Short answer: a high haematocrit means red blood cells make up more of your blood than your lab's normal range, which in UK labs usually tops out at 0.52 to 0.54 for men and 0.45 to 0.47 for women. The most common reasons are dehydration, smoking, testosterone (TRT or anabolic steroids), sleep apnoea and altitude; a rarer one is polycythaemia vera. One high result should be repeated, well hydrated. If it stays above 0.52 (men) or 0.48 (women), see your GP; on TRT, 0.54 is the level at which UK guidance reduces or pauses treatment, so speak to your prescriber.
Sources: British Society for Sexual Medicine guideline (Hackett et al., J Sex Med 2017); Endocrine Society guideline (2018); NHS Highland and South Tees NHS haematology pathways; North Bristol and Gloucestershire NHS reference ranges; Smit et al., Andrologia 2022. Written by James Smillie, TrueVitals, reviewed by the TrueVitals clinical review team. Sources checked November 2026.
Haematocrit (also called packed cell volume, PCV, or Hct) is the proportion of your blood volume taken up by red blood cells. UK labs write it as a fraction in litres per litre (L/L): 0.45 means red cells make up 45% of the blood. American sources tend to write the same number as a percentage, so 0.54 and 54% mean the same thing.
Haemoglobin (Hb) is the oxygen-carrying protein inside red cells, measured in grams per litre (g/L). The two usually rise and fall together, and both come from a standard full blood count (FBC), along with the red cell count and red cell size (MCV).
Because haematocrit is a ratio, it can rise for two quite different reasons: because you really have more red cells, or because you have less plasma (the liquid part of blood), for example when you are dehydrated. Doctors call these true and apparent (or relative) erythrocytosis. Telling them apart is the first job when a result comes back high.
There is no single national range. Each laboratory sets its own, so always read your result against the range printed on your report. Two NHS examples, checked November 2026:
| Laboratory | Haematocrit, men | Haematocrit, women | Haemoglobin |
|---|---|---|---|
| Severn Pathology (North Bristol NHS Trust) | 0.40 to 0.52 | 0.37 to 0.45 | Not listed on the same page |
| Gloucestershire Hospitals NHS Trust | 0.40 to 0.54 L/L | 0.37 to 0.47 L/L | Men 130 to 180 g/L; women 115 to 165 g/L |
A result just over the top of the range is common and often temporary. What matters more is whether it stays high on a repeat test, how far above the range it is, whether haemoglobin is also high, and whether there is an obvious cause.
NHS haematology pathways group the causes like this (South Tees Hospitals and NHS Highland guidance; GP Online review of raised haemoglobin):
Not drinking enough, a long sweaty session or a sauna before the test, diarrhoea, and diuretic tablets all shrink plasma volume and make haematocrit look higher. Heavy alcohol use, obesity and high blood pressure are also linked with this "apparent" rise.
Testosterone stimulates the bone marrow to make red cells, so a rise on TRT is expected and is the main reason TRT needs regular blood tests. Anabolic steroids do the same: in the HAARLEM study, haematocrit rose by an average 0.03 L/L during a cycle and returned to baseline three months after it (Smit et al., Andrologia 2022). See our TRT monitoring and blood test after a steroid cycle guides.
Carbon monoxide from cigarettes reduces how much oxygen your blood can carry, and the body responds by making more red cells. Smoking can also shrink plasma volume. Stopping smoking is one of the most effective ways to bring haematocrit down.
Anything that keeps oxygen low drives red cell production: obstructive sleep apnoea (loud snoring, pauses in breathing, daytime sleepiness), chronic lung disease, some heart conditions, and living at high altitude. Sleep apnoea is worth thinking about on TRT: the Endocrine Society guideline says to check for it when haematocrit rises too far.
NHS Highland lists SGLT2 inhibitors (a family of diabetes and heart failure medicines) and erythropoietin. Less commonly, kidney disease, a kidney transplant and some tumours make too much erythropoietin, the hormone that tells the marrow to make red cells.
A rare bone marrow condition in which the marrow makes too many red cells by itself, usually linked to a JAK2 gene change. It sometimes raises white cells and platelets too, and can cause itching after a warm bath. It is diagnosed by a haematologist with specific tests, not by a standard blood count.
These are the figures in the guidance we checked. They are what your GP or prescriber will usually work from; they are not instructions to act on yourself.
| Guidance | Threshold | What it means |
|---|---|---|
| NHS haematology pathways (South Tees; NHS Highland) | Persistently above 0.52 (men) or 0.48 (women) | Worth investigating if it stays raised for more than two months, ideally repeated without a tight tourniquet. |
| South Tees pathway | Above 0.60 (men) or 0.56 (women) | A true increase in red cells can be assumed. |
| NHS Highland pathway | Above 0.60, or any thrombosis | Emergency referral. |
| British Society for Sexual Medicine (2017), TRT | Above 0.54 | A reason not to start TRT; on treatment, reduce the dose or switch preparation, and if it stays high consider stopping and restarting at a lower dose. |
| Endocrine Society (2018), TRT | Above 54% | Stop testosterone until haematocrit falls to a safe level, look for low oxygen and sleep apnoea, then restart at a reduced dose. |
A review of haematocrit on testosterone therapy points out that 0.54 comes from the top of the reference range rather than from trials showing harm starts there (König et al., Sexual Medicine Reviews 2019, co-authored by the BSSM guideline's lead author). In practice that means 0.54 is a sensible safety line, not a cliff edge, and a rising trend matters even below it.
More red cells make blood thicker and slower-flowing. NHS Highland's haematology guidance puts it plainly: true polycythaemia matters because of "the thrombotic risk associated with a high haematocrit", meaning clots in veins (deep vein thrombosis, pulmonary embolism) or arteries (heart attack, stroke). A mildly raised result from dehydration doesn't carry the same risk, which is why the repeat test is so important.
Very thick blood can also cause symptoms of hyperviscosity. The South Tees pathway lists fatigue, headache, slowed thinking, blurred or double vision, bleeding from the gums or nose, chest or abdominal pain and muscle aches.
Get urgent help for chest pain, sudden breathlessness, a painful swollen calf, sudden weakness or numbness on one side, slurred speech or sudden loss of vision: call 999. For new headaches or blurred vision with a known high haematocrit, call NHS 111 or your GP the same day.
Repeat it before drawing conclusions. Drink normally the day before and the morning of the test, skip the sauna and the hard session beforehand, and if you can, test at the same time of day as last time.
Don't change, skip or split doses on your own. Your prescriber may adjust the dose, change the preparation, check for sleep apnoea or pause treatment, in line with BSSM guidance. See our blood test before TRT guide for why a pre-treatment baseline helps here.
It is confidential and changes how the result is read. The safest option is to stop using them; haematocrit returned to baseline on average three months after a cycle in the HAARLEM study.
Above 0.52 for men or 0.48 for women on repeat tests over two months is the point UK pathways investigate. Your GP may check oxygen levels, kidney function and iron, and refer you to haematology if polycythaemia vera is possible.
Home venesection is risky: fainting, infection, and iron deficiency, which can make you feel worse while hiding the real cause. When blood removal is needed, a medical team does it after the cause has been investigated.
Stop smoking (the NHS offers free support), ask your GP about sleep apnoea if you snore heavily or wake unrefreshed, and stay well hydrated.
Repeat within a few weeks, well hydrated. If it is still high, a further repeat over two months shows whether it is persistent, which is what UK pathways look for.
BSSM guidance checks haematocrit before treatment, at 3 to 6 months, at 12 months and then every 12 months, and says some men need checks every 3 months, including those with a high or rising haematocrit. Our TRT monitoring guide covers the full schedule.
Retest at around three months after the last dose. If it hasn't come down by then, see your GP.
Retest a few months after stopping smoking or starting sleep apnoea treatment, as your GP advises.
Monitoring is set by your haematology team, not by private testing.
It can show your haematocrit, haemoglobin, red cell count and red cell size, whether white cells and platelets are also raised (which points towards a marrow cause), and, with iron studies, whether low iron is hiding a tendency to make too many red cells. Alongside testosterone, oestradiol and kidney markers, it shows the most likely driver.
It can't measure your plasma volume, diagnose polycythaemia vera (that needs a JAK2 test and a haematologist), diagnose sleep apnoea, or tell you your personal clot risk. A normal result also doesn't make it safe to self-adjust a medicine.
The Ultimate panel (140 biomarkers, 139 for women) includes a full blood count with haematocrit and haemoglobin, iron studies with ferritin, testosterone, SHBG and oestradiol, kidney markers including eGFR and cystatin C, and lipids with ApoB, so a raised haematocrit is read alongside the markers that explain it. Samples go to Nationwide Pathology, a UKAS-accredited lab, and the clinician-reviewed report, written from your own lifestyle quiz, arrives within 4 working days of the lab receiving your sample.
When we're not the right choice:
Written by James Smillie and reviewed by the TrueVitals clinical review team, 2 November 2026. Thresholds come from the British Society for Sexual Medicine guideline on adult testosterone deficiency (Hackett et al., Journal of Sexual Medicine 2017), the Endocrine Society's 2018 testosterone therapy guideline as summarised by The Blood Project, the NHS Highland and South Tees haematology pathways, and a GP Online review of raised haemoglobin; reference ranges from North Bristol (Severn Pathology) and Gloucestershire Hospitals NHS trusts; steroid data from the HAARLEM study (Smit et al., Andrologia 2022). All checked in November 2026. TrueVitals is a UK blood testing company, so we have an interest in you testing; we've tried to say plainly when you don't need us.
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