High red blood cells: what they mean in your blood test
What do high red blood cells mean?
Having high red blood cells (erythrocytosis or polyglobulia) means that the red cell count is above your laboratory reference range: over 5.9 million/µL in men or 5.2 million/µL in women. It may be an apparent rise due to reduced plasma volume (dehydration) or a true increase in production (smoking, high altitude, sleep apnoea, COPD, polycythaemia vera).
Quick summary
- What they are: red blood cells or erythrocytes; they transport oxygen via haemoglobin.
- Normal range: men 4.5–5.9 million/µL · women 4.0–5.2 million/µL.
- Considered high: above 5.9 M/µL (men) or 5.2 M/µL (women).
- Most frequent causes: dehydration, smoking, altitude, sleep apnoea, COPD.
- When to seek prompt medical advice: red cells >6 M/µL, haematocrit >55%, headache, dizziness or blurred vision.
- First step always: repeat the blood test well hydrated before drawing conclusions.
This is one of the results that generates most questions at the pharmacy counter, almost always with the same sentence: "my red blood cells are high, is it serious?" The honest answer is that it depends how high, what other results sit alongside it and who you are. Let me put it in order.
Normal range and how to read your result
The count is expressed in millions per microlitre (10⁶/µL or ×10¹²/L). Typical ranges reported by laboratories are:
- Adult men: 4.5–5.9 ×10¹²/L.
- Adult women: 4.0–5.2 ×10¹²/L.
- Children 3 to 12 years: 3.9–5.3 ×10¹²/L.
- In the report it appears as: RBC, Erythrocytes or Red blood cells.
- Red cell lifespan: 120 days; the bone marrow produces about 2 million per second.
Two important points. First: compare with the range on your own report, not with generic values from the internet — analysers differ between centres and what is slightly high in one lab may fall within range in another. Second: this result is never read in isolation. Without haemoglobin, haematocrit and MCV alongside it, you are missing half of the information.
And there are physiological variations that confuse things: men naturally have more red blood cells than women because testosterone stimulates erythropoietin, and in pregnancy the count falls proportionally because plasma volume rises faster than red cell mass. That is not anaemia; it is normal adaptation.
What are the causes of high red blood cells?
The causes fall into two main groups, and the difference matters:
- Relative erythrocytosis: the number of red cells has not increased; there is less plasma so the concentration rises. Typical causes: dehydration, diuretics, vomiting or diarrhoea, heavy sweating.
- Secondary absolute erythrocytosis: the body makes more red blood cells in response to lack of oxygen. Causes include smoking, living at altitude above 1,500 m, obstructive sleep apnoea, COPD and heart disease with chronic hypoxaemia.
- Primary absolute erythrocytosis: the bone marrow overproduces because of an intrinsic problem. This is polycythaemia vera — uncommon but the condition that needs ruling out when levels are clearly raised and persistent.
- Other causes: use of testosterone or EPO, erythropoietin‑producing tumours, polycystic kidney disease.
Of all these, the two I see most often in practice are dehydration — very often after a blood test done following a night without drinking and in hot weather — and long‑term smoking, where carbon monoxide blocks part of the haemoglobin and the body compensates by making more red cells.
Sleep apnoea deserves a separate mention: many people present with high red blood cells without realising they snore with pauses. If you also have daytime sleepiness and fatigue, it is worth raising this with your GP.
What symptoms do high red blood cells cause?
Mild elevations usually cause no symptoms — they are picked up incidentally on a routine blood test. When the blood genuinely becomes more viscous, you may notice:
- Persistent headache, especially in the morning.
- Dizziness or a feeling of unsteadiness.
- Blurred vision or episodes of altered vision.
- Facial redness (plethora) and reddened mucous membranes.
- Generalised itching, classically after a hot shower in polycythaemia vera.
- Tingling in hands and feet, or burning pain in the palms.
Seek medical assessment without delay if your red blood cells are above 6 M/µL, your haematocrit is over 55%, or you develop severe headache, visual disturbance, chest pain or shortness of breath. Blood that is too viscous increases thrombotic risk and that needs managing by a doctor, not at home.
How to interpret them alongside the rest of the full blood count
This is the key reading approach that almost nobody explains:
- High red cells + high haemoglobin and haematocrit: pattern of true erythrocytosis. This is what should trigger a search for cause.
- High red cells with normal haemoglobin and low MCV: suggests microcytosis — this can point towards thalassaemia trait, especially if it has been stable for years.
- Very high haematocrit with only slightly raised red cells: a clue to dehydration.
- All raised + platelets and white cells also high: this combination raises suspicion of polycythaemia vera and warrants haematology assessment.
- Erythropoietin level in blood: low points towards polycythaemia vera; high towards a secondary cause due to hypoxia.
What to do if your red blood cells are high
The practical steps, in the order I usually suggest at the counter:
- Repeat the blood test well hydrated. Drink water as normal in the days before and avoid going after a night of heat or alcohol. A fair proportion of "high red cells" disappears at this stage.
- Review your day‑to‑day hydration. Especially in summer, if you take diuretics or if you do endurance sport.
- Stop smoking. This is the intervention with most real impact on the count if you smoke, and you see the effect over months.
- Mention snoring. If you snore with pauses or wake up unrefreshed, raise this with your GP: sleep apnoea is a treatable cause.
- Review medicines and supplements. Testosterone and diuretics have an effect. Take a full list to your doctor.
- Follow the medical pathway if it persists. With persistently raised levels, referral to haematology is appropriate — management there can include venesection (therapeutic phlebotomy), which has no pharmacy alternative.
I want to be clear about something because you will read the opposite online: no food supplement corrects high red blood cells. There is no supplement that lowers the count; be wary of anyone selling it to you on that basis. What does make sense is looking after overall cardiovascular health — diet, exercise, smoking cessation, blood pressure — because the risk we worry about with more viscous blood is precisely thrombosis.
In that broader context, omega‑3 EPA/DHA has evidence as cardiovascular support within a balanced diet, and if your doctor has already recommended it for that reason, in pharmacy I work with options at an appropriate concentration. But let me stress this: it is general cardiovascular support, not a treatment for erythrocytosis, and it does not replace investigating the underlying cause.
Pharmacist advice
If you have just discovered high red blood cells on a report, first of all do not try to interpret it on your own or in the heat of the moment. Look at the haematocrit and haemoglobin next to it, check whether this is new or has been similar for years, and ask yourself honestly: do you smoke, snore or drink very little water?
With those three answers and your results in hand, your GP appointment becomes far more productive. And if you would like me to look over them with you before that visit, come into the pharmacy or write to us: distinguishing between a trivial finding and one that needs urgency is exactly what I am here for as a pharmacist (registration number 1383, Cantabria).
A hug from Cantabria!
Comparative summary: High red blood cells
| Profile | Normal range (× 10¹²/L) | Considered high if above |
|---|---|---|
| Adult man | 4.5 – 5.9 | 6.0 |
| Adult woman | 4.0 – 5.2 | 5.4 |
| Pregnant woman | 3.5 – 4.8 | 5.0 (rare) |
| Child 3–12 years | 3.9 – 5.3 | 5.5 |
| Elite athlete | 4.8 – 6.0 | 6.2 (may be physiological) |
Frequently asked questions
Scientific references
