Haematocrit Blood Test: What High, Low and Normal Levels Mean

What is haematocrit?

Haematocrit (Hct) is the proportion of your blood volume made up of red blood cells. Australian labs report it as a fraction in litres per litre (L/L), so a haematocrit of 0.42 L/L means 42% of your blood is red cells and the rest is mostly plasma. A typical adult range is 0.40 to 0.54 L/L in men and 0.37 to 0.47 L/L in women.

Your haematocrit depends on how many red cells you have and how much plasma (the fluid part of blood) they sit in. It rises if you make more red cells or have less fluid on board, and falls if you have fewer red cells or more plasma.

Most labs now calculate haematocrit from your red blood cell count and their average size (MCV).

What does a haematocrit blood test show?

Haematocrit shows whether your blood has too few red cells, which can mean anaemia, or too many, which makes blood thicker. It usually moves in step with haemoglobin and red cell count, and the three are read together as part of a full blood examination.

Red cells carry oxygen, so a low haematocrit can leave you tired or breathless on exertion. Blood that stays thick raises clot risk. It is especially worth tracking if you have heavy periods, eat plant-based, smoke, train hard or take testosterone.

Normal does not always mean optimal. Iron stores can run down for months before haemoglobin and haematocrit fall, and our Medical Director, Dr Michelle Woolhouse, notes that a low ferritin can cause tiredness, poor sleep and restless legs while your blood count is still normal.

Haematocrit normal range

For adults, haematocrit is usually 0.40 to 0.54 L/L in men and 0.37 to 0.47 L/L in women.

Men run higher mainly because testosterone stimulates red cell production. Each lab sets its own range, so check the one on your report, and judge a borderline result against your hydration on the day and your previous results.

What does high haematocrit mean?

A mildly high haematocrit is most often caused by dehydration, which concentrates your blood without changing how many red cells you have. A haematocrit that stays high when you are well hydrated usually means your body is making more red cells, most often because of smoking, low oxygen levels or testosterone.

Common reasons:

• Dehydration, from not drinking before a fasting test, a hot day, hard training, a big night of alcohol, or vomiting and diarrhoea.
• Smoking. Carbon monoxide reduces how much oxygen your blood can carry, so your body makes more red cells to compensate.
• Testosterone therapy or anabolic steroids. See below.
• Some medicines, such as diuretics (fluid tablets) and SGLT2 inhibitor diabetes medicines.
• Altitude. Weeks at high altitude prompt more red cell production.

Medical causes a doctor will think about:

• Obstructive sleep apnoea, where repeated drops in oxygen overnight can drive red cell production.
• Chronic lung or heart conditions that lower blood oxygen.
• Polycythaemia vera, an uncommon bone marrow condition that makes too many red cells, checked with a blood test for a gene change called JAK2.
• Rarely, kidney conditions that overproduce erythropoietin, the hormone that drives red cell production.

If you take testosterone. A rising haematocrit is its most common side effect, especially with injections, and treatment is usually reviewed above 0.54 L/L. Have your testosterone and free testosterone checked alongside it, and keep your prescriber in the loop.

What does low haematocrit mean?

A low haematocrit usually means anaemia (fewer red cells or less haemoglobin than normal), and in Australia the most common cause is iron deficiency.

Common reasons:

• Low iron, from heavy periods, low dietary iron, a poorly planned plant-based diet, frequent blood donation or endurance training.
• Low vitamin B12 or folate, which makes red cells larger than usual (high MCV).
• Pregnancy. Plasma volume rises more than red cell numbers, so some dilution is expected.
• Endurance training, which expands plasma volume, so well-trained athletes often sit at the low end of the range.

Medical causes your GP will look for:

• Blood loss, including from the gut. Iron deficiency in men and post-menopausal women always needs a GP to find the cause.
• Chronic inflammation or kidney disease, which slow red cell production.
• Thalassaemia trait, an inherited cause of small red cells with normal iron.
• Rarely, bone marrow conditions or faster red cell breakdown.

What can affect your haematocrit result

Do you need to fast? Not for haematocrit. The Baseline includes fasting markers such as glucose and insulin, so you'll fast overnight, but water is allowed and encouraged. Drink normally the day before and have a glass or two of water on the morning of your test.

Other influences:

• Hydration. A dry morning after a hot day or a night of drinking pushes haematocrit up.
• Hard exercise the day before shifts fluid in and out of your blood.
• Recent blood donation lowers it for several weeks.
• Posture. It reads slightly higher after standing than after lying down.

How haematocrit fits with your other results

Read haematocrit with your other red cell and iron markers.

How to support healthy haematocrit levels

Lifestyle can correct the most common causes of an out-of-range haematocrit, such as dehydration, low iron and smoking, but not inherited traits or bone marrow conditions.

• Drink enough to keep your urine pale straw-coloured, more in hot weather or when training.
• Eat iron-rich foods most days, such as red meat, chicken, fish, legumes, tofu and leafy greens. Pair plant sources with vitamin C (citrus, capsicum, tomato) and keep tea and coffee away from meals, as they reduce iron absorption.
• Cover B12 if you eat a plant-based diet. B12 is found almost only in animal foods, so most vegans need fortified foods or a supplement.
• Get enough folate from leafy greens, legumes and bread (Australian bread-making flour is fortified with folic acid).
• If you smoke, stopping lowers a smoking-related high haematocrit over the following months.
• If you snore heavily or wake unrefreshed, ask your GP about sleep apnoea.

When to see your GP

See your GP if your haematocrit is above your lab's range on a repeat test when you are well hydrated, especially with headaches, dizziness, blurred vision, a red face, or itching after a hot shower. If you are on testosterone and your haematocrit is above 0.54 L/L, contact your prescriber promptly.

Also see your GP if your haematocrit is below range with tiredness, breathlessness or a racing heart, and promptly if it is below about 0.30 L/L. Black or bloody stools, very heavy periods or unexplained weight loss always need a GP. Chest pain or severe shortness of breath needs urgent care: call 000.

Testing haematocrit with Vively

Haematocrit is included in Vively's Baseline Health Check, as part of your full blood count, and we read it with your iron markers.

We arrange your referral through our GP network. You have one blood draw at one of 4,000+ accredited partner collection centres. Results arrive in the app, usually within a few days. Your practitioner tells you whether your haematocrit points to hydration, iron or something for your GP, builds a personalised plan, and you retest to track the trend.

Check your haematocrit and iron with the Baseline Health Check.

Frequently asked questions

Do I need to fast before a haematocrit test?

No, haematocrit doesn't need fasting. If it's part of a test with fasting markers, such as Vively's Baseline, you'll fast overnight, but you can and should drink water. Being well hydrated helps your haematocrit reflect your true red cell level rather than a dry morning.

Can dehydration cause high haematocrit?

Yes. Dehydration is the most common reason for a mildly high haematocrit. Less fluid in your blood makes the same number of red cells look more concentrated. If your result is slightly above range and you were short on fluids, drink normally for a few days and retest.

Is a haematocrit of 0.52 high?

For a man, 0.52 L/L is within the usual range of 0.40 to 0.54, though towards the top. For a woman it is above the usual range of 0.37 to 0.47. Either way, check your hydration, smoking and any testosterone use, and look at your haemoglobin and previous results.

Does testosterone therapy raise haematocrit?

Yes. Testosterone stimulates red cell production, and a rising haematocrit is its most common side effect. Guidelines recommend checking it before starting, at 3 to 6 months, then yearly. A result above 0.54 L/L usually means your doctor will review your dose or how you take it.

What is the difference between haematocrit and haemoglobin?

Haematocrit is the share of your blood taken up by red cells, in L/L. Haemoglobin is the oxygen-carrying protein inside them, in g/L. They usually move together. Haemoglobin is the main marker for anaemia, while haematocrit is especially useful for spotting thick blood.

Can I get a haematocrit test through Medicare?

Many blood tests can be Medicare-funded when a doctor orders them for a clinical reason. With Vively, we arrange the referral through our GP network as part of your Baseline Health Check, so you don't need a separate GP visit first.

References

• Royal College of Pathologists of Australasia (RCPA). RCPA Manual: Haematocrit.
• Pathology Tests Explained. Full blood count. https://www.pathologytestsexplained.org.au
• Healthdirect Australia. Full blood count. https://www.healthdirect.gov.au
• Bhasin S, Brito JP, Cunningham GR, et al. Testosterone therapy in men with hypogonadism: an Endocrine Society clinical practice guideline. Journal of Clinical Endocrinology and Metabolism. 2018;103(5):1715-1744.
• Healthy Male (Andrology Australia). Testosterone treatment. https://www.healthymale.org.au
• Food Standards Australia New Zealand. Folic acid fortification. https://www.foodstandards.gov.au