Fasting glucose and insulin test preparation
Start with the instructions for the tests actually ordered. A calculator cannot repair an unsuitable sample.
Confirm the fasting instructions
Check the requested duration with your laboratory. Insulin testing often involves an overnight fast of 8–12 hours; the instructions for your own tests take priority.
Source: MedlinePlus · Insulin in blood test (opens in a new tab)
Use paired values for fasting indices
Keep the glucose and insulin results from the same fasting sample together. Avoid combining a glucose reading from one day with an insulin result from another.
Source: Wallace et al. · Use and abuse of HOMA, 2004 (opens in a new tab)
Ask about medicines and supplements
Tell the ordering clinician what you take, including supplements. Ask whether any preparation changes are needed, and do not stop prescribed medicines independently.
Source: MedlinePlus · Insulin in blood test (opens in a new tab)
If the fast was interrupted
Tell the laboratory if you ate or drank something other than plain water. They can advise whether the sample should be collected or rescheduled.
Source: MedlinePlus · Fasting for a blood test (opens in a new tab)
Bring the full laboratory report
Keep values, units, collection conditions and any laboratory notes together. The calculator asks you to confirm fasting conditions and makes its unit conversions explicit.
Common follow-up questions
Do I need to fast for HOMA-IR?
HOMA uses paired fasting measurements. Follow your clinician’s and laboratory’s instructions; fasting glucose tests commonly require at least eight hours without food, with water allowed.
Source: NIDDK · Diabetes tests and diagnosis (opens in a new tab) · Related calculator → · Link to answer
Should I stop medication before a fasting insulin test?
Ask the ordering clinician about medicines and supplements. Do not stop prescribed medication on your own to obtain a calculator result.
Source: MedlinePlus · Insulin in blood test (opens in a new tab) · Related calculator → · Link to answer
Can I drink water before a fasting blood test?
Plain water is generally allowed during a blood-test fast. Follow the specific instructions provided for your ordered tests.
Source: MedlinePlus · Fasting for a blood test (opens in a new tab) · Related calculator → · Link to answer
Can I drink coffee during a blood-test fast?
General fasting guidance allows plain water and excludes other drinks, including coffee. If you consumed something else, tell the laboratory before the sample is collected.
Source: MedlinePlus · Fasting for a blood test (opens in a new tab) · Related calculator → · Link to answer
What if I accidentally eat before the test?
Tell the laboratory or clinician before the blood draw. They can decide whether to proceed or reschedule; do not label a nonfasting sample as fasting.
Source: MedlinePlus · Fasting for a blood test (opens in a new tab) · Related calculator → · Link to answer
Reference-linked educational content. Prepared 22 October 2026; independent medical review is pending.