Red cell mass studies

Red cell mass studies

• patients with Hct >0.60 (males) or >0.56 (females) can be assumed to have an absolute erythrocytosis, but in others RCM studies can be helpful to confirm an absolute erythrocytosis
• an RCM more than 25% above the mean predicted value is diagnostic of an absolute erythrocytosis

Red cell mass can be measured by the isotope dilution method. A small volume of the patient's red cells is incubated with 51-chromium and re-injected. Simultaneously, plasma volume is measured using 131-iodine-labelled albumin; it is injected and so diluted. Upon resampling, the volume of distribution can be determined for the known initial red cell concentration. Erythrocyte mass can then be calculated.

Obese patients can produce odd results and so their figures are often expressed in terms of ideal body weight or surface area.

A red cell mass greater than 125% of that predicted is diagnostic of polycythaemia.

Those with a raised Hct but an RCM within the normal range have an apparent erythrocytosis.

• 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. McMullin MF et al. Guidelines for the diagnosis, investigation and management of polycythaemia/erythrocytosis. Br J Haematol. 2005 Jul;130(2):174-95
2. 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, 176-191