What is RBC Count?

Haemoglobin describes what red cells contain and Haematocrit the volume they take up. Neither says how many cells there are, and that is what this count adds.

MCV, the average size of one red cell, multiplied by RBC Count gives Haematocrit, so any two of the three fix the third. When haematocrit moves, the count is what separates a change in how many cells there are from a change in how big they are.

The bone marrow makes new red cells while older ones are cleared at the end of their life, and the count sits between the two. A shortage of iron or illness in the marrow slows production; bleeding or faster destruction of cells raises the rate at which they are lost.

A red cell lives roughly 120 days, so a change in what the marrow is producing takes months to work through into the number circulating. Measurements a year apart are far enough separated that a genuine change in production or clearance shows in the count, rather than the day-to-day fluctuation between samples. This is Do Health's monitoring schedule, not a general recommendation for everyone.