The most common lab tests that require fasting are blood glucose tests, lipid panels (cholesterol and triglycerides), and metabolic panels. For most of these, you’ll need to avoid eating or drinking anything except water for 8 to 12 hours before your blood draw. Not every blood test requires fasting, though, so knowing which ones do can save you from an unnecessary empty stomach or, worse, inaccurate results.
Blood Glucose Tests
Fasting blood glucose is one of the primary tools for diagnosing diabetes and prediabetes. You need to fast for at least 8 hours beforehand, with nothing to eat or drink except water. The reason is straightforward: eating raises your blood sugar, and the whole point of the test is to see where your sugar levels sit when your body hasn’t processed a recent meal. That baseline number tells your provider whether your body is regulating glucose normally.
Not every glucose-related test requires fasting. The A1C test, which measures your average blood sugar over the past two to three months, does not require fasting at all. If you’re monitoring diabetes or being screened for it, your provider may order either test, so it’s worth confirming which one you’re getting.
Lipid Panel (Cholesterol and Triglycerides)
A lipid panel measures the fats circulating in your blood: total cholesterol, LDL (“bad”) cholesterol, HDL (“good”) cholesterol, and triglycerides. The traditional requirement is to fast for 10 to 12 hours before this test, which is a longer window than most other fasting labs.
Triglycerides are the main reason fasting matters here. After you eat, fatty particles from your meal stay elevated in your bloodstream for hours. That post-meal surge can inflate your triglyceride reading and throw off your calculated LDL cholesterol number, making your results look worse than they actually are at baseline.
That said, guidelines on lipid panel fasting are shifting. The Canadian Cardiovascular Society now recommends non-fasting lipid testing as the standard for both initial screening and follow-up, based on high-quality evidence that non-fasting results are reliable for most people. The exception is anyone with a history of very high triglycerides (above roughly 400 mg/dL), who should still fast before testing. In practice, many labs and providers in the U.S. still default to fasting lipid panels, so follow whatever instructions you’re given for your specific appointment.
Basic and Comprehensive Metabolic Panels
A basic metabolic panel (BMP) measures eight substances in your blood: glucose, calcium, sodium, potassium, carbon dioxide, chloride, and two kidney waste products (BUN and creatinine). A comprehensive metabolic panel (CMP) includes those same eight plus six additional markers for liver enzymes and proteins. Both panels typically require an 8-hour fast.
The glucose component is the primary reason for fasting here. The electrolytes and kidney markers on a metabolic panel are less sensitive to food intake, but because glucose is bundled into the same panel, the whole test gets a fasting requirement. If your provider orders a BMP or CMP, plan on fasting unless you’re told otherwise.
Liver and Kidney Function Tests
Liver function tests and renal (kidney) function panels sometimes require fasting, but not always. Liver tests measure proteins and enzymes that reflect how well your liver is working. Renal panels track waste products your kidneys filter out, and they’re commonly ordered for people with diabetes or suspected kidney disease.
Whether you need to fast for these depends on your provider’s preference and what else is being tested alongside them. If liver or kidney tests are bundled with a metabolic panel, fasting will be required for the panel as a whole. If they’re ordered on their own, your provider may or may not ask you to fast. When in doubt, check your lab order or call ahead.
Vitamin B12 and Iron Studies
A vitamin B12 blood test typically requires 6 to 8 hours of fasting. Iron studies, which measure how much iron is circulating in your blood and how well your body stores it, also generally require fasting because iron levels fluctuate after meals. If you’ve been referred for either test, assume you’ll need to fast unless your provider says otherwise.
Tests That Don’t Require Fasting
Many common blood tests have no fasting requirement at all. These include:
- A1C for diabetes monitoring
- Complete blood count (CBC), which looks at red and white blood cells and platelets
- Thyroid function tests like TSH
- Most hormone panels
- Inflammatory markers like C-reactive protein
If you’re having multiple tests drawn at the same appointment and even one of them requires fasting, you’ll need to fast for the entire blood draw.
What Fasting Actually Means
Fasting for bloodwork means no food and no beverages except plain water. Water is not only allowed, it’s encouraged. Staying hydrated makes your veins easier to find and the draw quicker.
Black coffee is where things get tricky. Some providers allow plain black coffee (no sugar, no cream) before a fasting blood test, but coffee can affect certain markers. Caffeine may influence glucose and cortisol levels. The safest approach is to stick with water only. If you can’t function without coffee, ask your lab or provider whether it’s acceptable for your specific test.
Chewing gum, even sugar-free varieties, can stimulate digestive processes and potentially affect glucose readings. Smoking can also alter blood sugar and certain lipid markers. If you want the cleanest results possible, avoid both during your fasting window.
How to Make Fasting Easier
The simplest strategy is to schedule your blood draw first thing in the morning. If your appointment is at 8 a.m., you stop eating by midnight (for an 8-hour fast) or 8 p.m. the night before (for a 12-hour fast). Most of your fasting window happens while you sleep.
If you take daily medications, the general guidance is to continue taking them with a small sip of water unless your provider specifically tells you to hold them. Some medications directly affect the markers being tested, so this is worth asking about when you schedule your lab work. Blood pressure medications, for example, are usually fine to take, while diabetes medications that lower blood sugar may need to be adjusted on the morning of a fasting glucose test.
If you accidentally eat or drink something during your fasting window, don’t skip your appointment automatically. Call the lab or your provider’s office. Depending on what you consumed and which tests are being run, they may still be able to draw your blood, or they’ll help you reschedule quickly.