Red blood cells: what they are, what they tell you and why we test them

Red blood cells are the cells that carry oxygen all around your body. The test counts them: it shows how many carriers you have, not how much oxygen they carry. That is why the count is read together with haemoglobin.

Why we test them

Red blood cells are made in the bone marrow. In your lungs they pick up oxygen and carry it to your muscles, heart and brain.

Their number alone says little. It is read with the rest of the blood count: haemoglobin shows whether there is anaemia, MCV how big the cells are.

Together they help show where a problem comes from. For example whether iron is short, or whether you carry thalassaemia trait, which is inherited and usually needs no treatment.

What they tell you, and what they do not

If they are low, haemoglobin is often low too: that is called anaemia, usually from too little iron. But it is haemoglobin that shows anaemia, not the red cell count.

If they are high, too little water or smoking is often behind it. If there are many but they are small, it can be an inherited trait that is common in Italy.

And more red cells does not mean more oxygen: you can have many small cells that carry little haemoglobin.

That is why they are read together with other tests:

Normal red blood cell levels

The lab decides what is normal. It is usually 4 to 5.2 million per microlitre for women and 4.6 to 5.6 for men. Your own report may print different limits.

The unit can be written in different ways: x10¹²/L, x10⁶/µL, or millions per µL or per mm³ (milioni/µL on an Italian report). They all say the same thing, and the numbers do not change.

Men have more, partly because testosterone pushes the bone marrow to make more. In pregnancy the count normally drops a little, because the blood gets more diluted.

In the Rosa report you also see your optimal range, read together with haemoglobin, haematocrit and MCV.

What your result means

Find the row for your result, in x10¹²/L, the same numbers as millions per µL. Which values are dangerous? For red cells, haemoglobin and haematocrit decide. A haemoglobin below 11 g/dL should be seen by a doctor, below 8 the same day. A haematocrit above 52% in men or 48% in women should be seen by a doctor, above 56% the same day.

If your red blood cells are high or low

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:

With Rosa, a haemoglobin below 11 g/dL is always flagged to a doctor, below 8 urgently. A haematocrit above 52% in men or 48% in women is always flagged too, above 56% urgently.

Frequently asked questions

Are erythrocytes, RBC and red blood cells the same thing? Yes, they are all names for red blood cells. On an Italian report you may also see GR. You find them in the blood count, next to haemoglobin (HGB), haematocrit (HCT) and MCV.

What is the difference between red blood cells and haemoglobin? Red blood cells are the cells, and haemoglobin is the protein inside them that carries oxygen. The first test shows how many cells there are, the second how much oxygen they can carry.

Do I need to fast for a red blood cell test? Not for red cells. They are usually tested with other things that do need fasting, though, so it is best to arrive fasted. You can drink water, and it helps keep the result accurate.

How often should they be checked? For people who feel well there is no fixed rule: Rosa measures them twice a year, with the blood count. If they are out of range, your doctor decides when to check again, often after about three months.

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