Transferrin Saturation Test

If you have ever been told your iron is "fine" based on a single ferritin number, you may be getting an incomplete picture. Ferritin tells you how much iron your body has in storage, but it says nothing about how much iron is actually circulating and available to your tissues right now. Transferrin saturation (TSAT) fills that gap. It shows what percentage of your blood's iron-delivery trucks are loaded with cargo, and that number predicts heart failure outcomes, kidney disease progression, and mortality risk in ways ferritin simply does not.

TSAT is calculated from two lab values you may already have: serum iron divided by TIBC (total iron-binding capacity, a measure of how much iron your blood could carry if every seat were full), multiplied by 100. The result is a percentage. In healthy adults, roughly 20 to 50% of those seats are occupied. When that number drops below about 20%, your tissues are starving for iron even if your storage looks adequate. When it climbs above 45 to 50%, excess iron may be spilling into forms that damage organs.

How Your Body Moves Iron

Your liver produces a protein called transferrin that acts as the sole dedicated iron shuttle in your bloodstream. Each transferrin molecule can carry two atoms of iron from where it is absorbed (your gut) or recycled (your spleen) to wherever it is needed, especially your bone marrow, which consumes most of the body's iron supply to build new red blood cells.

A hormone called hepcidin, also made by the liver, acts as a gatekeeper. When iron stores are high, hepcidin rises and slows iron absorption and release. When stores are low, hepcidin drops and the gates open. Inflammation throws a wrench into this system: it raises hepcidin even when iron is needed, trapping iron in storage and lowering TSAT. This is why someone with a chronic inflammatory condition can be iron-deficient at the tissue level while their ferritin reads normal or high.

Heart Failure Risk

Low TSAT is one of the strongest independent predictors of death and hospitalization in people with heart failure. In a study of roughly 2,050 heart failure patients, lower TSAT was significantly associated with higher risk of death and heart-failure-related hospital admissions, while ferritin showed no significant association at all. That finding is striking: the marker most doctors order first (ferritin) was not predictive, but the one they sometimes skip (TSAT) was.

A separate study of about 4,100 older adults with heart failure and new-onset anemia found that TSAT below 20% independently predicted more hospitalizations and deaths regardless of ferritin level. In yet another cohort of 661 heart failure patients, the combination of ferritin between 100 and 300 micrograms per liter with TSAT below 20% was the single most powerful predictor of worse survival.

Meta-analyses of intravenous iron trials in heart failure reinforce the point. The benefit of IV iron, including fewer cardiovascular deaths and heart failure hospitalizations, concentrates almost entirely in patients whose baseline TSAT was below 20%. Patients with TSAT at or above 20% saw little additional benefit, regardless of their ferritin. If you have heart failure, your TSAT may be more important than your ferritin for guiding treatment decisions.

Kidney Disease Risk

In non-dialysis chronic kidney disease (CKD), TSAT below 25% independently predicted higher all-cause mortality in a Korean cohort of about 2,150 patients. Another study of roughly 5,150 non-dialysis CKD patients found that iron deficiency defined by low TSAT was tied to higher risk of death and major cardiovascular events, with or without anemia. Even in CKD patients who were not anemic, TSAT at or below 20% increased the risk of kidney disease progression and death in a study of nearly 18,900 people.

For those on dialysis, the picture is similar. Among about 880 patients starting dialysis, TSAT at or below 20% carried roughly double the risk of cardiovascular and all-cause death compared to higher levels, partly explained by inflammation, enlarged heart chambers, and elevated cardiac stress markers. In maintenance hemodialysis patients, the combination of low TSAT (below 20%) with high ferritin was the strongest predictor of stroke, heart events, and death.

Coronary Artery Disease

In people with established coronary artery disease, TSAT shows a U-shaped relationship with death: risk is highest at both low and high ends, with an apparent sweet spot around 20 to 30%. In a study of 769 coronary artery disease patients, systemic inflammation partially explained the excess mortality seen at the low end of TSAT but not at the high end, suggesting different mechanisms drive risk at each extreme.

Type 2 Diabetes

A Mendelian randomization study, which uses genetic variants to test cause-and-effect relationships, found that genetically higher iron status (including higher TSAT) is causally linked to increased risk of type 2 diabetes. A large European observational study found the relationship is more nuanced: higher ferritin and transferrin levels raised diabetes risk, but higher TSAT was associated with lower risk in women, suggesting that the form in which iron exists in the body matters, not just the total amount.

Liver Disease and Cancer

Elevated TSAT is a hallmark of iron overload conditions. In hereditary hemochromatosis (an inherited condition where the body absorbs too much iron), persistently elevated TSAT above 50% during maintenance therapy was associated with worse joint and general symptoms, even when ferritin was controlled. In a large prospective cohort of about 132,500 people, hemochromatosis gene carriers had higher risks of diabetes, liver disease, and mortality, and these risks persisted even among carriers whose iron markers were normal, suggesting that the genetic variant itself carries risk beyond what TSAT and ferritin capture.

Among roughly 18,600 people with fatty liver disease who did not have hemochromatosis, elevated TSAT was significantly associated with increased risk of liver cancer. In myelodysplastic syndromes (a group of bone marrow disorders), TSAT above 80% independently predicted worse overall survival, faster disease progression, and higher risk of transformation to leukemia in a study of 718 patients.

Reference Ranges

The ranges below come from large population studies, primarily in Caucasian adults from the United States and Europe. Men carry roughly 15 to 20 percentage points higher TSAT than women from ages 20 through 70. Your own lab may use slightly different cutpoints depending on the assay platform and reference population.

Compare your results within the same lab over time. A single TSAT value near any threshold is unreliable because of the substantial day-to-day variation inherent in this calculation.

When Results Can Be Misleading

TSAT has significant within-person biological variability. A study of over 101,000 adults found that this variability was large enough that about one-third of people with confirmed hereditary hemochromatosis would be missed at standard screening cutoffs. Fasting before the test does not reliably reduce this variability.

• Inflammation and acute illness: Any condition raising CRP (infection, surgery, autoimmune flare, obesity) pushes hepcidin up and TSAT down, potentially masking adequate iron stores or creating a false impression of deficiency. Draw your blood when you are feeling well, not during or shortly after an illness.
• Recent iron supplements or iron-rich meals: Oral high-dose iron (65 mg of elemental iron) causes large transient spikes in serum iron and TSAT for several hours. Avoid iron supplements for at least 24 hours before testing.
• Blood transfusion: Receiving red blood cells causes a marked rise in serum iron and TSAT lasting up to 24 to 36 hours, which can mask underlying deficiency if blood is drawn too soon after transfusion.
• Menstrual cycle: In female athletes, TSAT and serum iron are lower during bleeding phases. If you menstruate, testing at the same cycle phase each time improves comparability.