FerritinvsSerum iron

Ferritin and serum iron both say "iron" on the report, but they answer different questions. Ferritin estimates how much iron your body has put away in storage. Serum iron measures how much is in transit in your bloodstream at the moment the needle goes in. For deciding whether someone is iron deficient, ferritin is the stronger test by a wide margin.

What is the difference between ferritin and serum iron?

Ferritin is a protein that holds iron inside cells, mostly in the liver, spleen and bone marrow. A small amount leaks into the blood, and in a healthy person that blood level tracks the size of the body's iron stores. Serum iron is the iron bound to transferrin, the protein that carries iron from the gut and from recycled red cells to the marrow and other tissues. Serum iron is a measure of traffic on the road; ferritin is a measure of what is in the warehouse.

That difference decides how much each number can tell you. A warehouse empties slowly, so ferritin falls over weeks to months as iron runs down. Traffic changes by the hour, so a serum iron result can look different from one morning to the next with no change in your stores at all. Full detail on each marker lives on its own page: ferritin and serum iron.

Serum ferritin radioimmunoassay was by far the most powerful test, with an area under the receiver operating characteristic curve of 0.95.Guyatt GH, et al. Laboratory diagnosis of iron-deficiency anemia: an overview. J Gen Intern Med 1992 ¹

Where TIBC and transferrin saturation fit

TIBC (total iron-binding capacity) measures how much iron the blood's transferrin could carry if every seat were filled. When the body is short of iron, the liver makes more transferrin, so TIBC tends to rise in iron deficiency and fall with inflammation. See TIBC for the ranges.

Transferrin saturation is serum iron divided by TIBC, as a percentage: the share of seats that are occupied. It is the most useful job serum iron does. A low saturation means too little iron is reaching the marrow, whatever the stores look like; a high saturation means transferrin is close to full, which is the first lab sign of iron overload. ⁶ The transferrin saturation page explains the cutoffs, and the tool below does the arithmetic if you have both numbers.

Why ferritin is the number that settles iron deficiency

No other blood test separates iron-deficient from iron-replete people as well as ferritin does. A 1992 review of 55 studies that checked blood tests against bone marrow iron, the old gold standard, found ferritin far ahead of serum iron, TIBC and saturation. ¹ The World Health Organization treats a ferritin below 15 µg/L as iron deficiency in otherwise healthy adults. ² Many clinicians use a higher line, because a cutoff of 15 misses people whose stores are low but not yet empty.

Ferritin also drops before hemoglobin does. The body spends its stores first and only then makes smaller, paler red cells, so a ferritin can be low for months while the blood count still reads normal. The iron deficiency guide covers that gap, and anemia covers what happens once hemoglobin falls.

Why serum iron is a poor test on its own

Serum iron is noisy. A Mayo Clinic study drew blood from 20 healthy adults at 8 AM, noon and 4 PM and again the next morning; morning iron was higher than afternoon iron in only half of them, and the swing from one day to the next was as large as the swing within a day. ⁵ Labs once insisted on morning draws for this reason, and the study found the restriction did not make results more reliable.

• Food: an iron-rich meal or a fortified cereal in the hours before the draw lifts serum iron.
• Supplements: an iron tablet taken the day before can push serum iron up for hours without changing ferritin.
• Inflammation and infection: the hormone hepcidin locks iron inside cells, so serum iron falls even when stores are full. ⁴
• Day-to-day variation: two draws a day apart can differ as much as two draws on the same day. ⁵

When ferritin and serum iron disagree

Disagreement between these two numbers is common and usually explainable. Read ferritin first, then saturation, then serum iron, and check whether anything inflammatory was going on when the blood was drawn. A CRP on the same panel is the usual way to check.

Low ferritin, normal serum iron

This is early iron deficiency in most cases. Stores are running down, but there is still enough iron moving through the blood, sometimes helped by a recent meal or supplement, to keep serum iron inside its range. The low ferritin carries the weight here; a normal serum iron does not cancel it. ¹

Normal or high ferritin, low serum iron

This pattern usually means inflammation. Infection, autoimmune disease, kidney disease and cancer raise ferritin, because ferritin is an acute-phase protein, and they lower serum iron through hepcidin. ⁴ The trap is that a real iron deficiency can hide underneath. For adults with infection or inflammation, the WHO guideline suggests a ferritin below 70 µg/L as the deficiency threshold (30 µg/L in young children), ² and a low transferrin saturation alongside a ferritin in the tens is a reason to ask whether stores are adequate. ⁴

High ferritin, normal saturation

A raised ferritin with a normal transferrin saturation is rarely iron overload. The American College of Gastroenterology guideline notes that ferritin is often raised in fatty liver disease and alcohol-related liver disease, and that these are more common than hereditary hemochromatosis among people with a high ferritin who do not carry the main hemochromatosis gene pattern. ⁶ Obesity, metabolic syndrome and recent illness do the same. See fatty liver for that side of the story.

High ferritin and high saturation

When ferritin and saturation are both high, iron overload moves up the list. Guidelines commonly treat a transferrin saturation above about 45%, especially when it stays high on a repeat fasting sample, as the point to consider testing for hereditary hemochromatosis; the European (EASL) guideline uses above 45% for women and above 50% for men, alongside a raised ferritin. ⁷ The ACG guideline treats a ferritin below 1,000 ng/mL at diagnosis as a marker of low risk for advanced liver scarring. ⁶ The hemochromatosis page covers the genetics and what happens next.

Which one to ask about

If the question is whether you are low in iron, ferritin is the test to ask about, ideally with a full iron panel (serum iron, TIBC and saturation) and a complete blood count on the same draw. Serum iron alone is the weakest choice: it is the result most likely to change between two mornings and the least able to tell you about your stores. If ferritin comes back high, ask for transferrin saturation and a liver panel before anyone draws a conclusion about overload.

• Was my ferritin read alongside a CRP or another sign of inflammation?
• What is my transferrin saturation, and does it agree with my ferritin?
• If I am low, what is the likely source of the loss, and does it need looking into?
• Did an iron supplement or a meal before the draw affect my serum iron?
• When would a repeat test tell us something new?

For the wider picture of how an iron panel sits inside routine bloodwork, the CBC guide explains the red cell numbers that change once stores run out, and why a trend beats a single reading applies with extra force to serum iron.

The plain rule for an iron panel: ferritin tells you about stores, saturation tells you about supply, and serum iron mostly feeds the saturation calculation. When ferritin and serum iron point in different directions, ferritin, read with an eye on inflammation, is usually the one to trust.

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