Further investigations of erythrocytosis

Useful investigations for distinguishing primary and secondary erythrocytosis include:

• FBC:
  • the following are present only in primary erythrocytosis:
    • white cell count shifted to left (neutrophils)
    • platelets and nucleated red blood cells increased
      
      
• ESR is usually low (less than 5mm/h) in both primary and secondary disease
  
  
• leukocyte alkaline phosphatase:
  • raised in primary disease but may also be raised in infection or thrombosis
    
    
• vitamin B12 levels are increased in primary disease, predominantly due to increased transcobalamin III levels
  
  
• chest X-ray
  
  
• abdominal ultrasound or CT:
  • to detect renal, hepatic or uterine pathology, and/or hepatosplenomegaly may be positive in secondary disease
    
    
• arterial oxygen tension (PaO2) and O2 saturation measurement - decreased in secondary disease
  
  
• haemoglobin O2 affinity (P50) - may be raised in secondary disease if high affinity abnormal haemoglobins
  
  
• radioimmunoassays, ELISA - to measure serum levels of erythropoietin; raised in secondary erythrocytosis
  
  
• in vitro culture of erythroid progenitors - occurs without addition of erythropoietin in primary disease; only available in specialist labs