Iron, TIBC, ferritin and transferrin in cats and dogs

What it shows

Think of iron in the body as goods in a warehouse and on delivery trucks. Serum iron is what is on the trucks: iron carried in the blood by transferrin, a transport protein made in the liver. It changes from day to day and poorly reflects stored iron.

TIBC is the total capacity of the trucks — an indirect measure of transferrin. The lab adds excess iron, measures how much stayed unbound and adds serum iron. In health roughly a third of the binding sites are occupied; many panels also print % saturation = iron ÷ TIBC × 100. Some forms list transferrin itself, the protein that TIBC reflects.

Ferritin is the storage protein — the warehouse. The small amount in serum reflects body stores better than serum iron, and a fall in stored iron is the only known cause of low ferritin. But ferritin is also an acute phase protein, one of the proteins whose blood level rises with inflammation, and in animals it correlates only weakly with iron stores.

Higher than reference — what it may be a sign of

• Iron — random variation; steroid medicines in dogs (up to about twice); acute liver cell damage; faster red cell breakdown, as in hemolytic anemia; hemolysis in the tube; rarely iron overload after repeated transfusions.
• TIBC — uncommon and often unexplained: severe lipemia or very high serum iron as an artefact, necrotizing hepatitis in dogs.
• Ferritin — inflammation; some tumours, such as histiocytic sarcoma in dogs; iron overload after massive transfusions.

Lower than reference

• Iron — most often transient variation or inflammation: inflammatory signals lock iron away in stores. True iron deficiency is less common; in adult animals its most common cause is chronic bleeding into the gut.
• TIBC — the liver makes less transferrin (liver failure, a portosystemic shunt), transferrin is lost with albumin through the kidneys or gut, or inflammation lasts more than 24–48 hours (transferrin falls during inflammation).
• Ferritin — depleted iron stores.

The pattern matters more than one number. Low iron with low TIBC and normal saturation is typical of inflammation lasting more than a day or two. Low iron with normal TIBC and low saturation has many causes and points to iron deficiency anemia only together with chronic external blood loss and small, pale red cells.

Look at together with

• MCV and MCHC — fully developed iron deficiency anemia makes red cells small and pale; early on these indices can still be normal.
• Reticulocytes and CH — new red cells are the first to show a shortage of iron.
• Albumin — when TIBC is low, a low albumin suggests protein loss.

Units and conversion

Iron is reported in µmol/L (SI units, мкмоль/л on Russian-language forms) or in µg/dL. To convert, µg/dL × 0.179 = µmol/L, so 100 µg/dL is about 17.9 µmol/L. TIBC uses the same units and the same factor, because it is expressed as the amount of iron that can be bound. Ferritin is reported in ng/mL or µg/L — the same number, the factor is 1. Transferrin is reported in g/L. In the app each row keeps the unit printed on the form, and the chart converts to one unit.

Reference range

The reference range is on your lab’s form. The iron results are read as a set, not one by one. Iron deficiency develops in stages: stores run down first while blood iron is still in range, then iron and saturation fall, and only then does anemia appear. That is why one value inside the range does not rule out a developing shortage, and one low value does not prove it. Ferritin requires species-specific reagents, and eClinPath notes that most veterinary laboratories do not offer it.

Cats and dogs

Sources

1. eClinPath (Cornell University) — Iron
2. eClinPath (Cornell University) — TIBC
3. eClinPath (Cornell University) — Ferritin
4. eClinPath (Cornell University) — Iron panels
5. eClinPath (Cornell University) — % Saturation of transferrin
6. eClinPath (Cornell University) — Development of iron deficiency anemia
7. Merck Veterinary Manual — Nonregenerative Anemias in Animals