Hct
Haematocrit (Hct, packed cell volume, PCV) is the share of blood volume occupied by red blood cells, expressed as a percentage. It tells how "concentrated" the blood is in red cells: a low haematocrit is a sign of anaemia or blood dilution; a high haematocrit indicates dehydration or erythrocytosis and makes blood more viscous.
What haematocrit shows
Blood consists of plasma (water with proteins, salts, nutrients and hormones) and cells. Red blood cells make up almost the entire cellular part; white cells and platelets together occupy less than 1 % of the volume. Haematocrit therefore reflects two things at once: the number and size of red blood cells and the volume of plasma in which they are suspended.
Because red cells carry oxygen, haematocrit is closely related to the oxygen-carrying capacity of blood. It also determines blood viscosity: viscosity rises steeply — almost exponentially — as haematocrit increases. A moderate haematocrit is the physiological compromise between carrying enough oxygen and keeping blood fluid enough to flow through small vessels.
How it is measured
The reference method is the microhaematocrit: blood in a thin capillary tube is spun in a centrifuge and the length of the packed red-cell column is read. Modern haematology analysers do not centrifuge; they count red cells and measure their mean volume (MCV) and then calculate haematocrit:
Because haematocrit and haemoglobin usually change together, haematocrit (%) is roughly three times the haemoglobin concentration in g/dL. A marked deviation from this ratio suggests abnormal red-cell indices (for example very small or hypochromic cells) or a measurement error.
Reference ranges
| Group | Typical range | Comment |
|---|---|---|
| Adult men | 41–50 % (0.41–0.50 L/L) | Many laboratories use 40–52 % |
| Adult women | 36–44 % (0.36–0.44 L/L) | Many laboratories use 36–48 % |
| Pregnancy | ≈ 33–41 % | Lower because of plasma expansion |
| Newborns | ≈ 45–61 % | Falls over the first two months |
| Children 1–10 years | ≈ 33–41 % | Gradually rises with age |
Low haematocrit
A haematocrit below the reference range has the same causes as low haemoglobin, and both are interpreted together:
- Anaemia of any cause — iron deficiency, chronic blood loss, chronic kidney disease, chronic inflammation, vitamin B12 or folate deficiency, haemolysis, thalassaemia, bone-marrow disorders.
- Acute bleeding — note that in the first hours after a haemorrhage haematocrit may still be normal, because red cells and plasma are lost together; it falls as the body refills the circulation with fluid.
- Dilution (haemodilution) — large volumes of intravenous fluid, heart or kidney failure with fluid retention, and normal pregnancy.
- Physiological — endurance athletes often have a slightly lower haematocrit because of plasma expansion.
Symptoms are those of anaemia: fatigue, shortness of breath on exertion, palpitations, pallor, dizziness and poor concentration. The lower the value and the faster it falls, the more pronounced the symptoms.
High haematocrit
Main causes
- Dehydration — vomiting, diarrhoea, heavy sweating, low fluid intake, diuretics (relative erythrocytosis)
- Chronic hypoxia — smoking, high altitude, COPD, sleep apnoea, cyanotic heart disease
- Hormonal — testosterone therapy, anabolic steroids, erythropoietin, some kidney and liver tumours
- Polycythaemia vera — a bone-marrow neoplasm with a JAK2 mutation
Why it is dangerous
- Thicker blood flows more slowly through small vessels and raises the risk of stroke, heart attack and venous thrombosis
- Possible symptoms: headache, dizziness, tinnitus, blurred vision, red face, itching after a bath
- In polycythaemia vera, keeping haematocrit below 45 % reduced cardiovascular death and major thrombosis about four-fold compared with 45–50 % (CYTO-PV trial)
What affects the result
- Hydration: a single day of poor drinking or a stomach upset can raise haematocrit by several percentage points.
- Posture and tourniquet: standing for a long time or a tight tourniquet held for more than a minute concentrates blood in the vein.
- Altitude and smoking: both raise haematocrit through chronic hypoxia.
- Laboratory technique: calculated haematocrit depends on MCV, which can be falsely raised in old samples or very high blood glucose.
When to see a doctor
a low haematocrit is accompanied by signs of bleeding (black stools, vomiting blood, heavy menstrual or other bleeding), chest pain, fainting or severe breathlessness — or if a high haematocrit is accompanied by sudden weakness, speech or vision problems, chest pain or a painful swollen leg.
haematocrit is repeatedly outside the reference range. Repeat the test after drinking normally, and discuss with your doctor whether further tests are needed (iron studies, vitamin B12, kidney function, oxygen saturation, erythropoietin, JAK2 testing).
References
- Billett HH. Hemoglobin and hematocrit. In: Walker HK, Hall WD, Hurst JW, eds. Clinical Methods: The History, Physical, and Laboratory Examinations. 3rd ed. Boston: Butterworths; 1990. Chapter 151. ncbi.nlm.nih.gov/books/NBK259
- Marchioli R, Finazzi G, Specchia G, et al. Cardiovascular events and intensity of treatment in polycythemia vera. N Engl J Med. 2013;368(1):22–33. doi:10.1056/NEJMoa1208500
- Arber DA, Orazi A, Hasserjian R, et al. The 2016 revision to the World Health Organization classification of myeloid neoplasms and acute leukemia. Blood. 2016;127(20):2391–2405. doi:10.1182/blood-2016-03-643544
- McMullin MF, Harrison CN, Ali S, et al. A guideline for the diagnosis and management of polycythaemia vera. A British Society for Haematology Guideline. Br J Haematol. 2019;184(2):176–191. doi:10.1111/bjh.15648
- World Health Organization. Guideline on haemoglobin cutoffs to define anaemia in individuals and populations. Geneva: WHO; 2024. who.int/publications/i/item/9789240088542
This article is for general information and does not replace a consultation with a doctor. Reference ranges vary between laboratories; interpret your result together with a healthcare professional.
Published on 3 June 2019