Q. In pathoma it's mentioned that-"Although Hb and Hct and RBC count are usually used as a surrogates for RBC mass, but in some circumstances they are not a good estimate of RBC mass". Can anybody cite hematological disorder with brief explanation that clarifies Hb, hct and RBC count can't always be used as a good surrogate for RBC mass.What exactly is RBC mass? Is it the total no. of RBCs in circulation? Thanks.