What is Haemoglobin?

The amount of haemoglobin in a given volume of blood sets how much oxygen that blood can carry. The same protein also carries some carbon dioxide from the tissues back to the lungs.

RBC Count gives the number of red cells and Haematocrit the volume they occupy, so a change in haemoglobin can be set against how many cells there are and how much space they take up. Ferritin reflects the iron stores the bone marrow draws on, and those stores limit how much haemoglobin the marrow can build into a new cell.

The marrow cannot build haemoglobin without iron. The marrow also raises production when the blood is delivering less oxygen. That is why living at altitude, where there is less oxygen to take up, prompts more haemoglobin over time. Smoking works through the same signal, by reducing how much oxygen the blood delivers.

New haemoglobin reaches the blood only inside a new red cell, so the total in circulation changes no faster than those cells are replaced, across a lifespan of roughly 120 days. A change in what the marrow is producing takes most of that time to show in full, and yearly measurements sit far enough apart to catch that change complete rather than half way through. This is Do Health's monitoring schedule, not a general recommendation for everyone.