Transferrin saturation from serum iron and TIBC
Enter serum iron plus whichever second value your panel reports. Labs differ: some print TIBC, some UIBC, some transferrin. Switch the toggle to match your report rather than converting by hand.
Educational tool, not a diagnosis. TSAT is interpreted alongside ferritin, not instead of it — ferritin rises with inflammation and can look normal while iron stores are genuinely low, which is exactly when TSAT is most useful. A persistently high TSAT, particularly above 45%, is the standard trigger for investigating hemochromatosis. Most US laboratories print a normal range of roughly 20 to 50 percent; the 25 to 35 percent band above is the tighter zone where iron supply is comfortable rather than merely inside the lab range. Reference intervals vary between laboratories; compare against the range printed on your report.
How to calculate TSAT
Transferrin saturation is not measured directly. It is a ratio, calculated from two numbers your lab already printed:
TSAT (%) = serum iron ÷ TIBC × 100
Both values are in micrograms per deciliter on a US panel. A serum iron of 90 with a TIBC of 320 gives 90 ÷ 320 = 0.281, so a saturation of 28.1 percent. That is the whole calculation — the difficulty is almost never the arithmetic, it is that your report may not contain a line called TIBC at all.
When your lab prints UIBC or transferrin instead
Three assays are in common use, and most calculators only handle the first one. All three get you to the same place:
TIBC — use it directly. UIBC, the unsaturated portion, adds to serum iron to give TIBC: 90 + 230 = 320. Transferrin in grams per litre converts with a factor of 125, because one milligram per deciliter of transferrin binds about 1.25 micrograms per deciliter of iron and one gram per litre is 100 milligrams per deciliter: 2.56 × 125 = 320.
A factor of about 25 also circulates for the transferrin path, and it is correct in its own context: it gives TIBC in micromoles per litre, for use with a serum iron also reported in micromoles per litre. Applied to a US panel, where serum iron is in micrograms per deciliter, it produces saturations far above 100 percent — physiologically impossible, and a common way to frighten yourself over nothing. The toggle above picks the right path so the question does not arise.
Reading the percentage you get
Most US laboratories print a normal range of roughly 20 to 50 percent. The tighter 25 to 35 percent band shown with your result is the zone where iron supply is comfortable rather than merely inside the lab range — the distinction between a normal result and a good one, which is the distinction most panels never make.
| TSAT | How it is usually read |
|---|---|
| Under 20% | The classic iron-deficiency pattern. Can appear before hemoglobin falls, so it often precedes anemia. |
| 20–25% | Inside most flagged intervals but below the typical middle. Not diagnostic alone; read with ferritin and symptoms. |
| 25–35% | Where most adults sit. Iron supply and carrying capacity are broadly matched. |
| 35–45% | Still within many reported ranges. A recent iron supplement or a non-fasting draw can put a normal person here. |
| 45% and above | The usual threshold for investigating iron overload, including hereditary hemochromatosis, when it persists on a repeat fasting sample. |
The width of that lab interval is the point. A report that flags nothing between 20 and 50 percent prints a result of 21 percent and a result of 45 percent without comment, despite the two pointing in opposite directions. Where you sit inside the interval, and which way it moves between panels, carries more information than whether a flag appeared.
Below 20 percent is the classic deficiency pattern and can appear before hemoglobin falls. Above 45 percent, repeated on a fasting sample, is the usual trigger to investigate iron overload including hereditary hemochromatosis. Two things move the number without your iron having changed: a non-fasting draw or a recent iron supplement pushes it up, and anything that shifts transferrin — pregnancy, oral contraceptives, liver disease — moves the ratio from the denominator. A fasting morning draw is the one worth comparing over time.
Why TSAT catches what ferritin misses
Ferritin is the marker most people know, and it is a good one — but it has a well-documented blind spot. Ferritin is an acute-phase reactant, meaning it rises with inflammation, infection, obesity and liver disease regardless of how much iron you actually have. That is how someone can be told their ferritin is normal while being genuinely iron deficient. Transferrin saturation is not distorted the same way, so a low TSAT alongside a normal-looking ferritin is one of the most informative patterns on a panel — the case covered in ferritin versus iron saturation.
Reading it alongside the rest of your panel
TSAT is rarely interpreted alone. Low TSAT with normal hemoglobin suggests depletion that has not yet become anemia. Low TSAT with a low MCV and MCH points toward established iron-deficiency anemia. If you have been supplementing without improvement, the reasons are covered in why iron supplements stop working.
One saturation is a snapshot of a value that moves. The pattern worth having is the same fasting draw tracked across several panels, with ferritin beside it — which is what Merios is built to do from the reports you already have.