Most routine blood tests came out about the same whether people had eaten a few hours earlier or fasted overnight, according to a study of 9,755,547 lab results from 101,148 adult outpatients published September 28 in JAMA Internal Medicine. Only a few tests shifted enough to matter, led by blood sugar and triglycerides.
What this means for you: before your next blood draw, it is reasonable to ask your doctor whether that specific panel needs a fast. The study does not mean you should skip a fast you were told to do. Some tests still need one, and your doctor knows which ones were ordered.
Blood sugar is the clearest exception. Whether a glucose monitor can spot heart risk your blood tests miss is a separate question, but for a standard fasting glucose, the fast still counts.
Key takeaways
- Of 121 lab tests analyzed, 27 changed enough with fasting time to exceed accepted error limits when different people were compared, and only 10 held up once each person was compared with themselves.
- Glucose, triglycerides, GGT (a liver enzyme) and lipase (a pancreas enzyme) ran higher when people had fasted less than 8 hours.
- No test differed meaningfully between an 8 to 12 hour fast and a fast longer than 12 hours.
What the study found
Researchers at a 670-bed university hospital near Seoul, South Korea, pulled lab records from February 2021 to December 2023. Each patient’s fasting time was recorded and sorted into three groups: less than 8 hours, 8 to 12 hours, and more than 12 hours.
They then asked a practical question. Was the difference between groups bigger than the error labs already accept for that test, based on a European standard for how much a result naturally varies?
When they compared groups of different people, 27 tests crossed that line. When they compared the same patients tested at different fasting times, only 10 held up. The researchers said this suggests the rest came from differences between patients, not from food.
Compared with longer fasts, people who had fasted less than 8 hours had:
- Glucose about 10.8% higher
- Triglycerides about 20.1% higher
- GGT about 14.8% higher
- Lipase about 36.4% higher
Some results moved the other way. Total bilirubin was about 17% lower and vitamin D was about 10% lower in people who had fasted less than 8 hours, meaning both ran higher after a fast longer than 12 hours.
Stretching a fast past 12 hours bought nothing. No test crossed the error limit between the 8 to 12 hour group and the more than 12 hour group.
In an accompanying commentary titled “Time to Stop Fasting Before Blood Sampling,” researchers from Copenhagen University Hospital wrote that “most people spend the majority of the day in a nonfasting state,” so a nonfasting test better reflects a person’s usual state.
Dr. Kumar’s take
The routine instruction to fast 8 to 12 hours before any blood test is a habit more than a rule. This study shows the fast matters for a short list of tests, not the whole panel.
The useful version is a checklist, not a blanket order. A fasting glucose test needs a fast by definition. Triglycerides rise after a meal, so a high triglyceride result on a nonfasting draw can be rechecked fasting. GGT and lipase also moved. For most of the rest of a routine panel, the fast changed little.
The longer fast is not free either. For someone on insulin or other drugs that lower blood sugar, skipping food for that long can push blood sugar too low.
There are real limits. This was one hospital in one country. It was a snapshot of records, not a trial where people were assigned to eat or fast. About 90% of results came after a fast longer than 12 hours, so the short fasting group was much smaller. The researchers also could not check what people ate or confirm the fasting times patients reported.
What it means for you
- When a blood draw is scheduled, ask your doctor which tests were ordered and whether any of them need a fast.
- If you were told to fast for a glucose or triglyceride test, keep the fast.
- There is no gain from going past 12 hours. An 8 to 12 hour fast did as well.
- If you take insulin or other diabetes medicine, ask your doctor how to handle your doses before any fasting draw.
A routine panel is often part of a blood pressure or heart checkup. If yours is, which exercises lower blood pressure the most over a full day is worth reading next.