High Hematocrit Without TRT: Causes in Men and What to Do

Hematocrit (Hct) is the percentage of blood volume taken up by red blood cells. A raised value means the blood is 'thicker' than usual, which increases its viscosity and — with large elevations — the risk of thrombotic complications. Reference ranges depend on the laboratory and sex; in men the upper limit is usually around 50–52%. In the WHO criteria for polycythemia vera the threshold is hemoglobin above 16.5 g/dL or hematocrit above 49% in men, but a value above this threshold does not by itself mean bone marrow disease — it means the cause must be found.

Among biohackers and health-conscious people, hematocrit is associated almost exclusively with testosterone therapy, which we cover broadly in the entry on a safe hematocrit during TRT. Yet many people with a raised result do not take any androgens. For them the diagnostic path is different: instead of asking how to reduce the dose, the question becomes why the body produces too many red cells, or why there are too many of them relative to plasma.

This article covers causes and the order of workup in men not on TRT. It does not discuss managing hematocrit during testosterone therapy or treating polycythemia vera. If you are interested in hemoglobin itself as a value and how it is interpreted in women and men, we devote a separate text to high hemoglobin.

The first step in thinking about a high hematocrit is to ask whether there really are more red blood cells (absolute erythrocytosis) or whether their concentration is simply higher because plasma volume has fallen (relative, also called apparent, erythrocytosis). In the latter situation the red cell mass is normal, and the result rises because the blood is concentrated.

Relative erythrocytosis is caused mainly by dehydration (too little fluid intake, heat, fever, diarrhea, vomiting), diuretics, heavy sweating, for example after a sauna or training, and drinking alcohol the day before. In some people an elevated hematocrit on a single test is simply the result of a morning without a glass of water. That is why the first, simplest and cheapest recommendation is to repeat the blood count after proper hydration — the effect of the amount of water drunk on hematocrit is described in a separate article about drinking a lot of water before a test.

In absolute erythrocytosis the bone marrow really does make more red cells. This happens for one of two reasons: either an external stimulus (hypoxia, excess erythropoietin, drugs) makes the marrow work harder — then we speak of secondary erythrocytosis — or the marrow, independently of stimuli, produces too many because of a stem cell mutation (polycythemia vera, a group of myeloproliferative neoplasms). The great majority of cases are the secondary type.

A historical look at your results also matters. If hematocrit has been raised for years at a roughly constant level, we look rather for chronic causes (congenital, lung, heart, apnea). If it has risen over a few months, we need to think of acquired causes: a new drug, starting to smoke, weight gain with apnea, less often a tumor producing erythropoietin. So it helps to have old blood counts at hand.

In practice several causes are often on the list at once. A man who is overweight, smokes, sleeps poorly and snores, drinks little water, and recently started an SGLT2 drug for diabetes has at least four independent reasons for a rising hematocrit. Removing one of them — for example quitting smoking — can lower the result without any further intervention.

Obstructive sleep apnea is a common but usually undiagnosed cause of chronic nocturnal drops in blood oxygenation. Hypoxia stimulates the kidneys to produce more erythropoietin, and that prompts the bone marrow to make red blood cells. The question is how often this leads to clinically visible erythrocytosis and whether treating apnea improves the results.

Several things follow. First, across the whole population with apnea erythrocytosis is uncommon (a few percent) but rises with apnea severity. Second, effective treatment of apnea (CPAP) lowers hematocrit by about one percentage point on average — not much, but in a sensible direction. Third, this effect comes from single-arm trials, so its size should be treated cautiously. The practical conclusion: in a man with a high hematocrit, snoring, daytime sleepiness, morning dry mouth or hypertension, it is worth considering a workup for apnea (questionnaire, polysomnography or polygraphy).

A broader discussion of apnea and its diagnosis can be found in the entry on obstructive and central sleep apnea. It is also worth knowing that apnea itself raises cardiovascular risk, so the diagnosis is valuable regardless of its effect on hematocrit.

Sodium-glucose cotransporter 2 (SGLT2) inhibitors are drugs used in type 2 diabetes, heart failure and chronic kidney disease. Besides lowering blood glucose, they reduce plasma volume and probably increase erythropoietin production, which favors a rise in hematocrit. The phenomenon is now well known, and some authors regard it as part of the mechanism of these drugs' cardiovascular benefit.

For the patient this means that with a raised hematocrit it is worth checking which medications you take, and not stopping them on your own. If hematocrit rises after an SGLT2 inhibitor is introduced, the decision about what to do next — observation, hydration, a possible dose reduction or change of drug, or starting a hematology workup — belongs to the treating doctor, who will weigh the cardiorenal benefits against the risk. It is worth knowing that combining SGLT2 inhibitors with testosterone therapy further increases the risk of erythrocytosis, so monitoring should be more frequent in people taking both.

Other drugs and substances that can raise hematocrit include erythropoiesis-stimulating agents (used in anemia), androgens, and — in sport — erythropoietin and related products. If you use anything outside medical care, tell your doctor: that information shortens the workup by many steps and is nothing to be embarrassed about from a medical standpoint.

Polycythemia vera is a myeloproliferative neoplasm in which the bone marrow produces too many red cells (often also white cells and platelets) because of a mutation, most commonly JAK2 V617F or one in exon 12. Unlike secondary causes, erythropoietin secretion is usually low, because the marrow works independently of the signal from the kidneys. The disease increases the risk of clots, and in the long term may progress to marrow fibrosis or acute leukemia, but when recognized and treated it has a good outlook.

In practice the workup is usually done in stages. First, repeat the blood count with good hydration and assess the obvious factors (smoking, drugs, apnea). If hematocrit stays raised, the next tests are usually serum erythropoietin and the JAK2 mutation test, and depending on the picture — blood gas analysis or overnight pulse oximetry, assessment of lung and heart function, an abdominal ultrasound with a look at the kidneys and spleen, and saturation. With low erythropoietin and a JAK2 mutation, polycythemia vera is very likely and requires referral to a hematologist. High or normal erythropoietin suggests a secondary cause.

Keep in mind that erythropoietin levels are interpreted differently depending on the laboratory and that they are only one piece of the puzzle. That is why context and the further steps set by the doctor matter, not your own interpretation of a single value.

The opposite also happens: someone with a result just above normal plays it down for years even though it comes with snoring and hypertension — the classic picture of undiagnosed apnea. In such a case the hematocrit number itself is not dangerous, but it is a lead to the diagnosis of a condition that really matters for the heart and vessels. So neither panic nor dismissal is a good choice.

It is also worth distinguishing two concepts: high hematocrit as a laboratory feature and erythrocytosis as a state requiring workup. A slight excess over the upper limit in a healthy, active man living at altitude is a different case from a hematocrit of 55% in a smoker with hypertension. Interpretation should always take into account the value, its trend, symptoms and risk factors.

Remember that many of the listed symptoms have other, more common causes. Having them together with a high hematocrit does not mean a diagnosis, only that a more urgent assessment is justified.

High hematocrit without TRT is a good example of a result that calls for a methodical rather than emotional approach. First we check what is simple and reversible: hydration, smoking, drugs, sleep. Only then, if the result persists, do we reach for erythropoietin and the JAK2 test. This order saves unnecessary stress and cost while not missing rare, serious diagnoses.