Iron Saturation Calculator

What Transferrin Saturation Measures

Transferrin saturation, or TSAT, is calculated from two numbers on a standard iron panel: serum iron (the amount of iron currently bound to transferrin in the bloodstream) and total iron-binding capacity, or TIBC (the capacity of transferrin to carry iron). The calculation is simple:

TSAT % = (Serum Iron / TIBC) × 100

Unlike ferritin, which reflects stored iron, TSAT reflects iron in transit — the iron that is actually available for making red blood cells, supporting cellular energy metabolism, and dozens of other functions. It is the single most useful marker on an iron panel for distinguishing iron deficiency, where available iron is low, from iron overload, where transferrin is carrying more iron than it would ideally saturate.

Reference Ranges and Categories

Source: Adams PC et al., AASLD Practice Guideline — Hemochromatosis Diagnosis and Management, Hepatology 2019.

Low TSAT: Absolute vs Functional Iron Deficiency

A TSAT below 16% is one of the most specific signals of iron deficiency. The next question is why. Reading TSAT together with ferritin (and CRP when available) distinguishes two fundamentally different states:

• Low TSAT + low ferritin — absolute iron deficiency. The body is genuinely short of iron, most often due to blood loss (heavy menstruation, GI bleeding, frequent blood donation), malabsorption (coeliac disease, inflammatory bowel disease, post-bariatric surgery), or dietary inadequacy. Responds well to iron supplementation once the cause is identified.
• Low TSAT + normal or elevated ferritin (especially with elevated CRP) — functional iron deficiency or anaemia of chronic disease. Here the body has iron, but hepcidin — an inflammation-driven hormone — locks iron in storage and blocks release to transferrin. Oral iron is often poorly absorbed in this setting; treatment typically focuses on the underlying inflammation. Common in chronic kidney disease, rheumatoid arthritis, inflammatory bowel disease, and chronic infections.

High TSAT: Iron Overload and Hemochromatosis

A persistently elevated TSAT is the initial screening finding for hereditary hemochromatosis, the most common genetic disorder in people of northern European descent. The AASLD 2019 Hemochromatosis Guideline (Adams et al., Hepatology) recommends that a TSAT above 45%, particularly combined with elevated ferritin, should prompt further evaluation including HFE gene testing. Higher specificity is achieved at TSAT above 55% in men or 50% in women. Once hemochromatosis is confirmed, phlebotomy (therapeutic blood removal) is the mainstay of treatment and can entirely prevent progression to organ damage if initiated early.

Not all elevated TSAT is hemochromatosis. Other causes include:

• Recent iron supplementation — oral iron taken within 24 hours can raise serum iron sharply.
• Recent blood transfusions — each unit contains ~250 mg of iron.
• Haemolysis — breakdown of red cells releases iron into circulation.
• Liver disease — especially with secondary iron accumulation.
• Non-fasting, non-morning sampling — iron can be transiently elevated.

Diurnal Variation: Draw Iron Fasting in the Morning

Serum iron shows substantial circadian variation — often peaking in the morning and falling 30–50% through the day. Because TSAT is derived from serum iron, the same applies. To get a comparable and interpretable result, an iron panel should ideally be drawn fasting, in the morning, and before any oral iron supplement that day. Ferritin does not share this variability and can be drawn at any time of day.

TIBC vs UIBC vs Transferrin

Three closely related terms cause frequent confusion:

• TIBC (Total Iron-Binding Capacity) — maximum amount of iron that transferrin could bind. Rises in iron deficiency (the liver makes more transferrin to scavenge iron); falls in inflammation and chronic disease.
• UIBC (Unsaturated Iron-Binding Capacity) — the portion of TIBC not currently carrying iron. Some labs report UIBC instead; TIBC = Serum Iron + UIBC.
• Transferrin — the protein itself, usually reported in mg/dL or g/L. TIBC is derived from transferrin (roughly: TIBC in µg/dL ≈ Transferrin in mg/dL × 1.389, though methods differ).

When to Repeat

A single abnormal TSAT — especially one taken non-fasting, in the afternoon, or soon after an iron supplement — is rarely a diagnosis. Most clinicians would repeat a borderline or abnormal result with a fasting morning draw, alongside ferritin and CRP, before acting. For persistently elevated TSAT above 45% with elevated ferritin, the AASLD pathway leads to HFE genetic testing (C282Y and H63D mutations); for persistently low TSAT, a search for the cause of blood loss or malabsorption follows.