Blood volume measurement in pseudopolycythaemia--when and why?

Upper reference limits for packed cell volume (PCV) for a hospital population were established by analysis of 14,000 consecutive blood count profiles. These were 0.50 for men and 0.45 for women. In 86 patients, pseudopolycythaemia was identified by an increased PCV with red cell mass (RCM) less than 125% of predicted normal. We have proposed a modified classification of pseudopolycythaemia and identified three subsets on the basis of RCM and plasma volume (PV) values. Although all patients had raised PCV at the time of referral, by the time of blood volume study a lower PCV value was noted in a proportion of patients. Patients with a low PV (less than 80% predicted normal, Group 1) and those with a 'high normal red cell mass' (RCM 113-125% predicted normal, Group 2) had higher and more persistently elevated PCV values on repeated measurement, compared with the patients in Group 3 ('transient pseudopolycythaemia'). These latter patients have a normal RCM and PV, and although initially fulfilling the criteria for pseudopolycythaemia on the basis of a raised PCV and normal RCM, a high proportion subsequently showed fluctuation in PCV which often fell below the upper reference limit. It is important to diagnose pseudopolycythaemia because of the associated increase in morbidity and mortality, and our observations suggest that whereas a blood volume study is essential to diagnose pseudopolycythaemia reliably, this investigation need only be performed when a patient shows a persistently increased PCV on repeated measurement.