Incremental value of red cell mass estimation by 51Cr labeled erythrocytes over hematocrit in establishing the clinical diagnosis of polycythemia vera (PCV)

Objectives Hematocrit values alone are not reliable for diagnosis of PCV owing to possible masking consequent to plasma volume (PV) expansion. We studied the role of in vitro 51-Cr-labelled RBC mass estimation technique in confirming the diagnosis of PCV in clinically suspected cases where hematocrit values were inconclusive.

Methods 111 patients (97 male and 14 female) with clinically suspected PCV underwent a 51Cr-labelled RBC mass (RCM) estimation study. In vitro labeling was done with 1.5-3.5 kBq of 51-Cr and samples were counted in a calibrated well counter. The estimated values were compared with the reference standard values of 25 and 30 ml/kg for females and males respectively for RCM, while that for PV 40 and 39 ml/kg.

Results Mean hemoglobin of 97 males was 16.86 ± 1.69 (range=13.8 - 23, median=16.8). Mean hematocrit was 0.51 ± 0.06 (range=0.41 - 0.71, median= 0.5). Of these, 31 patients had an increased RCM of which 20 patients had normal PV (absolute polycythemia) while 10 had an increased PV (Polycythemia with PV expansion) and 1 patient had a decreased PV (apparent/relative polycythemia). 18 patients had an increased PV with normal RCM. Mean hemoglobin of 14 females was 15.88 ± 2.37 (range=12.3 - 22, median=15.9), mean hematocrit was 0.51 ± 0.09 (range=0.37 - 0.74, 0.5). 7 patients had an increased RCM of which 4 patients had normal PV (absolute PCV) while 2 had an increased PV (PCV with PV expansion) and 1 patient had a decreased PV with normal RCM (apparent/relative polycythemia). 27 out of 111 patients, had haematocrit < 60 % but elevated RCM.

Conclusions RCM estimation is crucial for accurately diagnosing Polycythemia,in an appropriate clinical setting and only hemoglobin or hematocrit values cannot be relied upon. Moreover, in view of the findings of the Polycythemia Vera Study Group greater utility of RCM estimation appears to be in those patients with haematocrit less than 60 %.