What is the formula for percent transferrin saturation?

• A. (Serum iron / TIBC) × 100
• B. (TIBC / Serum iron) × 100
• C. (Serum ferritin / TIBC) × 100
• D. (Serum iron × TIBC) / 100

Answer: (A)

Percent transferrin saturation shows how much of transferrin’s iron-binding sites are actually occupied by iron. To express this as a percentage, you compare the amount of circulating iron (serum iron) to the total capacity to bind iron (TIBC) and multiply by 100. In other words, it’s the fraction of transferrin that is carrying iron.

Why this formula is the best fit: serum iron is the iron circulating bound to transferrin, while TIBC represents how much iron transferrin could bind. Taking serum iron and dividing by TIBC gives the proportion of binding sites filled, and multiplying by 100 converts that proportion to a percent.

Why the other approaches aren’t correct: using the inverse ratio (TIBC divided by serum iron) would invert the relationship and no longer reflect the fraction of occupied sites. Ferritin reflects stored iron, not the current occupancy of transferrin’s binding sites, so it isn’t part of saturation calculation. Multiplying serum iron by TIBC and dividing by 100 yields a product that isn’t a true occupancy ratio and lacks a meaningful interpretation.