Both measure the same protein. TIBC counts every iron-binding seat on your transferrin; UIBC counts the empty ones. One equation connects them.
TIBC measures every iron-binding site on your transferrin proteins, occupied or not. UIBC measures only the empty ones. They are linked by a single equation: UIBC = TIBC − serum iron. Most labs measure UIBC directly and calculate TIBC from it, so seeing one and not the other means nothing is missing from your report.
Transferrin is the protein that carries iron through your bloodstream. Each transferrin molecule has binding sites that iron can occupy, and at any moment some are full and some are empty. TIBC and UIBC are two ways of counting the same thing.
Think of transferrin as a bus. TIBC is the total number of seats. Serum iron is how many passengers are aboard. UIBC is the number of empty seats. Because the three quantities describe one bus, knowing any two gives you the third, which is why no lab measures all three separately.
Which one your lab prints comes down to assay cost, not clinical preference. Measuring unoccupied binding sites is the cheaper and more direct assay, so most labs run UIBC and report TIBC as the sum of UIBC and serum iron. A report showing UIBC but no TIBC is complete.
| TIBC | UIBC | |
|---|---|---|
| What it counts | All iron-binding sites on transferrin | Only the empty binding sites |
| How it is obtained | Usually calculated (UIBC + serum iron) | Usually measured directly |
| Typical adult range | 250–450 μg/dL | 131–425 μg/dL |
| In iron deficiency | High | High |
| In iron overload | Low or low-normal | Low |
| In chronic inflammation | Low | Low or normal |
| Moves with transferrin | Directly — TIBC is a proxy for transferrin concentration | Indirectly — also depends on how much iron is aboard |
Reference ranges vary between laboratories and assay methods. Each range is established independently by the lab rather than derived arithmetically from the others, so the endpoints above will not subtract to line up. Always read your own report's stated range.
TIBC and UIBC are two of four numbers that only mean something together. These are the typical adult ranges for all four.
| Marker | Typical adult range | What it tells you |
|---|---|---|
| Serum iron | 60–170 μg/dL | Iron circulating right now. Fluctuates through the day and after meals, so a single value is weak evidence on its own. |
| TIBC | 250–450 μg/dL | How much transferrin is available to carry iron. Rises when stores are low, falls in inflammation and liver disease. |
| UIBC | 131–425 μg/dL | The unused portion of that capacity. Moves in the same direction as TIBC in iron deficiency. |
| Transferrin saturation | 20–50% | The percentage of seats occupied: (serum iron ÷ TIBC) × 100. The number that resolves the picture. |
TIBC and UIBC describe capacity. They do not tell you whether that capacity is being used, and this is where reading either one in isolation goes wrong. Transferrin saturation supplies the missing half.
A saturation below 20% means iron is not reaching the tissues that need it, whatever the TIBC says. Above roughly 45–50% raises the question of iron overload, and the American College of Gastroenterology's haemochromatosis guideline treats a raised transferrin saturation together with a raised ferritin as the trigger for HFE genetic testing. Between those two figures, capacity numbers are rarely acting alone.
This is why a lab can report a normal-looking TIBC on a report that still describes a real iron problem, and why the pattern below matters more than any single value.
The same TIBC value means opposite things depending on what sits beside it. These are the patterns worth recognising.
| Pattern | Serum iron | TIBC / UIBC | Saturation | What it suggests |
|---|---|---|---|---|
| Iron deficiency | Low | High / High | Low | Stores are depleted and the liver is making extra transferrin to scavenge what is left. Ferritin is usually low and confirms it. |
| Iron overload | High | Low / Low | High | Most seats are taken. Haemochromatosis is the cause to rule out, especially with a raised ferritin. |
| Anaemia of inflammation | Low | Low / Low or normal | Low or normal | Iron is present but locked in storage by hepcidin. Ferritin is normal or high, which separates it from true deficiency. |
| Liver disease or malnutrition | Variable | Low / Low | Variable | The liver is producing less transferrin, so total capacity falls without iron itself being the problem. |
| Low TIBC and UIBC, normal iron | Normal | Low / Low | Normal or slightly high | Fewer seats carrying a normal number of passengers. Points to reduced transferrin production rather than an iron disorder. Check ferritin and CRP. |
The row people get stuck on is the last one. Low capacity numbers alongside a normal serum iron look contradictory if you read TIBC as an iron measurement. It is a transferrin measurement, and transferrin drops for reasons that have nothing to do with iron. For the direction-by-direction detail, see the full UIBC blood test guide.
Inflammation moves TIBC, but TIBC is not a test for inflammation. Transferrin is a negative acute-phase protein: during an inflammatory response the liver shifts production toward proteins such as CRP and makes less transferrin, so TIBC falls.
That makes a low TIBC a useful clue that inflammation may be in the picture, particularly when serum iron is also low and ferritin is normal or raised. It is not a substitute for measuring inflammation directly. CRP does that job, and TIBC is far too sensitive to iron status to be read as an inflammatory marker on its own.
Neither one alone. The useful request is an iron panel: serum iron, ferritin, and either TIBC or UIBC, with transferrin saturation calculated from them. Ferritin is the marker most often left off and the most informative about stores, so it is the one worth naming explicitly.
If your report already shows UIBC without TIBC, or the reverse, nothing is missing. One is derived from the other and your serum iron result, and your doctor reads the same picture either way.
TIBC, UIBC, serum iron, ferritin and transferrin saturation only mean something together. Upload your results and FixFirst reads all of them at once, flags which pattern they form, and tells you which marker to address first.
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