What happens if I have fasted for over 12 hours?
The ideal fasting window for most blood tests is 8 to 12 hours. This helps ensure the most accurate and clinically meaningful results. But if you've gone a little over, don’t worry — your results are still likely to be useful.
Why the 8–12 Hour Window Matters
Sticking within the 8–12 hour fasting window helps standardise your test results, particularly for markers like:
Glucose
Insulin
Cortisol
Triglycerides
Fasting for too long (over 12 hours) may slightly alter some of these values due to your body’s stress and energy adaptation responses.
What to Do If You’re Over 12 Hours
If you’re approaching 12 hours of fasting and your appointment is still a little while away, it’s better to have a small snack and remain in the optimal range, rather than continuing to fast excessively.
A small snack — something light, low in sugar and fat — can prevent your results from being skewed by an extended fast. Let the pathology provider know when you last ate.
Fasting Duration Guidelines
Fasting Duration | What to Do |
|---|---|
Up to 16 hours | Go ahead with the test — results are usually still reliable. Let the pathology nurse know how long you've fasted. |
Over 16 hours | Inform the pathology nurse. They’ll note this for the interpreting doctor. But best to reschedule. |
Over 24 hours | Reschedule your appointment. Extended fasting may significantly affect accuracy for certain markers. |
When to Be Cautious
Feeling faint, dizzy or unwell? Sit down, sip some water, and don’t push through — safety first.
If you’re diabetic or prone to hypoglycemia, don’t extend your fast beyond 8–12 hours. It’s not safe or necessary.
Pro Tips for Easier Fasting
Set an alarm to remind yourself when to start your fast the night before
Book early morning appointments (7–9 AM) to shorten the fasting window
Stay hydrated – water doesn’t break your fast and helps you feel better
Have a small snack if you're going past the 12-hour mark — this is better than over-fasting
Bottom line: We know fasting isn’t always easy — do your best to stick within the 8–12 hour window, and if you go over, a small snack can help bring you back into that ideal range. Always let your pathology provider know how long you fasted so your results can be interpreted accurately.