Transferrin saturationTSAT · Iron saturation
What percentage of your iron-carrying capacity is actually occupied.
Transferrin saturation is serum iron divided by total iron binding capacity, expressed as a percentage: of all the iron-carrying seats available, how many are filled.
It matters because it is a better guide to iron availability than serum iron alone, and because it is the standard screening figure for iron overload as well as deficiency. Persistently high saturation is one of the first clues to haemochromatosis.
In practice it inherits serum iron's daily swing, so it is measured on a fasting morning sample where it matters, and read alongside ferritin.