Polycythaemia (apparent)

Apparent polycythaemia (apparent erythrocytosis) describes an normal or minimally raised red cell volume with a reduced plasma volume. Causes include:

• transient apparent polycythaemia:
  • dehydration
  • diuretic therapy
• chronic apparent polycythaemia:
  • spurious polycythaemia
  • stress erythrocytosis - Gaisbock's syndrome

Management of apparent erythrocytosis patients include

• spontaneous improvement is the rule
• targeting in reduction or elimination of precipitating factors: smoking, obesity, hypertension and high alcohol consumption (1)
• if no such factors are identified, venesection should be considered in the following patients
  • a recent history of thrombosis, or with additional risk factors for thrombosis
  • when the PCV is chronically greater than 0.54 (1).

Notes:

• true increased red cell mass (i.e. not apparent erythrocytosis) can be assumed when the haematocrit is over 60% in males and 56% in females (2)
• haematocrit returns to within the normal range in up to 30% of patients with untreated apparent erythrocytosis if serial measurements are undertaken (3)
• those with a raised Hct but an Red Cell Mass studies within the normal range have an apparent erythrocytosis (4)
  • a relative erythrocytosis, found in states of dehydration, can be confirmed when the RCM is within the normal range and plasma volume is below normal
  • patients with a relative or apparent erythrocytosis require no further investigation

Reference:

1. Tefferi A, Barbui T. Polycythemia vera: 2024 update on diagnosis, risk-stratification, and management. Am J Hematol. 2023 Sep;98(9):1465-87.
2. Pearson, T.C. Evaluation of diagnostic criteria in polycythemia vera. Seminars in Hematology 2001;38(1 Suppl 2): 21-24.
3. Messinezy, M. and Pearson, T.C. A retrospective study of apparent and relative polycythaemia: associated factors and early outcome. Clinical and Laboratory Haematology 1990;12(2): 121-129.
4. McMullin MF et al. A guideline for the diagnosis and management of polycythaemia vera. A British Society for Haematology Guideline.British Journal of Haematology, 2019, 184, 176-19