Apparent polycythaemia: diagnosis, pathogenesis and management.

The term "apparent polycythaemia" is applied to a group of patients who have a raised PCV (> 0.51 in males, > 0.48 in females) but a normal red cell mass (less than 25% above their predicted mean normal value). Some have additionally a marked reduction in plasma volume and can be defined as a subgroup: relative polycythaemia. Smoking, hypertension and to a lesser extent obesity, excessive alcohol, low-dose diuretic therapy and hypoxaemia have all been associated with apparent polycythaemia but the mechanism is both uncertain and likely to be complex. This group of patients is unlikely to be uniform in pathogenesis and may well include some normal individuals. Investigation requires exclusion of factors associated with other types of polycythaemia. The possibility of an increased vascular occlusive risk is uncertain in these patients except at the higher PCV values. Reduction of PCV by venesection is sensible at PCV > 0.54 or where there is perceived to be an increased risk of vascular occlusion. The remaining patients should be managed by regular observation to detect further rise in PCV or evolution to absolute polycythaemia (raised red cell mass). In some, the PCV returns to normal.