Polycythemia

Differential Diagnosis

Primary
- Polycythemia vera (PV) (JAK2 positive)
- Myelofibrosis, CML, or CMML
- Hypereosinophilic syndrome
Secondary
- Chronic hypoxia
  - Respiratory
    - Chronic CO poisoning, including smoking
    - Obstructive sleep apnea
    - IPF
    - Cystic fibrosis
    - Severe chronic lung disease
  - Cardiac
    - Congenital heart defects
    - Other causes of significant chronic shunt
- Neoplasms (autonomous Epo production)
  - Renal cell carcinoma
  - Hepatocellular carcinoma (HCC), hemangioblastoma, pheochromocytoma, uterine leiomyomata
  - Cerebellar tumours
- Decreased plasma volume
  - Hydrochlorothiazide (iatrogenic)
- Others
  - Renal transplant, renal artery stenosis, cysts, hydronephrosis
  - Doping
  - Cobalt toxicity
  - POEMS syndrome

Repeat CBC to confirm
JAK2, for myeloproliferative disorders such as polycythemia rubra vera (PRV)
Erythropoietin level (Epo)
- If elevated, suggests secondary cause
Urinalysis r/o RCC
Second-tier investigations:
- Ultrasound abdo for splenomegaly and RCC
- Bone marrow biopsy