Before your blood test: what can skew your results
A wrong result is usually a preparation problem, not a lab problem. One page per factor: which markers it moves, in which direction, for how long, and what to do — each claim tied to its primary source.
Fasting
Do you need to fast before a blood test — and which tests does eating affect?
Fast 8–12 hours (water is fine) for fasting glucose, fasting insulin and a lipid panel — eating raises glucose and triglycerides for hours.
Biotin supplements
Does biotin affect blood test results?
Yes, and seriously.
Alcohol
How does alcohol affect blood test results?
A drinking session in the 24–48 hours before a draw can raise triglycerides, liver enzymes (especially GGT and AST), uric acid and glucose, and depress blood sugar hours later.
Exercise
Should you exercise before a blood test?
Skip strenuous exercise for 24–48 hours before a draw.
Thyroid medication timing
Should you take levothyroxine before a thyroid blood test?
Draw before that day's dose when the order includes Free T4 or total T4: a levothyroxine tablet raises T4 in the blood for several hours afterward, while TSH barely moves within a day.
Iron supplements
Should you stop iron supplements before an iron blood test?
Hold your iron pill on the morning of the draw and test fasting: a supplement taken hours earlier can push serum iron and transferrin saturation well above your true baseline.
Vitamin D supplements
Should you take vitamin D before a vitamin D blood test?
You don't need to stop.
Time of day
Does the time of day change blood test results?
For several hormones, yes — a lot.
Menstrual cycle day
What day of your cycle should you get hormone blood tests?
It depends on the hormone.
Recent illness
Should you postpone a blood test if you've been sick?
For most routine and screening tests, yes — wait about two weeks after a cold, flu, COVID or any infection.
Hydration
Does dehydration affect blood test results?
Yes.
Common medications
Which common medications affect blood test results?
Several everyday medicines change results in predictable ways: statins can raise CK and liver enzymes; oral contraceptives raise SHBG and lower free testosterone; metformin lowers vitamin B12 over time; long-term acid reducers (PPIs) lower magnesium and B12; steroids raise glucose and white cells and suppress cortisol; and finasteride roughly halves PSA.