GPAtlas🧭 When to suspect
Erythrocytosis (a raised haematocrit or haemoglobin) is a common, often incidental finding on the full blood count. The first task is to separate an apparent (relative) rise – a contracted plasma volume with a normal red cell mass – from a true (absolute) increase in red cells, and then to split the absolute group into primary (polycythaemia vera) and secondary (erythropoietin-driven) causes.
The concern throughout is thrombosis: a high haematocrit raises blood viscosity and the risk of arterial and venous clots. Haematocrit is the more reliable marker than haemoglobin, and the rise must be persistent (confirmed on a repeat, fresh, uncuffed sample) before it is acted upon.
| Venous haematocrit (persistent > 2 months) | Interpretation | Action |
|---|---|---|
| ≤ 0.52 (men) / ≤ 0.48 (women) | Within range | No action unless symptomatic |
| > 0.52 (men) / > 0.48 (women) | Erythrocytosis | Repeat (fresh, uncuffed sample) → investigate the cause |
| > 0.60 (men) / > 0.56 (women) | Assume absolute (true) erythrocytosis; hyperviscosity risk | Discuss / refer haematology urgently |
A haemoglobin > 185 g/L (men) or > 165 g/L (women) supports the diagnosis, but the haematocrit is the better guide. Arterial thrombosis or hyperviscosity symptoms warrant urgent referral regardless of the exact figure.
| Category | Mechanism | Common causes |
|---|---|---|
| Apparent (relative) | Reduced plasma volume; red cell mass normal | Dehydration, diuretics, alcohol excess, obesity (Gaisböck) |
| Absolute – primary | Clonal marrow overproduction | Polycythaemia vera (JAK2 V617F), congenital |
| Absolute – secondary | Driven by raised erythropoietin (EPO) | Chronic hypoxia (COPD, OSA, heavy smoking, altitude, cyanotic heart disease); EPO-secreting tumours (renal cell, hepatocellular); testosterone / anabolic steroids; SGLT2 inhibitors |
|
🧠 Clinical pearl In primary care the commonest reasons for a raised haematocrit are apparent (relative) and secondary causes – not polycythaemia vera. Before launching a full work-up, correct the reversible drivers (dehydration, alcohol, obesity, a diuretic, heavy smoking) and repeat the count. Many "abnormal" results settle, sparing the patient unnecessary referral and worry. |
Source: British Society for Haematology
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