Medical procedure - Red Blood Mass

Today I was at the hospital having my Red Blood Mass checked. This involved having a sample of blood taken, tagged with a radioactive Chromium isotope, re-injected and then two more samples taken at 20 & 40 mins after injection. Presumably these two samples then have their radiation levels measured.

I am not exactly why this procedure was ordered - something to do with a very slightly elevated haemaglobin(sp?) level. I also forgot to ask (hospitals make me a little nervous) (a) What exactly is being measured and (b) how does it work.

Can any Dopers enlighten me?