What it is
The Ganzoni equation estimates the total body iron deficit in milligrams so you can plan a course of parenteral (IV) iron replacement. It is the classic dose-targeting formula behind most IV-iron protocols: it accounts both for the iron needed to correct the circulating haemoglobin and for the iron needed to replenish stores (the “depot”).
Formula
$$ Iron\ deficit\ (mg) = weight,(kg) \times (target\ Hb - actual\ Hb),(g/dL) \times 2.4 + depot\ iron $$
The 2.4 is not arbitrary: it equals 0.0034 (the iron content of haemoglobin, ~0.34%) x 0.07 (blood volume as a fraction of body weight, ~7%) x 10,000 (unit conversion). The depot iron term is 15 mg/kg for body weight < 35 kg and a flat 500 mg for >= 35 kg — this is the pediatric-aware variant the calculator uses.
When to use
When you have confirmed iron-deficiency anaemia biochemically (low ferritin / low transferrin saturation) and have decided on IV rather than oral repletion — malabsorption, intolerance, ongoing losses, inflammatory bowel disease, CKD, or a need to correct quickly. It tells you the total to give; you then split it across infusions according to the specific product’s single-dose ceiling.
Worked example
A 25 kg child, actual Hb 7 g/dL, target Hb 12 g/dL:
- Replacement component = 25 x (12 - 7) x 2.4 = 300 mg
- Depot (weight < 35 kg) = 15 x 25 = 375 mg
- Total iron deficit = 675 mg
That 675 mg is the full course, not a single dose — you divide it across infusions per the product label.
Pitfalls
- It is a planning target, not a single-shot dose. No IV-iron product is given as 675 mg in one push to a 25 kg child. Respect each formulation’s maximum single dose and infusion-rate limits and spread the total accordingly.
- Confirm iron deficiency first. The equation assumes a true deficit; using it on anaemia of inflammation or thalassaemia (where ferritin can be normal or high) risks iron overload. Check ferritin and transferrin saturation before dosing.
- Depot rule matters. Forgetting the 15 mg/kg vs 500 mg switch at 35 kg either under-fills stores (in a small child you wrongly used 500) or over-estimates them — match the patient’s weight band.
- Newer formulations (ferric carboxymaltose, ferric derisomaltose) come with their own simplified or weight-and-Hb-banded dosing tables; some guidelines now prefer those over Ganzoni, which is known to slightly under-estimate requirements in some cohorts.
- Re-check after repletion — recovery Hb and ferritin guide whether the calculated dose actually closed the gap.
Run it: Total Iron Deficit (Ganzoni)
Decision support for qualified clinicians only — verify against current primary guidelines and your clinical judgement.