Fasting Blood Tests: Which Tests Actually Require Fasting?

Why Some Tests Need Fasting

Eating raises the level of some substances in the blood for several hours — most notably glucose and, to a lesser extent, certain fats such as triglycerides. For tests where the result is interpreted against a fasting reference value, eating beforehand would make the result difficult to interpret. For most other tests, a normal meal has little effect, and fasting is unnecessary.

Tests That Usually Require Fasting

Fasting blood glucose (FBS)

This test measures blood sugar after a period without food and is used in diagnosing and monitoring diabetes. A fast of at least eight hours is usually required, often overnight. Our English Health Education site has a detailed guide on fasting hours for the FBS test.

Oral glucose tolerance test (OGTT)

This test measures how the body handles a measured glucose drink, with blood taken before and after. It requires fasting beforehand and specific instructions on the day, and is usually performed at a laboratory rather than at home.

Lipid profile (cholesterol test) — sometimes

Many current guidelines accept non-fasting samples for routine cholesterol testing, because fasting makes little difference to most of the results. However, a fasting sample may still be requested — for example, when triglycerides have previously been high, or according to the laboratory's or doctor's practice. Follow the instructions on your test request. Details are available in our guide to the lipid profile.

Tests That Usually Do Not Require Fasting

The following common tests do not normally require fasting, unless the doctor has a specific reason to request it:

• Full blood count (FBC)
• HbA1c (average blood sugar over recent months)
• Kidney function tests, including serum creatinine and electrolytes
• Liver function tests, in most situations
• Thyroid function tests (TSH), although timing in relation to thyroid medicine may matter
• CRP and ESR (markers of inflammation)
• Dengue tests

Some specialised tests have their own preparation requirements, such as timing or avoiding certain foods or supplements beforehand. The requesting doctor or laboratory should explain these.

What Fasting Means in Practice

• No food for the stated number of hours.
• Plain water is usually allowed and encouraged, unless you have been told otherwise.
• Avoid tea, coffee, milk, juice and soft drinks, including "sugar-free" drinks, during the fasting period unless the instructions allow them.
• Avoid chewing gum, sweets and smoking during the fast.
• Do not over-fast. Fasting for much longer than required can also affect some results and may make the patient feel unwell. For an overnight fast, arranging the test early in the morning is usually easiest.

Medicines and Fasting

Most regular medicines can be taken with water as usual on the morning of a fasting test. Some exceptions exist, so check with the doctor who requested the test.

When Several Tests Are Requested Together

If a fasting test and non-fasting tests are requested at the same time, all the samples can usually be taken during the same fasting visit. There is no need to fast for every test separately. For older or frail patients, especially those with diabetes, discuss with the doctor whether fasting is truly necessary or whether an alternative test would give the needed information.

If You Accidentally Eat

If you eat or drink something other than water before a fasting test, tell the person collecting the sample. For fasting glucose or a fasting lipid test, the test may need to be rescheduled. For other tests on the same request, the sample can often still be taken.

Related Guides

• Home Blood Tests: How to Prepare and What to Expect
• How to Check Blood Sugar at Home and Avoid Common Errors
• Understanding Common Blood Tests Ordered During Home Medical Care
• When Repeat Blood Tests Are Needed During Recovery at Home