High and low monocytes: what they are and what they tell you

Monocytes are the white blood cells that clean up your tissues and show your other defences what they have found. Your report usually shows them twice, as a percentage and as an absolute count. When they are high, your body is often finishing off an infection.

Why we test them

Monocytes are one of the five types of white blood cell in a full blood count, in the part called the differential. They travel in the blood for a few days, then move into the tissues and become macrophages.

There they swallow bacteria, old cells and debris, and show the rest of your defences what they have found.

A value that stays high over several tests can signal inflammation that lasts, or the tail end of an infection. It says more than a single result out of range.

What they tell you, and what they do not

If they are high, a recent infection, lasting inflammation or smoking are often behind it. After an infection, they usually return to normal on their own.

If they are low, it usually means nothing.

And they are a small share of your white cells, so the count is less precise than for the more common ones. A result just out of range says little: look at the absolute count, and how it changes over time.

That is why they are read together with other tests:

Normal monocyte levels

Your report usually lists monocytes twice. The percentage says what share of your white cells are monocytes. The absolute count says how many there are in a microlitre of blood. If there is only the percentage, the absolute count is the percentage times your white cell count, divided by a hundred.

The lab decides what is normal, and prints its limits next to each line. They vary a little from lab to lab.

The absolute count is written as x10⁹/L or x10³/µL: it is the same number. Some reports give it per mm³, multiplied by a thousand. It tells you more than the percentage, because it does not shift when the other white cells change.

In the Rosa report you see your monocytes next to the rest of your blood count. Rosa also calculates the absolute count, from your total white cells.

If your monocytes are high or low

What causes high monocytes

This is called monocytosis. It is usually temporary and settles on its own. Check the absolute count first: if it is normal, a high percentage is usually nothing to worry about.

Symptoms of high or low monocytes

High or low monocytes cause no symptoms on their own. If there are any, they come from the cause.

After an infection you may feel tired for a while. A fever that does not go away, night sweats, aching joints or weight loss without a reason should be seen by a doctor.

Low monocytes: what it means

Usually nothing. Monocytes are few, and small dips are often down to the count itself.

They can drop with cortisone, severe stress or some infections, and return to normal once the cause passes.

It matters more if they stay low on two tests, together with other blood count values out of range. Then your doctor looks for the cause: sometimes a medicine, more rarely an autoimmune disease or a bone marrow problem.

What to do if they are out of range

Usually nothing urgent. Where to start:

What can skew the result

A few things change the number for a while. It helps to know them before you read it.

When to worry

Talk to your doctor if:

Frequently asked questions

What are monocytes? White blood cells that clear bacteria and old cells from your tissues, and show your other defences what they have found. In the tissues they become macrophages.

What are normal monocyte levels? The lab decides, and your report prints its limits for both the percentage and the absolute count. The most useful is the absolute count, read with the rest of your blood count.

High monocytes: when should I worry? Usually you do not need to: they are often the tail end of an infection and come down on their own. Talk to your doctor if they stay high on a second test. Do the same if you have a fever that does not go away, night sweats or weight loss without trying.

High monocytes and lymphocytes: what does it mean? Often that your body has just fought off a virus. They usually return to normal within a few weeks: if they stay high, talk to your doctor.

Can high monocytes mean leukaemia? Rarely. Far more often a recent infection, inflammation or smoking is behind it. If they stay high for months without a reason as you get older, your doctor looks for the cause with other tests.

How often should they be checked? They are part of the blood count: Rosa measures them twice a year. If they are out of range, your doctor decides whether and when to repeat them.

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