Do You Have to Fast Before Bloodwork?

Which tests typically require fasting?

Fasting lipid panel (cholesterol test): Food — especially fat — temporarily raises triglyceride levels, which is why a fasting sample has traditionally been preferred for lipid panels. Guidelines from the American Heart Association and the ACC now acknowledge that non-fasting lipid testing is acceptable for routine screening in many situations 1Ref 1Grundy SM, Stone NJ, Bailey AL, et al. (2019).2018 AHA/ACC/AACVPR/AAPA/ABC/ACPM/ADA/AGS/APhA/ASPC/NLA/PCNA Guideline on the Management of Blood Cholesterol.Non-fasting lipid testing is acceptable for routine cardiovascular risk screening in many situations per ACC/AHA guidance, but many clinicians and labs still specify a fasting draw. Your order will state whether fasting is required.

Fasting glucose: Blood sugar rises after eating, so a fasting glucose captures a baseline rather than a post-meal peak. The fast is typically 8 hours 3Ref 3National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) (2023).Diabetes Tests.Fasting plasma glucose ≥126 mg/dL on two tests indicates diabetes; fasting is defined as no food or drink (except water) for at least 8 hours; standard diagnostic criteria for diabetes and prediabetes using fasting glucose. The American Diabetes Association uses fasting plasma glucose as one of the standard criteria for diagnosing diabetes and prediabetes — a result of 126 mg/dL or higher on two separate tests indicates diabetes 2Ref 2American Diabetes Association Professional Practice Committee (2024).Standards of Care in Diabetes—2024.Fasting plasma glucose is a standard diagnostic criterion for diabetes and prediabetes per ADA Standards of Care.

Iron studies: Iron levels fluctuate after meals, so some iron panels are more reliably interpreted when drawn fasting. Your order should specify.

Basic or comprehensive metabolic panel (BMP/CMP): Usually does not require fasting, but your clinician may prefer it fasting if glucose is a specific focus.

Which tests do not require fasting?

Most blood tests are not affected by a recent meal. These include:

• Complete blood count (CBC)
• Thyroid tests (TSH, free T4)
• Hemoglobin A1c — this measures average blood sugar over 2–3 months and is unaffected by what you ate this morning 2Ref 2American Diabetes Association Professional Practice Committee (2024).Standards of Care in Diabetes—2024.Fasting plasma glucose is a standard diagnostic criterion for diabetes and prediabetes per ADA Standards of Care
• Most hormone panels
• PSA
• Vitamin levels (B12, D, folate)
• Kidney function markers (creatinine, BUN)
• Liver enzymes
• Inflammatory markers (CRP, ESR)
• Most infectious disease tests and cancer markers

If your annual panel includes both fasting and non-fasting tests, your clinician will typically ask you to fast so all tests can be drawn in one visit.

What does fasting actually mean?

Fasting for bloodwork means no food and no caloric beverages for the required number of hours before the draw 3Ref 3National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) (2023).Diabetes Tests.Fasting plasma glucose ≥126 mg/dL on two tests indicates diabetes; fasting is defined as no food or drink (except water) for at least 8 hours; standard diagnostic criteria for diabetes and prediabetes using fasting glucose. This includes juice, coffee with milk or sugar, and most flavored drinks.

Water is almost always fine — and recommended. Staying hydrated makes veins easier to find and draw. Plain water does not affect any standard lab test.

Black coffee (no milk, no sugar) is generally not thought to significantly affect most tests, but plain water is the most universally accepted option if you want to be certain.

How long to fast depends on the test. Fasting lipid panels often call for 9–12 hours; fasting glucose is often 8 hours 3Ref 3National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) (2023).Diabetes Tests.Fasting plasma glucose ≥126 mg/dL on two tests indicates diabetes; fasting is defined as no food or drink (except water) for at least 8 hours; standard diagnostic criteria for diabetes and prediabetes using fasting glucose. Your lab order or clinic instructions will specify.

If you take morning medications that must be taken with food, ask your clinician ahead of time how to handle this before your lab appointment.

What if you accidentally ate before a fasting test?

Let the lab know before they draw. They may still collect the sample but mark it as non-fasting — which means your clinician will need to interpret the result differently. Or they may ask you to reschedule.

Do not guess or try to conceal it. Eating before a fasting glucose or lipid panel can produce results that appear abnormal and lead to unnecessary follow-up. Honesty with the lab protects you from avoidable downstream testing.

Practical tips for fasting labs

• Schedule as early in the morning as possible. Sleeping through most of the fast makes it manageable for nearly everyone.
• Drink plenty of plain water the night before and the morning of your draw.
• Confirm medication timing with your clinician before your appointment if you take daily medications.
• Tell the lab if you have diabetes before your draw — they can often prioritize you so you can eat immediately afterward.
• Bring your lab order or have a photo of it so there is no ambiguity about which tests were ordered.