Iron Deficit Calculator: Ganzoni Replacement Dose

The Ganzoni equation estimates how many milligrams of elemental iron a patient is missing, splitting the answer into circulating iron and storage iron. The multiplier 2.4 is not arbitrary: blood volume is taken as 7 percent of body weight, or 0.07 L/kg, and hemoglobin carries 3.4 mg of iron per gram. Multiplying 0.07 by 3.4 and converting deciliters to liters and grams to milligrams gives 2.38, rounded to 2.4. So weight times the hemoglobin gap times 2.4 yields the iron locked up in the missing red cell mass, and a storage term of 500 mg for adults over 35 kg (or 15 mg/kg below that) is added so the reservoir is refilled rather than left empty. Typical results run from about 1,000 to 2,000 mg. A 70 kg patient moving from 8 to 13 g/dL needs 840 mg for hemoglobin plus 500 mg of stores, or 1,340 mg total. Diagnosis should precede calculation: ferritin below 30 ng/mL confirms depletion in an uninflamed patient, but inflammation and chronic kidney disease push the cut-off to 100 ng/mL or higher, with transferrin saturation below 20 percent used alongside. The formula is known to underestimate requirement, which is why fixed-dose schedules based on weight and hemoglobin band are now common. Delivery is capped by product: iron sucrose gives about 200 mg per visit, ferric carboxymaltose up to 750 mg in the United States and 1,000 mg in Europe, ferumoxytol 510 mg, while low-molecular-weight iron dextran can be given as a single total-dose infusion.

Parenteral iron predates the formula. Iron dextran was introduced in the 1950s and made total-dose infusion feasible, but the high-molecular-weight preparations of that era carried a real anaphylaxis risk that made clinicians reluctant to give large amounts. What was missing was a defensible way to decide how much a given patient actually needed rather than dosing by habit. Ambrogio Maria Ganzoni, working in Switzerland, published the deficit equation in Schweizerische Medizinische Wochenschrift in 1970 in the context of intravenous iron-dextran therapy, and its physiological derivation from blood volume and the iron content of hemoglobin is why it survived while the drug it was written for fell out of favor. The equation became the labelling standard for successive intravenous iron products through the 1990s and 2000s as iron sucrose, ferric gluconate, ferumoxytol, ferric carboxymaltose and ferric derisomaltose arrived with far better safety profiles and progressively larger single doses. Its weakness then became visible: clinical trial work in inflammatory bowel disease published by Evstatiev and colleagues in 2011 compared Ganzoni-calculated dosing with a simplified fixed-dose table and found the simplified schedule at least as effective, with markedly fewer patients left underdosed and better adherence to the full course. Simplified banding is now embedded in many gastroenterology, nephrology and patient blood management protocols, while the Ganzoni result remains the reference estimate of true total body deficit.