How to prepare for a blood test to get reliable, comparable results

Many things besides your health change blood test results. Preparing the same way each time makes your results more reliable and easier to compare. Always follow the instructions from your clinician or lab first.

Before the test

• Fasting. Glucose, insulin and triglycerides are affected by food. When fasting is required, it usually means 8–12 hours without food or drinks other than water. Fast the same way each time.
• Time of day. Testosterone and cortisol are highest in the morning, and iron also varies during the day. Morning tests (often before 10 am) are standard for these; test at a similar time each time.
• Hard exercise. A hard workout in the day or two before a draw can raise CK, AST and ALT, creatinine and inflammation markers.
• Biotin. Biotin (vitamin B7), common in hair, skin and nail supplements, can interfere with many lab tests, including thyroid tests, hormones and troponin. Labs often advise stopping it for a few days before testing: ask your lab or clinician.
• Alcohol can affect triglycerides, liver enzymes and other markers. Avoid it the day before.
• Hydration. Drinking water normally helps the draw and avoids falsely concentrated results.
• Medications. Keep taking them as prescribed unless your clinician tells you otherwise: some (such as thyroid medication) are sometimes taken after the draw, but only on medical advice.

Things to note down

• Recent illness or vaccination: CRP, ferritin and white blood cells can rise for days or weeks after an infection.
• Menstrual cycle day: estradiol, progesterone, LH and FSH depend on it.
• Poor sleep or unusual stress before the test.

Making results comparable over time

• Use the same lab when possible: methods and ranges differ between labs.
• Test at the same time of day and in the same fasting state.
• Look at trends over several results. Every test varies naturally from day to day: around 10% for LDL cholesterol, 20–30% for triglycerides, and much more for hs-CRP.

In UnitedMarkers you can record supplements, medications, exercise and sleep next to your results, so you can see what was going on before each blood draw.

More guides

• Why Quest and Labcorp show different reference ranges for the same test
• How to read a Labcorp blood test report
• How to read a Quest Diagnostics blood test report
• Blood test units explained: mg/dL vs mmol/L and how to convert

This guide is educational and does not replace advice from a healthcare professional.