Why you can be low in iron before a blood test says so

Tiredness is one of the most common reasons people get tested, and one of the vaguest symptoms there is. Iron is a useful place to start, not because it explains every case, but because it is easy to measure, usually fixable, and often missed.

Your stores empty before the blood count changes

Anaemia is diagnosed by measuring haemoglobin, the part of your blood that carries oxygen. But haemoglobin is the last thing to drop. Your body spends its stored iron first, and ferritin, which measures that store, can fall a long way while a standard blood count still looks completely normal.

So you can have a normal blood count and an empty iron store at the same time. If nobody measured the store, nothing looks wrong.

Ferritin also rises when you are inflamed

This is where reading it properly matters. Ferritin goes up when there is inflammation in the body, so the number can look reassuring while your stores are genuinely low. Read next to inflammation results and the rest of your iron results, the picture usually becomes clear. Read on its own, it can mislead you either way.

Measure, treat, measure again

If your stores are low, what happens next depends on why. Iron that keeps dropping is worth asking questions about, rather than just topping up the number. Testing again after a while is part of the job: it shows whether the treatment worked, and whether the real cause was dealt with.