High red blood cell count: what it means, the common causes and when it matters

Your red blood cell count, your hemoglobin and your hematocrit are three readings of one thing: how much red cell sits in each drop of blood. When one is high the other two are usually high with it, and doctors call the finding erythrocytosis (the older word is polycythemia). The first job is to work out whether the body holds more red cells than normal, or whether the same red cells are packed into less fluid, because that decides whether you need a repeat test or a full workup.

What counts as a high red blood cell count, hemoglobin or hematocrit

High hemoglobin and high (elevated) hematocrit mean the same thing as a high red blood cell count. Medically it is erythrocytosis, or polycythemia; the rare marrow form, polycythemia vera, has its own page.

A red blood cell count (RBC) counts cells. Hemoglobin measures the oxygen-carrying protein inside them, and hematocrit measures the percentage of blood volume taken up by red cells. The usual reference ranges are below ¹. A result a little above the top of the range is common and often settles on a repeat test; the cutoffs that trigger a workup sit higher.

Relative or absolute: more red cells per drop, or more red cells in the body

A high red blood cell count means one of two things. Relative erythrocytosis means the plasma (the fluid part of blood) has shrunk, so the same number of red cells is more concentrated. Vomiting, diarrhea, heavy sweating, skipping water before a fasting draw, diuretic water pills and alcohol the night before can all do it ². It often comes with a slightly high albumin or urea (BUN on US reports) on the same panel, and it disappears on a repeat test.

Absolute erythrocytosis means the body is making more red cells. It is either secondary, driven by the kidney hormone erythropoietin (EPO) because tissues sense too little oxygen or because a tumor, kidney cyst or medicine pushes EPO up, or primary, where the bone marrow makes red cells on its own, as in polycythemia vera ².

The common causes of a high red blood cell count

Most absolute rises are secondary. The CMAJ review by Mithoowani and colleagues lists smoking, testosterone, sleep apnea, lung and heart disease, SGLT2 inhibitor diabetes drugs and kidney transplantation among the usual drivers ². The table runs roughly from most to least common.

High RBC count with low MCV: often thalassemia trait

A high red blood cell count with small red cells (a low MCV, mean corpuscular volume) and a normal or slightly low hemoglobin usually means many small cells carrying the normal amount of hemoglobin. That pattern points most often to thalassemia trait, an inherited and harmless carrier state. Iron deficiency also makes cells small but usually lowers the count, so it fits less often, though it can when iron runs low in someone whose count was already high. The red cell mass is normal, so this is not polycythemia.

The Mentzer index, described by William Mentzer in 1973, divides MCV by the RBC count: a result under 13 leans toward thalassemia trait and over 13 toward iron deficiency. For example, an MCV of 68 and an RBC of 6.0 gives 11.3, which leans toward thalassemia trait. Hemoglobin electrophoresis (a test that sorts the types of hemoglobin and spots thalassemia trait) and a ferritin test settle it. Our page on a low MCV goes through the rest of that pattern.

How testosterone raises hematocrit, and the 54% line

Testosterone raises red cell production by raising EPO and lowering hepcidin, the hormone that locks iron away, so more iron reaches the marrow. Short-acting injections raise hematocrit more than gels or patches, and older men and higher doses see the biggest rises ⁵. The Endocrine Society guideline advises checking hematocrit before starting testosterone, again at 3 to 6 months and at 12 months, and then yearly, and stopping therapy when hematocrit rises above 54% until it comes down, then restarting at a lower dose ⁵. The same guideline advises against starting testosterone in a man whose hematocrit is already high ⁵. A total testosterone result shows whether the hematocrit is tracking a high level.

For how testosterone treatment is monitored overall, see testosterone and low testosterone.

When it is the bone marrow: polycythemia vera in brief

Polycythemia vera (PV) is a rare, slow-growing blood cancer, one of a group of conditions where the marrow overproduces blood cells (myeloproliferative neoplasms). The WHO criteria start at a hemoglobin above 16.5 g/dL in men or 16.0 g/dL in women, or a hematocrit above 49% in men or 48% in women, and add a JAK2 gene mutation and bone marrow findings ⁴. A JAK2 mutation is found in about 95% or more of people with PV ⁴. The clues on an ordinary blood count are a high red count together with high platelets or a high white count. Itching after a warm bath, burning hands and feet, and an enlarged spleen are the clues in the history.

In PV, keeping hematocrit low cuts clots sharply. In the CYTO-PV trial, a hematocrit target under 45% lowered cardiovascular death or major clots to 2.7%, against 9.8% with a target of 45 to 50%, over a median of about 31 months ⁶. Treatment is covered on the polycythemia vera page.

Symptoms of a high red blood cell count

A mildly high red blood cell count usually causes no symptoms and turns up on a routine test. When hematocrit is high enough to thicken the blood, people describe headache, dizziness, a red face, blurred vision, ringing in the ears and tiredness. Itching after a warm shower and burning pain in the hands or feet are more typical of polycythemia vera than of secondary causes. Thicker blood flows more slowly through small vessels, which is why a high hematocrit raises the risk of clots in the legs, lungs, heart and brain ³.

What the rest of your blood test says

A high red count is easier to read beside the other lines on the complete blood count (CBC) and the chemistry panel. Our CBC guide explains each line; the common patterns are below.

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What happens next: the usual workup

1. 1Repeat the blood countA repeat CBC when well hydrated removes most relative highs. Guidance acts on a hematocrit that stays above 52% in men or 48% in women for more than two months ³.
2. 2Take the historySmoking, testosterone or steroids, SGLT2 inhibitors, diuretics, altitude, snoring, lung or heart disease, and a kidney transplant cover most secondary causes ².
3. 3Check oxygenA finger oxygen reading, and an overnight study if there is snoring or daytime sleepiness, look for low oxygen at night.
4. 4Measure EPOA low serum EPO points strongly toward polycythemia vera; a high EPO points toward a secondary cause; a normal EPO does not rule polycythemia vera out, so JAK2 testing may still be needed ²³.
5. 5Test for JAK2A JAK2 mutation with a high hemoglobin or hematocrit makes polycythemia vera very likely ⁴. A high EPO without low oxygen leads to kidney and liver imaging instead.

See a doctor within a few weeks for a hematocrit that stays above the lines in the first table, a high red count with high platelets or white cells, or a high hematocrit on testosterone. A mildly high value with a clear reason, such as a fasting draw after a stomach bug, can wait for a routine recheck.

Can you bring a high red blood cell count down?

The count comes down when the driver goes away. Stopping smoking, treating sleep apnea, changing the testosterone dose or form under the prescriber, and drinking normally before draws usually bring it down, though the change takes time: red cells live about 120 days, so a retest 2 to 3 months after a change shows the real effect. Removing blood (venesection) is first-line treatment for polycythemia vera ³ and is used only in selected secondary cases, always under a clinician.

For more on the red cell count itself, see the red blood cell count guide and the wider blood count topic.

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