3 min readNew DelhiJun 26, 2026 03:00 PM IST
Will a 13-hour fast affect your blood sugar test results? Answering this Quora query, “I did my blood sugar test for 13 hours of fasting, whereas my doctor had told me to fast for 8-12 hours. Does it affect my test?”, Dr Vijay Negalur, HoD, diabetology, KIMS Hospitals Thane, noted that fasting longer than the recommended duration can sometimes affect blood sugar readings. However, the impact is usually slight for most people.
“For fasting blood sugar tests, an overnight fast of 8 to 12 hours is typically advised because it creates a standard window to evaluate how the body manages glucose without food. When the fasting period extends beyond this range, the body activates its natural energy-maintenance systems. In response, the liver may release stored glucose into the bloodstream to maintain energy, especially for vital organs like the brain,” said Dr Negalur.
DISCLAIMER: This article is based on information from the public domain and/or the experts we spoke to. Always consult your health practitioner before starting any routine.
As a result, some people may notice slightly higher blood sugar levels, even after fasting longer, Dr Negalur continued.
Can longer hours of fasting affect your readings? (Photo: Getty Images/Thinkstock)
“For others, particularly those without diabetes, the difference may be minimal. Factors such as age, medications, stress, sleep quality, physical activity, and overall metabolic health can also affect the final reading,” said Dr Negalur.
Here’s what the query stated (Photo: Quora)
A 13-hour fast is just a little longer than the recommended upper limit and, in most situations, is unlikely to cause a significant change in the results. “Therefore, the test can still provide useful clinical information. However, if the reading seems borderline or does not match the individual’s overall health profile, the doctor may interpret it cautiously or suggest repeating the test under standard fasting conditions,” said Dr Negalur.
The key takeaway is that consistency is important. For future tests, try to stick to the fasting duration your doctor or laboratory recommend. “This helps ensure that results can be accurately compared over time and decreases the chance of variations caused by differences in fasting duration rather than actual changes in blood sugar control,” clarified Dr Negalur.
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DISCLAIMER: This article is based on information from the public domain and/or the experts we spoke to. Always consult your health practitioner before starting any routine.