Fasting is a common requirement for many routine laboratory blood tests, with patients often asked to avoid eating or drinking anything other than water for 8 to 12 hours before their sample is collected.
However, according to a new study, this step may be unnecessary for a wide range of tests.
Aside from a few notable exceptions, the findings suggest that many patients could eat and drink in the hours before their tests without skewing the results.
These results alone may not yet warrant a change in the pre-test fasting standard, nor do they mean that you should ignore medical instructions regarding fasting. But they provide compelling evidence to reassess the wisdom of defaulting to fasting before routine blood tests.
“Our comprehensive analysis suggests that most routine blood tests are minimally affected by short-term fasting, and that fasting beyond 12 hours is not associated with significant additional differences for the tests examined,” the study authors write.
“These results suggest that traditional rigid fasting requirements may warrant reconsideration in favor of a more nuanced, analyte-specific approach to pre-test preparation.”
These rigid fasting rules originally arose out of concerns that patients’ diets might compromise clinical accuracy. Blood test results often form the basis of important medical decisions, and some measurements are actually very sensitive to variables that occur before the test, such as food intake.
To minimize this variability, clinicians began widely asking patients to fast for eight to 12 hours before blood sampling – “a practice that originated in lipid and glucose testing,” the researchers explain, “but is now applied by default to much broader test panels.”
Yet despite the prevalence of this practice, recommendations regarding the optimal duration of fasting remain inconsistent. Research suggests that fasting helps in some tests, but its value in a diverse panel of tests has not been systematically studied with clinical data.
Research on pre-test fasting comes largely from lipid tests, while comparable data are limited for tests focused on parameters such as liver enzymes, pancreatic enzymes, hematologic indices, hormones, and vitamins, the authors note.
Thus, fasting recommendations may vary from one laboratory to another.
Given the limitations of existing research on the topic – as well as the burden that fasting can place on patients – the authors of the new study took a closer look at this clinical tradition.
Led by Sunghwan Shin, a laboratory physician at the University of Ulsan Asan Medical Center in Seoul, the researchers conducted a retrospective cross-sectional study using laboratory data collected between 2021 and 2023 at a single 670-bed university hospital.
The study subjects were all adult outpatients with documented fasting durations before blood tests, which the authors of the new study divided into three categories: less than eight hours, eight to 12 hours, or more than 12 hours.
Their final dataset included more than 9.7 million test results from more than 101,000 patients, which researchers screened for signs of fasting-related differences beyond established analytical performance specifications (APS).
After performing between-group comparisons and intra-individual analyses, they found a link between fasting duration and differences beyond PSA for only 10 of the 121 analytes targeted by the blood tests.
Fasting appears to help for these few analytes – primarily glucose, triglycerides, gamma-glutamyl transferase and lipase, all of which were higher in patients who fasted for less than eight hours.
However, for many other analytes, the results suggest that fasting for eight hours or more might be an unnecessary burden.
“These results support analyte-specific rather than uniform fasting requirements,” the authors write.
In an accompanying commentary, clinical biochemist Liv Tybjærg Nordestgaard of Copenhagen University Hospital and colleagues note that the study “provides strong support for the accumulating evidence that fasting blood sampling is not necessary.”
The research was published in JAMA Internal Medicine.
This article was fact-checked by Rebecca Dyer and edited by Rebecca Dyer. Although we are proud of our process, we are only human. If you spot an error, please let us know.
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