Hematocrit

Hematocrit is the share of your blood made up of red blood cells, given as a percentage. Testosterone raises it, and keeping an eye on it is one of the main reasons people on masculinizing hormones have regular blood tests. Spelled haematocrit in the UK; sometimes reported as packed cell volume (PCV).

Where you'll meet it

On the full blood count that comes with most testosterone monitoring, and in conversations with your prescriber about dose, especially in the first year or when switching between injection types.

Why it goes up

Testosterone tells the bone marrow to make more red cells, which is why cis men typically have a higher hematocrit than cis women. On testosterone, a trans person's figure usually moves into the male range, and that is expected.

When it matters

If it climbs too high, a condition called erythrocytosis, the blood becomes thicker, and the risk of clots and heart problems rises. Guidelines differ on the exact cut-off, but a figure above roughly 50 to 54 per cent is commonly treated as a reason to review. The fix is usually a change of dose, form or timing, which is your prescriber's call, not something to adjust yourself.

What pushes it higher

Smoking, sleep apnoea, living at altitude, dehydration on the day of the test, and injections with high peaks. Blood tests and monitoring covers what else gets checked.

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