High hematocrit in men: the causes that matter

Hematocrit is the part of your blood volume made up of red blood cells; a result of 0.48 l/l means 48 percent red cells. The value is not spun but calculated, from the red blood cell count multiplied by their mean volume, which is why it moves with everything that affects that volume.

For men the reference band sits higher than for women. Star-shl publishes 0.40 to 0.50 l/l for adult men and 0.35 to 0.45 l/l for women. That difference of roughly 0.05 l/l is no measurement artefact but largely hormonal: testosterone stimulates red cell production in the bone marrow and at the same time suppresses hepcidin, the hormone that limits iron uptake, so that more iron becomes available. More available iron plus a stronger production signal gives a higher value.

That relationship is dose-dependent, which is what makes hematocrit useful as a monitoring value: as the hormonal signal rises, the value rises with it. This is why the test sits as standard in the monitoring schedule for testosterone therapy. Note that not every laboratory uses a male band; some report one range for everyone, so that a man at 0.47 falls inside the male band and still gets a flag. The range on your own report is the range your result was compared against.

Always read the result alongside your haemoglobin. Those two should move roughly in proportion, and if only the hematocrit climbs, look first at your fluid balance on the day of the draw.

With a high hematocrit the cause is more interesting than the number. There are five routes, and they call for very different follow-up.

The first is dehydration, and it is immediately the most common. Less plasma means a higher concentration: a hard session, a sauna or a day of drinking too little lifts the result while nothing about the blood itself has changed. This is what you always exclude first.

The second is hormonal. Testosterone therapy and anabolic steroid use raise the value dose-dependently, and this is the best documented side effect of testosterone treatment. The 2018 Endocrine Society guideline sets a clear boundary: at a hematocrit above 54 percent therapy is withheld until the value normalises, after which it may be resumed at a reduced dose, and the same step advises evaluation for hypoxia and sleep apnoea. That 54 percent is a decision threshold for your clinician and no target for you. Never adjust a dose on your own initiative.

The third route is low oxygen. Obstructive sleep apnoea is the classic missed cause in men here: falling oxygen levels at night drive red cell production, year after year, without anyone making the connection. Chronic lung disease works the same way.

The fourth is smoking. Carbon monoxide occupies part of the haemoglobin, so the body compensates by producing more cells. Altitude, or a recent period at altitude, belongs in the same category.

The fifth is a primary bone marrow cause such as polycythaemia vera, often with a JAK2 mutation. That is rare and requires assessment by a haematologist; interpreting it yourself serves no purpose.

A low hematocrit is less common in men than a high one, and for that reason more often relevant: men have no structural route of blood loss outside menstruation, so a persistently low value warrants earlier questions about blood loss, iron, vitamin B12 and folate. Immediately after an acute bleed the value is still normal, by the way; it only falls hours later, once the body has redistributed fluid.

Anyone not on hormonal treatment mainly meets this value in a periodic check or when symptoms appear; headache, a flushed face or itching after a hot shower are then the usual triggers.

If you use testosterone, the picture changes. Hematocrit then belongs to the fixed monitoring values, alongside testosterone itself, and your clinician sets the schedule, which normally starts with a baseline measurement before treatment. That baseline matters more than people think: without one you cannot tell whether a value of 0.50 l/l is a rise or was always your normal. So measure before you start.

The conditions around the draw deserve attention in this group in particular. Strength training, drinking little and a sauna all three raise the concentration, so measure on a quiet morning with normal fluid intake and skip a heavy session in the preceding 24 hours.

If a raised result persists with good hydration, repeating it under the same conditions is the logical step. A single high measurement justifies no conclusion; a series under comparable conditions does.