What is measured
- Ferritin: the body iron store
- Serum iron: iron circulating in the blood at that moment
- TIBC and transferrin saturation: how much of the iron transport capacity is filled
Ferritin shows the long-term store, while serum iron shifts hour to hour. That is why all three are read together rather than one at a time.
How the test is done
One blood sample, and a morning slot is suggested because serum iron shifts through the day.
If you are taking an iron supplement, tell the doctor before the test. Supplements can raise serum iron temporarily and make the picture look better than it is. The doctor will say whether to pause it and for how long before the draw.
An inflammatory marker such as hs-CRP is usually drawn at the same time, for the reason explained below.
Why a normal ferritin does not always mean enough iron
Ferritin is an acute phase protein. It rises with inflammation, infection and liver disease, even when iron stores are genuinely low.
That means someone with low-grade inflammation can return a comfortable looking ferritin while their stores are already thin. This is why ferritin is read next to an inflammatory marker rather than on its own.
The reverse matters too. A high ferritin is not proof of good iron status. It needs assessment, because the cause can be inflammation, a liver condition, or iron overload.
