What blood tests require fasting, and which don’t?

The short answer

Most of them. A complete blood count, an HbA1c, a thyroid panel and, usually, liver and kidney tests can be drawn without fasting. The short list that still needs a fast is led by fasting glucose, and it has grown shorter since 2016, when European guidance dropped the fast for cholesterol.

The catch is that many orders mix both kinds. Here are the blood tests that don’t require fasting, the few that still do, and how to tell which rule applies to your own appointment.

Tests that need no fasting

A complete blood count does not appear on the lists of tests that need a fast, so it can be drawn at any time of day. HbA1c needs no fast either: US guidance says you can eat and drink before it, because it reflects your average blood sugar over the past three months rather than this morning’s breakfast.

A thyroid panel needs no fast, but the time of day matters more than food. In a study of more than 20,000 TSH results, the hour of the draw shifted TSH enough to matter clinically. A morning draw, at a similar hour each time, makes results easier to compare.

Liver and kidney tests are usually drawn without fasting too, although US guidance notes that some may need a fast depending on the exact test. When in doubt, your order decides.

The short list that still does

Fasting glucose is the one clear case. US guidance defines it as a sample taken after at least 8 hours with nothing to eat or drink except sips of water, usually in the morning, since a meal changes blood sugar for hours afterwards.

A basic metabolic panel, a common bundle of eight measurements, includes glucose, so it may need an 8-hour fast as well. Iron tests are usually done in the morning, and your provider may ask for a 12-hour fast.

For men being tested for low testosterone, the Endocrine Society recommends measuring total testosterone in a fasting, morning sample, and confirming a low result with a second fasting morning sample.

Beyond those, a specific request may carry its own instruction, which is always the one to follow.

The lipid panel changed sides

For decades a cholesterol test meant an overnight fast. Since 2016 the European Atherosclerosis Society and the European Federation of Clinical Chemistry and Laboratory Medicine have jointly recommended the opposite: measure lipids without fasting, as the default.

The differences are small. After an ordinary meal the largest average changes are about +26 mg/dL for triglycerides and −8 mg/dL for LDL cholesterol, while HDL cholesterol, apolipoprotein B and lipoprotein(a) are largely unaffected, and non-fasting values predict cardiovascular disease just as well. Guidelines and statements in the United States, Canada, the United Kingdom and elsewhere endorse non-fasting profiles.

A fast comes back in one situation: when non-fasting triglycerides are above 440 mg/dL (5 mmol/L). If your doctor ordered a non-fasting lipid panel, that is not a shortcut; for patients, laboratories and clinicians alike, it is a simplification that does not weaken the result’s value for prevention.

Can you eat before a blood test?

Yes, if nothing on your order requires a fast. Eat as you normally would; a usual breakfast before a blood count, an HbA1c or a thyroid panel does not get in the way.

If the order mixes fasting and non-fasting tests, the fasting ones set the rule for the whole appointment. Even within the glucose family there is an exception: US guidance says a random blood sugar test can be done at any time, without fasting.

Why you were told to fast anyway

Because the instruction is usually given for the whole order rather than line by line. One fasting glucose on a sheet of fifteen tests makes the entire appointment a fasting one, which leaves the impression that all fifteen required it.

Habits also differ. Laboratory specialists have pointed out that definitions of fasting vary between institutions, and some US laboratory guidance still lists the lipid panel and the basic metabolic panel among tests that may need a fast. Follow the instruction on your own order rather than a table online, this one included.

What if you fast when you didn’t need to?

For most tests that need no fast, an unnecessary fast simply costs you breakfast. The result is not spoiled by it.

The exception is safety. If you take insulin or a diabetes pill that lowers blood sugar, fasting while taking your usual dose raises the risk of a low, so do not fast unless you were asked to, and if you were, ask the prescriber what to do that morning.