Haematocrit
| Clinical | Optimal | |
|---|---|---|
| Male | 0.38 - 0.54 L/L | 0.40–0.52 L/L (40–52%) |
| Female | 0.36 - 0.48 L/L | 0.36–0.48 L/L (36–48%) |
What is Haematocrit?
Haematocrit (also called packed cell volume or PCV) measures the proportion of your blood that is made up of red blood cells, expressed as a percentage. If your haematocrit is 42%, it means that 42% of your blood volume is red blood cells and the remaining 58% is plasma (the liquid portion containing water, proteins, and other dissolved substances).
Think of it like a smoothie — haematocrit tells you the ratio of fruit (red blood cells) to liquid (plasma). More red blood cells mean a thicker, more concentrated smoothie.
Haematocrit closely tracks with haemoglobin and red blood cell count, as all three reflect the same basic concept: how many red blood cells you have relative to the total blood volume. Changes in hydration status directly affect haematocrit, making it one of the most fluid-sensitive blood markers.
Why Haematocrit Matters for Your Health
Haematocrit provides a quick assessment of blood viscosity and oxygen-carrying capacity. Too low, and your blood is thin and cannot deliver enough oxygen. Too high, and your blood is too thick, straining your heart and increasing clot risk.
For athletes and active individuals, haematocrit is a key performance indicator. It reflects the balance between red cell production, hydration, and training load. Elite endurance athletes often have slightly lower haematocrit due to plasma volume expansion (sometimes called 'sports anaemia'), which is actually a healthy adaptation.
From a longevity perspective, haematocrit at either extreme is associated with increased cardiovascular risk. An optimal haematocrit ensures efficient oxygen delivery without excessive blood viscosity — the sweet spot for cardiovascular health and cellular function.
Haematocrit& Your Wearable Data
Haematocrit measures the proportion of blood volume occupied by red blood cells. It directly reflects blood viscosity and oxygen-carrying capacity, both of which influence wearable-tracked cardiovascular metrics. Optimal haematocrit supports efficient oxygen delivery visible as good VO2 max, appropriate exercise heart rates, and quick recovery on your wearable.
Haematocrit is one of the most hydration-sensitive blood markers. Dehydration from exercise, heat exposure, or inadequate fluid intake — all inferrable from wearable data — can elevate haematocrit by 5-10%, increasing blood viscosity and cardiovascular strain. High haematocrit combined with wearable data showing elevated resting heart rate may indicate chronic dehydration.
Endurance training causes plasma volume expansion, which lowers haematocrit even while total red cell mass increases. This is a healthy adaptation. Wearable data showing improving fitness metrics (rising VO2 max, falling resting heart rate) alongside slightly lower haematocrit is a reassuring sign of cardiovascular adaptation rather than developing anaemia.
What High Haematocrit May Suggest
A high haematocrit means a greater proportion of your blood is composed of red blood cells. This increases blood viscosity, making your heart work harder to pump blood and increasing the risk of blood clots, stroke, and heart attack.
The most common cause of a mildly elevated haematocrit is dehydration — less plasma means a higher red cell concentration. Other causes include smoking, chronic lung disease, sleep apnoea, living at high altitude, testosterone replacement therapy, and polycythaemia vera.
If your haematocrit is mildly high, rehydrate thoroughly and retest before assuming a genuine elevation. Persistently high haematocrit above 0.52 in men or 0.48 in women should be investigated by your GP.
What Low Haematocrit May Suggest
A low haematocrit indicates that red blood cells make up a smaller proportion of your blood than normal, suggesting anaemia or overhydration. The causes mirror those of low haemoglobin and low red blood cell count.
Common causes include iron deficiency, B12 or folate deficiency, chronic disease, blood loss, bone marrow disorders, and excessive fluid intake (which dilutes the blood). Pregnancy naturally lowers haematocrit due to plasma volume expansion.
Symptoms of low haematocrit include fatigue, weakness, pallor, breathlessness, dizziness, and cold extremities. The underlying cause needs to be identified to guide appropriate treatment.
How to Optimise Your Haematocrit
Food
If haematocrit is low, focus on nutrient-dense foods that support red blood cell production: iron-rich foods (red meat, liver, shellfish, lentils), B12 sources (meat, fish, eggs), and folate sources (leafy greens, legumes). If haematocrit is high, ensure adequate fluid intake — aim for at least 2 litres of water daily. Reduce processed and salty foods that can contribute to dehydration.
Lifestyle
Stay well hydrated throughout the day, especially if you exercise or live in a warm environment. Regular moderate exercise supports healthy haematocrit levels. If high, address contributing factors: stop smoking, investigate sleep apnoea, and discuss testosterone therapy dosing with your GP if applicable. If low, identify and treat the underlying cause with your GP's guidance.
Supplements
For low haematocrit due to iron deficiency, iron bisglycinate with vitamin C supports red blood cell production. B12 and folate supplements as appropriate for confirmed deficiencies. For high haematocrit, no supplements are needed — focus on hydration and addressing the underlying cause. Avoid iron supplements if haematocrit is already elevated.
When to Speak to Your GP
See your GP if haematocrit is persistently outside the reference range after accounting for hydration status. Low haematocrit with anaemia symptoms warrants investigation. High haematocrit above 0.52 (men) or 0.48 (women) on repeat testing should be assessed. Seek urgent advice if you experience vision changes, severe headaches, or chest pain with abnormal haematocrit.
References
- NHS. Blood tests — Types. Updated 2024. nhs.uk
- WHO. Haemoglobin concentrations for the diagnosis of anaemia and assessment of severity. Geneva: WHO; 2011. who.int
- NICE. Polycythaemia/erythrocytosis — Clinical Knowledge Summaries. cks.nice.org.uk
- BMJ. Tefferi A. Anemia in adults: a contemporary approach to diagnosis. Mayo Clin Proc. 2003;78(10):1274-1280. pubmed.ncbi.nlm.nih.gov
Medical Disclaimer— This content is for general educational and informational purposes only. It does not constitute medical advice, diagnosis, or treatment. Omniwo Ltd is a wellness information service and is not a medical device, clinical laboratory, or regulated healthcare provider under MHRA guidelines. The “optimal ranges” presented on this page are based on published clinical guidelines (WHO, NICE, NHS) and peer-reviewed research; they represent functional wellness targets and may differ from standard laboratory reference ranges. Individual results should always be interpreted by a qualified healthcare professional (such as your GP) who understands your full medical history. Do not start, stop, or change any medication or supplement based solely on this information. If you are experiencing symptoms, seek medical attention promptly.