Q.Relation of RBC mass and Hct/Hb/RBC count

[sgsh]

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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.

 

[Silverish]

RBC mass is exactly what it sounds like. More RBC = higher RBC mass (i.e polycythemia vera). More salt in a spoon is higher salt mass. Sometimes when you have a proportional loss of solvent from the blood, you can have a rise in hct. In such a case, RBC mass didn't change, but the solution it was in DID change. I believe that's what it's referring to.

 

[sgsh]

Thanks silverish for such a good explanation of RBC mass. Can anybody give example of any disorder which shows Hct/Hb/RBC count doesn't always hold proportional relationship with RBC MASS?

 

[Jabbed]

Acute hemorrhage (most important), polycythemia (except vera), hypoalbuminemia, heat stroke

 

[Entadus]

Dehydration, pregnancy

 

[sgsh]

Thanks all for the examples and explanations.

 

[EmetyreUSMLEStep1Tutor1]

Thanks silverish for such a good explanation of RBC mass. Can anybody give example of any disorder which shows Hct/Hb/RBC count doesn't always hold proportional relationship with RBC MASS?

Dehydration?

 

[tvelocity514]

Dehydration?

Yes, which is a cause of relative polycythemia .