Polycythaemia Interpreter — High Haemoglobin or Haematocrit
Polycythaemia Interpreter — High Haemoglobin or Haematocrit
Enter a haemoglobin or haematocrit, the patient’s sex and what is known — whether it has been repeated, JAK2, serum erythropoietin, smoking, hypoxia, testosterone, SGLT2 inhibitors, dehydration. It reads the value against the WHO and ICC 2022 polycythaemia vera thresholds and the British Society for Haematology’s, which differ for men, and separates polycythaemia vera from secondary and relative erythrocytosis.
High haemoglobin or haematocrit
Hb or Hct + JAK2 + EPO → causeMan, haematocrit 0.51, confirmed on repeat, JAK2 V617F positive, erythropoietin not measured, no secondary cause
Thresholds for polycythaemia vera, side by side
| WHO 2022 | ICC 2022 | BSH 2019 (JAK2-positive) | |
|---|---|---|---|
| Haemoglobin, men | Above 165 g/L (16.5 g/dL) | Above 165 g/L (16.5 g/dL) | Not used |
| Haemoglobin, women | Above 160 g/L (16.0 g/dL) | Above 160 g/L (16.0 g/dL) | Not used |
| Haematocrit, men | Above 0.49 | Above 0.49 | Above 0.52 |
| Haematocrit, women | Above 0.48 | Above 0.48 | Above 0.48 |
| Red cell mass | More than 25% above predicted | More than 25% above predicted | More than 25% above predicted |
| Also needed | Marrow, JAK2; or first two major + low EPO | As WHO | JAK2 mutation |
Is it real, is it primary, and which thresholds apply?
A high haemoglobin or haematocrit raises three questions, in order. Is it real? A single result can reflect a long tourniquet time, dehydration or a diuretic, and the NHS Highland pathway only investigates a persistently raised haematocrit. A reduced plasma volume — relative or apparent erythrocytosis — raises the numbers without any extra red cells.
Is it primary? Polycythaemia vera is a myeloproliferative neoplasm, nearly always carrying a JAK2 mutation, with the erythropoietin usually suppressed. Far more often the cause is secondary: smoking, hypoxia from lung disease or sleep apnoea, testosterone, renal lesions and erythropoietin-secreting tumours. SGLT2 inhibitors are a newer cause: a Mayo series found a median haematocrit rise of 7.4% that resolved when the drug stopped.
Which thresholds? The WHO 5th edition and the International Consensus Classification, both 2022, use the same lines: haemoglobin above 165 g/L in men and 160 g/L in women, or haematocrit above 0.49 and 0.48. The British Society for Haematology’s 2019 guideline uses a haematocrit above 0.52 in men. The page shows both, and for a man with a haematocrit between 0.49 and 0.52 it says so rather than choosing.
A very high haematocrit also affects laboratory tests: above 0.55 it prolongs clotting times unless the citrate is adjusted (see the citrate volume adjustment calculator). For newborns, polycythaemia is a different problem: see the neonatal partial exchange calculator. To convert between haemoglobin and haematocrit, see the haematocrit from haemoglobin calculator.
Frequently asked questions
What haematocrit suggests polycythaemia vera?
WHO and ICC 2022 use a haematocrit above 0.49 (49%) in men and above 0.48 in women, or a haemoglobin above 165 g/L and 160 g/L. The British Society for Haematology uses a haematocrit above 0.52 in men and 0.48 in women, with a JAK2 mutation. The value is only one criterion; JAK2 and, for WHO and ICC, a marrow or low erythropoietin complete it.
Are the WHO and ICC 2022 polycythaemia vera criteria the same?
For the blood count thresholds, yes: both use haemoglobin above 16.5 g/dL in men and 16.0 g/dL in women, or haematocrit above 49% and 48%, or a red cell mass more than 25% above predicted, with marrow and JAK2 criteria and a low erythropoietin as the minor criterion. The BSH 2019 guideline differs for men.
What causes secondary polycythaemia?
Anything that drives erythropoietin: hypoxia from lung disease, sleep apnoea, cyanotic heart disease or altitude; carbon monoxide from smoking; testosterone and anabolic steroids; SGLT2 inhibitors; renal cysts, renal artery stenosis and transplantation; and erythropoietin-secreting tumours.
Should a high haematocrit be repeated?
Yes. A single result can be raised by dehydration, a diuretic or a long tourniquet time. Repeat on a fresh sample with the patient well hydrated; investigation starts when it is persistently raised.
Do SGLT2 inhibitors cause polycythaemia?
They raise haemoglobin and haematocrit. In a Mayo series of 30 patients with JAK2-unmutated erythrocytosis on an SGLT2 inhibitor, the median haematocrit rose by 7.4% and normalised in all 6 who stopped. A later series of 100 found no association between the haematocrit and thrombosis.
Related calculators
References
- Khoury JD, Solary E, Abla O, et al. The 5th edition of the World Health Organization Classification of Haematolymphoid Tumours: myeloid and histiocytic/dendritic neoplasms. Leukemia. 2022;36(7):1703–1719. — polycythaemia vera: haemoglobin above 16.5 g/dL in men and 16.0 g/dL in women, or haematocrit above 49% and 48%, or red cell mass more than 25% above predicted; marrow panmyelosis; JAK2 V617F or exon 12; minor criterion subnormal serum erythropoietin; diagnosis with all three major, or the first two major plus the minor. Essential thrombocythaemia: platelets 450 × 10⁹/L or more. (Thresholds as tabulated side by side with the ICC by GIMEMA, 2024.)
- Arber DA, Orazi A, Hasserjian RP, et al. International Consensus Classification of myeloid neoplasms and acute leukemias: integrating morphologic, clinical, and genomic data. Blood. 2022;140(11):1200–1228. — the same haemoglobin, haematocrit, red cell mass and erythropoietin criteria for polycythaemia vera as the WHO 5th edition.
- 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. — JAK2-positive PV: haematocrit above 0.52 in men or above 0.48 in women, or red cell mass more than 25% above predicted, with a JAK2 mutation. JAK2-negative PV: red cell mass raised or haematocrit 0.60 or more in men and 0.56 or more in women, no JAK2 mutation, no secondary cause, compatible marrow, plus further criteria including low serum erythropoietin (criteria as reproduced by GPnotebook).
- NHS Highland. Polycythaemia / erythrocytosis guidelines. Right Decisions, accessed September 2026. — investigate a persistently raised haematocrit above 0.48 in women and 0.52 in men, on a fresh uncuffed sample; secondary causes include hypoxia, sleep apnoea, smoking, renal disease and drugs ("SGLT2 inhibitors, erythropoietin, testosterone"); iron replacement can raise the haematocrit rapidly.
- Gangat N, Szuber N, Alkhateeb H, Al-Kali A, Pardanani A, Tefferi A. JAK2 wild-type erythrocytosis associated with sodium-glucose cotransporter 2 inhibitor therapy. Blood. 2021;138(26):2886–2889. — 30 patients; median haematocrit rise 7.4%; peak haemoglobin 17.9 g/dL and haematocrit 52.9%; resolution in all 6 who stopped the drug.
- Gangat N, et al. Sodium-glucose co-transporter-2 inhibitor use and JAK2 unmutated erythrocytosis in 100 consecutive cases. Am J Hematol. 2023;98(7):E165–E167. — haemoglobin and haematocrit were not associated with thrombosis.
Not medical advice. For healthcare professionals and education. Reference intervals vary by laboratory and assay — always use your own laboratory's. Never base a dose or a treatment decision on this page alone. Full disclaimer at calcengines.com/disclaimer/