Formula used :  iron replacement (mg) = ((15-hgb) *( wt in kg * 3)) - 250 mg * number of units of blood administered

For every unit of blood administered the Hemoglobin will increase by approximately 1 grams/dl

Facts about Iron....

Absorption and Distribution:
The average dietary intake of iron is 12 to 20 mg/day for males and 8 to 15 mg/day for females; however, only about 10% of this iron is absorbed (1 to 2 mg/day) in individuals with adequate iron stores. Percent absorption is affected by salt form, the amount administered, and size of iron stores. Different salt forms vary in the amount of elemental iron, as indicated in the table below (Alpers et al, 1995; Stoklosa & Ansel, 1991).
 
IRON SALT                         % IRON
Ferrous fumarate                          33
Ferrous gluconate                        11.6
Ferrous sulfate                             20
Ferrous sulfate, anhydrous           30

Iron is primarily absorbed from the duodenum and upper jejunum by an active transport mechanism. The ferrous salts (sulfate, gluconate, fumarate) are absorbed almost on a milligram-for-milligram basis but differ in the content of elemental iron. Sustain-release or enteric-coated preparations reduce the amount of available iron. Absorption from these dose forms is reduced because iron is transported beyond the duodenum. Dose also influences the amount of iron absorbed. The amount of iron absorbed increases progressively with larger doses; however, the percentage absorbed decreases. Food with dairy products can decrease the absorption of iron by 40 to 66%; however, gastric intolerance may often necessitate administering the drug with food.1 Absorption is enhanced when the iron storage is depleted or when erythropoiesis occurs at an increased rate. Subjects with normal iron stores absorb 10% to 35%; iron deficient patients absorb 80% to 95% (Harju, 1989).

Laboratory Tests
Iron studies: A patient with any one of the following criteria requires iron therapy: serum ferritin level less than 100 micrograms (mcg)/L, transferrin saturation less than 20%, or percentage of hypochromic red blood cells greater than 10%. Iron supplementation should be used with caution in patients with serum ferritin greater than 1000 mcg/L or with transferrin saturation greater than 50%, due to possible toxicity. Iron supplementation should aim for a serum ferritin level of 300 to 500 mcg/L and transferrin saturation of 25% to 35% (Drueke et al, 1997).2 A more recent study found that a serum ferritin less than 400 mcg/L has the greatest sensitivity/specificity relationship (100%/87.9%) for determining iron deficiency requiring iron supplementation. The authors concluded that serum ferritin less than 100 mcg/L was too restrictive a criterion for treatment; patients with serum ferritin between 100 and 400 mcg/L and those with serum ferritin less than 100 mcg/L had the same increase in hemoglobin with iron treatment (Silva et al, 1998)

Dosage and Administration
Iron deficiency anemia: For iron replacement therapy, the usual oral dose in adults is 2 to 3 mg/kg/day of elemental iron in three divided doses. In children, 2 to 12 years of age, the usual oral dose is 3 mg/kg/day of elemental iron in 3 to 4 divided doses. In children 6 months to 2 years of age, 6 mg/kg/day of elemental iron in 3 to 4 divided doses is recommended. For infants, a dose of 10 to 25 mg of elemental iron daily in 3 to 4 divided doses has been used; however, a maximum daily dose of 15 milligrams should not be exceeded (Johnson, 1991). However, higher doses do not result in a more rapid hematologic response. Therapy may continue for 4 to 6 months depending upon the severity and cause of the iron deficiency.

For patients who sustain intermittent blood loss, quantitative estimates of the patient's blood loss and hematocrit prior to the blood loss provide a convenient method for calculation of the required iron replacement dose. Assuming that 1 mL of normocytic, normochromic red cells contains 1 mg of elemental iron (Prod Info Infed(R), 1996): Replacement Oral Elemental Iron (mg) = Blood Loss (mL) x Hematocrit x 10 The factor of 10 is required to correct for the 10% absorption of oral iron.

Miscellaneous

For optimal response to erythropoietin, maintenance of serum ferritin greater than 100 mcg/L, transferrin saturation greater than 20%, and hypochromic red blood cells less than 10% are indicative of adequate iron stores.1 Supplemental iron therapy is recommended for hemodialysis patients when the iron/TIBC ratio is less than 0.2, regardless of the serum ferritin (Meisels & Roy-Chaudhury, 1996).

Info from Maryland Department of Health and Mental Hygiene
Pharmacy and Therapeutics Medication Review May 2004