Before your blood test
Should you postpone a blood test if you've been sick?
Short answer
For most routine and screening tests, yes — wait about two weeks after a cold, flu, COVID or any infection. Illness raises inflammation markers (CRP, ESR, ferritin), shifts white cell counts, can lower iron and cholesterol, and disturbs thyroid results. Tests ordered to evaluate the illness itself are the exception.
Medically reviewed by Youmna DiStefano, MD · Last reviewed September 2026
Which results does it affect?
| Marker | Direction | How long | What to do |
|---|---|---|---|
| CRP, hs-CRP, ESR | raises | Days to 2+ weeks after an infection | Wait 2 weeks (hs-CRP needs a well baseline) [source] |
| Ferritin | raises | 1–3 weeks (acute-phase response) | Wait 2 weeks — illness can mask iron deficiency [source] |
| Iron (serum), transferrin saturation | lowers | During and after infection | Wait 2 weeks [source] |
| White blood cells and differential | raises or lowers | Days to a week | Wait until symptoms resolve [source] |
| TSH, Free T3 | raises or lowers | Weeks (non-thyroidal illness) | Wait 4–6 weeks after significant illness before judging thyroid [source] |
| Total and LDL cholesterol | lowers | Weeks after serious illness | Wait 6–8 weeks after a major illness for a lipid baseline [source] |
The acute-phase response
When the body fights an infection, the liver reprograms what it makes: CRP and ferritin go up, albumin and iron-binding proteins go down, and iron is locked away. That's useful physiology and useless for a baseline. A ferritin drawn during a cold can look 'normal' in someone who is genuinely iron-deficient, and an hs-CRP meant to estimate heart risk is meaningless while you're recovering.
Thyroid tests are the slow ones: significant illness suppresses T3 and can move TSH for weeks, a pattern called non-thyroidal illness. Give it four to six weeks after a hospitalization or serious infection before drawing thyroid tests you intend to act on.
Common questions
Can I get blood work with a mild cold?
For CBC, inflammation, iron or thyroid tests, better to wait two weeks. For an A1c, lipid panel (if you're eating normally) or a titer, a mild cold rarely matters.
How long after COVID should I wait for a blood test?
Two weeks after symptoms resolve for routine tests; longer (4–6 weeks) for thyroid and lipid baselines after a significant illness.
Educational content, not a diagnosis. Lab results are one input; a clinician interprets them with your history and exam.