Been ill? What that does to your blood test result

1. Which markers does illness move?

When you fight an infection, your liver switches production. It makes more of some proteins and fewer of others. Those are called acute phase proteins, and several of them sit on a standard panel.

• CRP goes up, often a lot. C-reactive protein is the body’s general inflammation signal, and it is the marker most obviously affected. Our CRP guide covers what the numbers mean.
• Ferritin goes up. Ferritin is your iron savings account, and it is also an acute phase protein. This is the one that causes real trouble, and section three explains why.
• Albumin goes down. The liver deprioritises it during an illness, so a slightly low albumin after a fever is common and usually temporary.
• Thyroid numbers drift. Being properly unwell can push TSH and free T3 around without anything being wrong with the gland.
• Testosterone tends to fall. Illness, poor sleep and not eating all push it down, which is why one low reading proves little on its own.

Your red and white cell counts change too. Our guide to the full blood count explains what those cells are doing while you are fighting something off.

2. How long until things settle?

CRP is the fastest mover in both directions. It starts climbing within about six hours of an insult and peaks around 36 to 48 hours. Once the trigger clears it drops just as sharply. Its half-life in the blood is roughly 19 hours, which is the figure the standard reference on CRP reports.

In practice that means CRP is close to baseline within a few days of you feeling better. Ferritin lags behind it, sometimes by weeks. Thyroid markers take their own time.

There is no single official waiting time in the UK, and any page quoting one precisely is guessing. The workable rule is about two weeks clear of symptoms for a routine panel, and longer if iron is the question you care about.

A mild sniffle with no fever is different from a week of flu. Judge it by how ill you actually were.

3. Why ferritin is the sneaky one

CRP being high after a cold is obvious. Everyone expects it. Ferritin is the problem, because it moves quietly and in the wrong direction.

Inflammation pushes ferritin up. If your iron stores were genuinely low, the illness can lift the number into the normal range and hide the shortage completely. You get a reassuring result and no answer.

This is exactly why ferritin should be read next to CRP, never on its own. A normal ferritin with a raised CRP is a result to repeat. Our guide to low ferritin with normal haemoglobin goes through the same trap from the other side.

The markers that are not fooled the same way are transferrin saturation and total iron binding capacity. If you have been unwell recently and iron is your question, those two carry the weight.

4. Should you delay your blood test?

For thyroid, NICE does not leave it to judgement. The Association for Laboratory Medicine summary of the guideline is direct: “Do not measure TFTs in acutely ill patients, unless there is strong clinical suspicion of thyroid disease.”

That principle travels well. If your test is routine, and you are trying to see your normal, wait. If you are testing because something specific is wrong and you need an answer now, do not delay on account of a cold. Test, and read the result knowing what was happening.

Two related points worth checking before you post a sample. Dehydration from a fever or a stomach bug concentrates your blood and nudges several markers up, which our guide to dehydration and blood test results covers. And anything you took while poorly counts, from high-dose vitamin C to a supplement you added, as the guide to supplements that affect blood tests sets out.

The wider checklist lives in our blood test preparation guide.

5. What if you already tested while unwell?

Nothing is wasted. You have a snapshot taken under known conditions, and that is still information. Read it with three questions in mind.

• Is CRP raised? If so, treat the inflammation-sensitive markers with caution.
• Does anything look reassuring that you expected to be low? Ferritin is the usual suspect.
• Is anything low that illness explains, such as testosterone or albumin?

Then retest the specific markers once you are properly better, at the same time of day, so the two are comparable. Our General Energy & Wellness panel (£149) carries CRP alongside thyroid, vitamin D, B12 and cholesterol, which is the combination that makes a result readable rather than confusing. If iron is the question, the Vitamins & Minerals panel (£159) measures the full iron workup.

These tests measure your biomarkers and give you a wellness picture. They do not diagnose a condition. A result outside its range is one to take to your GP or a qualified clinician.

Frequently asked questions