours?
Glucose, triglycerides, gamma-glutamyl transferase, and lipase showed the largest within-person differences with shorter fasting. For example, glucose averaged 13.0 mg/dL higher and triglycerides 17.1 mg/dL higher with less than 8 hours of fasting compared with 8-12 hours.
Is fasting longer than 12 hours better for lab results?
No. The study found that prolonged fasting beyond 12 hours conferred no measurable advantage for any analyte, and lipid results were no better with more hours of fasting.
Who was included in the study?
The study included 101,148 adult outpatients at a single center in Korea, with 9,755,547 test results across 372 test items. The median age was 61 years and 43% were women, which limits how far the results generalize to younger or non-East Asian populations.
Will fasting requirements for blood tests change because of this?
It is not yet clear. The authors and an accompanying editorial support analyte-specific fasting rules, but changing laboratory practice would require guideline bodies and institutions to revise their recommendations, likely supported by further research in broader populations.
Source: rss
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