Ferritin
An iron storage protein whose blood level reflects the body's reserves, and the earliest marker of deficiency, well before haemoglobin.
Ferritin is the protein that stores iron in the liver, spleen and bone marrow. Its blood level is the most faithful reflection of the body’s iron stores, which makes it the central marker for assessing iron status.
It falls before haemoglobin does. This is what gives it its clinical value: the body draws on its reserves first, and only once these are exhausted does haemoglobin fall and anaemia appear. A low ferritin with a still-normal haemoglobin signals a deficiency in the making, the moment when correcting it is easiest.
This explains a frequent reason for consultation: persistent fatigue, hair loss or breathlessness on exertion with a “normal” blood count. Without a ferritin test, the depletion of reserves goes unnoticed.
Take care with interpretation, though: ferritin is also an inflammation protein. It rises with infection, chronic inflammation or liver disease, which can mask a real deficiency. This is why it is interpreted alongside CRP and the rest of the tests, never in isolation. A high level is not harmless either and should be investigated.