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.
Caffeine
Does coffee or caffeine affect blood test results?
Black coffee before a fasting test is a gray area: it has almost no calories, but caffeine raises cortisol and catecholamines, can nudge glucose and free fatty acids, and is a mild diuretic that concentrates the sample.
Smoking and nicotine
How does smoking or vaping affect blood test results?
Smoking changes several routine results in a way that can look like disease: it raises hemoglobin, hematocrit and white cells, lowers HDL and raises triglycerides, increases CRP and fibrinogen, lowers vitamin C and folate, and — within hours — raises glucose, cortisol and blood pressure.
Sleep deprivation
Does lack of sleep affect blood test results?
One bad night moves the stress and metabolic markers: cortisol is higher and its morning peak shifts, glucose and insulin resistance rise, testosterone falls (by roughly 10–15% after a week of five-hour nights in young men), and CRP, appetite hormones and TSH can drift.
Keto and high-protein diets
How do keto, carnivore and high-protein diets affect blood test results?
Very-low-carb and high-protein diets change several markers predictably: LDL cholesterol often rises (sometimes dramatically in lean people), triglycerides fall and HDL rises, BUN climbs with protein intake, uric acid rises in the first weeks, sodium and potassium can dip during the transition, and ketones appear in urine.
Creatine supplements
Does creatine affect blood test results like creatinine and eGFR?
Creatine supplements raise serum creatinine — creatine breaks down into creatinine — so a creatinine-based eGFR reads lower than the kidneys' true filtration, usually by a modest amount at a 3–5 g daily dose.
Oral contraceptives and estrogen
How does birth control (the pill) affect blood test results?
Combined pills and other estrogen-containing contraceptives raise the liver's binding proteins, which changes many hormone results without changing the active hormone: SHBG rises (so free testosterone reads low), total T4 and total cortisol rise while free levels stay normal, and total copper and vitamin A rise.
Peptides and GLP-1s
Do peptides affect blood work — and which labs should you check while using them?
Yes — by doing what they do, not by interfering with the assays.
Ashwagandha and thyroid-support supplements
Does ashwagandha affect thyroid blood tests?
It can.
Antidepressants and antipsychotics
Do SSRIs and antipsychotics raise prolactin on a blood test?
Yes — medication is the most common non-pregnancy cause of a high prolactin.
Ibuprofen and other NSAIDs
Does ibuprofen affect kidney function blood test results?
Regular NSAID use — ibuprofen, naproxen, high-dose aspirin, diclofenac and others — can raise creatinine and BUN and lower eGFR, because these drugs reduce blood flow through the kidney's filters.
Aspirin and blood thinners
How do aspirin and blood thinners affect clotting and platelet tests?
It depends on the drug.
Recent blood donation
Does donating blood affect blood test results?
Yes, for weeks.
Metformin
Does metformin affect blood test results — and should you take it before a blood test?
Take your metformin as usual unless the prescriber or the lab has told you otherwise — holding a dose for a routine draw is not needed, and a fasting draw is simply taken before the morning dose.
Prednisone and other corticosteroids
Does prednisone affect blood test results?
Yes, and the changes are real rather than lab errors.
Antibiotics
Can you take antibiotics before a blood test — and which results do they change?
For most blood tests, finish or continue your antibiotics as prescribed — they do not interfere with routine chemistry, lipids, thyroid or vitamin assays.
Vaccines (flu, COVID-19, shingles, MMR)
Should you get blood work before or after a vaccine — and does a vaccine affect test results?
A vaccine sets off a deliberate, short immune response, and the blood shows it: CRP, ESR and ferritin can rise and white cells shift for several days to about two weeks, and lymph nodes near the injection site can swell.
GLP-1 medications (semaglutide, tirzepatide)
Do Ozempic, Wegovy, Mounjaro and Zepbound affect blood test results?
Mostly in the ways they are meant to: GLP-1 and GIP/GLP-1 medicines lower fasting glucose and A1c, and as weight falls, triglycerides, LDL, liver enzymes and blood pressure usually improve.
Testosterone therapy (TRT)
When should you get blood work on testosterone therapy — and what does TRT change?
Timing is the whole result on TRT.
Antihistamines and allergy medicines
Do antihistamines affect allergy blood test results?
Not the blood tests.
Cannabis (THC and CBD)
Does marijuana show up on a blood test — and how long does THC stay detectable?
Routine blood work does not test for marijuana: a CMP, CBC, lipid panel, thyroid or hormone test will not detect or report THC, and cannabis use does not meaningfully change those results — though an edible the night before a fasting draw breaks the fast.
The night before (what you eat and drink)
What should you avoid the night before a blood test — and how long does a big meal show up in your results?
A fasting draw cares about the last 8–12 hours; what you ate the evening before matters for a shorter list than people fear.
PSA test (what not to do before)
What should you avoid before a PSA blood test — and what doesn't matter?
A PSA test needs no fasting, and coffee, alcohol and food do not meaningfully change it.
Vitamins and supplements (when to stop)
How long should you stop vitamins and supplements before a blood test?
Most supplements do not need to be stopped, and stopping some makes the result less useful.
Trying to lower a result before a test (A1c, glucose, cholesterol, testosterone)
Can you lower your A1c, blood sugar or cholesterol right before a blood test?
Mostly no — and the parts you can move are the parts that mislead.