Red Cell Mass Measurement in Polycythemia: Evaluation of Performance and Added Value.

Background : Current diagnosis of polycythemia vera (PV) is based on a set of clinical and laboratory criteria that were adopted more by consensus rather than because of support from systematic evidence. Accordingly, one major criterion for the diagnosis of PV requires the demonstration of increased red cell mass (RCM) as measured by radionuclide dilution methods. In order to adjust for the influence of obesity on RCM expressed in mL/kg, an expert radionuclide panel of the International Committee for Standardization in Haematology (ICHS) has recommended that the results be expressed in reference to body surface area and specific formulae for the prediction of normal values as well as guidelines for the interpretation of measured values have been proposed (J Nuclear Med 1980;21:793, BJH 1995;89:748). Nevertheless, there is limited data on either the performance or added value of RCM measurement, following these revised recommendations, for the diagnosis of PV in current clinical practice. Methods : The current study looks at a single institution experience with RCM measurement over the last 10 years involving patients in whom the test was performed to consider the diagnosis of PV. The study excluded patients that were previously treated with either phlebotomy or cytoreduction. Designation of diagnostic categories was based on both a retrospective and prospective analysis of clinical data, bone marrow histology, and other laboratory parameters including leukocyte count, platelet count, serum erythropoietin (EPO) level, serum B12 level, and leukocyte alkaline phospatase (LAP) score. A diagnosis of secondary polycythemia (SP) required the presence of a condition known to be associated with SP. Apparent polycythemia (AP) was represented by patients in whom the diagnosis of either PV or SP could not be made and the stability of hematocrit values was documented by serial measurements. Measurement and interpretation of RCM values were according to the aforementioned published criteria and separate analyses were performed for males and females. Results : i) Evaluation of test performance : The study cohort consisted of 105 patients (60 males; median age 62 years, range 16–89) including 25 with PV, 35 with SP, 38 with AP, and 7 with essential thrombocythemia (ET). Table 1 outlines the percentage of patients, in each disease category, whose measured values exceeded the 98–99% limits of the reference range (i.e. ±25% of the normal predicted mean for an individual patient). Table 1 | Diagnosis | % with increased RCM (m2) | % with normal RCM (m2) | % with decreased plasma volume | % with increased plasma volume | |:--------- | ------------------------- | ---------------------- | ------------------------------ | ------------------------------ | | PV (n=25) | 80 | 20 | | 20 | | ET (n=7) | 57.1 | 42.9 | | 29 | | SP (n=35) | 20 | 80 | 2.9 | 5.7 | | AP (n=38) | 21.6 | 78.4 | 5.4 | 5.4 | The results reveal that RCM measurement was neither adequately sensitive nor specific in distinguishing PV from the other disease categories. In addition, based on the aforementioned ICHS criteria, chronically contracted plasma volume appears to be an infrequent phenomenon in AP. ii) Evaluation of added value for the diagnosis of PV : Among the 19 PV patients with elevated RCM, serum EPO was measured in 17 and the results showed decreased levels in 16 (94%). Bone marrow biopsy was available for review in 9 patients and the results were consistent with PV in all instances (100%). LAP score was performed in 12 patients and 11 had LAP scores above 130 (92%). In none (0%) of the 19 patients was RCM measurement found to be vital for the diagnosis of PV. Conclusion : In the current retropsective study, RCM measurement was found to be neither diagnostically accurate nor essential for the diagnosis of PV.