# Erythrocytosis (primary vs secondary)

Source: https://onco.cc/terms/erythrocytosis/  
OnCo record `erythrocytosis` (Term). Data CC BY-NC 4.0, attribute "Data from OnCo (onco.cc)"; commercial use needs a licence.

## TL;DR

Too many red cells. In polycythaemia vera the marrow itself is at fault (primary); far more often the cause is something else driving it, such as smoking, low oxygen, sleep apnoea, a kidney tumour or testosterone (secondary).

## Summary

Erythrocytosis simply means a raised red cell mass. Secondary erythrocytosis is the body's response to low oxygen or to excess erythropoietin and is not a cancer; primary erythrocytosis, of which polycythaemia vera is the main form, comes from a clonal marrow disorder. The two are separated by the serum erythropoietin level (low in PV, normal or high in secondary causes), the JAK2 mutation and the marrow picture. Getting this right matters because the treatments are entirely different.

## Fields

- Kind: Term
- Last checked: 2026-09-16
- Tags: polycythaemia-vera; mpn

## Sources

- Wikipedia: https://en.wikipedia.org/wiki/Polycythemia
- Wikipedia: https://en.wikipedia.org/wiki/Polycythemia

## Connected records

- cancers: [Polycythaemia vera (PV)](https://onco.cc/cancers/polycythaemia-vera/)

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JSON: https://onco.cc/api/v1/entities/erythrocytosis.json