Iron Deficiency Calculator

Understanding Iron Studies: Ferritin, TIBC, and Transferrin Saturation

Iron metabolism involves a complex interplay of storage, transport, and utilization. Ferritin is the body's main iron storage protein, primarily located in the liver, spleen, and bone marrow. Serum ferritin reflects total body iron stores with remarkable fidelity—each 1 ng/mL of ferritin corresponds to approximately 8-10 mg of stored iron. In a healthy adult, total iron stores range from 3000 to 4000 mg in men and 2000 to 3000 mg in women, with menstrual losses accounting for the difference. Ferritin below 15 ng/mL signals complete depletion of iron stores, making it the most specific marker of iron deficiency.

Total iron binding capacity (TIBC) measures the blood's capacity to bind iron with transferrin, the iron transport protein. Normal TIBC ranges from 250 to 450 μg/dL. In iron deficiency, the liver synthesizes more transferrin to capture any available iron, raising TIBC above 400 μg/dL. Conversely, iron overload or chronic disease suppresses transferrin production, lowering TIBC below 250 μg/dL. Serum iron measures the amount of iron currently bound to transferrin, typically 60-170 μg/dL. Serum iron fluctuates widely throughout the day and after meals, making it less reliable than ferritin but still useful when combined with TIBC.

Transferrin saturation (TSAT) is the calculated ratio: (serum iron ÷ TIBC) × 100. It represents the percentage of transferrin binding sites occupied by iron. Normal TSAT is 20-50%. When TSAT drops below 20%, insufficient iron reaches the bone marrow for hemoglobin synthesis, even if ferritin is borderline. This pattern defines functional iron deficiency—stores may exist, but mobilization is impaired. TSAT above 45% suggests excess circulating iron, raising concern for hemochromatosis, particularly if ferritin exceeds 200-300 ng/mL. The combination of ferritin, TIBC, and TSAT provides a comprehensive picture of iron economy that no single test can match.

Interpreting these values requires clinical context. A ferritin of 25 ng/mL with TSAT of 15% strongly suggests iron deficiency in an otherwise healthy person. But in someone with rheumatoid arthritis or chronic kidney disease, ferritin can remain in the 30-100 ng/mL range despite true deficiency because inflammation artificially elevates ferritin. In such cases, TSAT becomes the more reliable indicator; values below 20% justify iron supplementation even when ferritin appears adequate. The calculator synthesizes these parameters to generate clinically actionable interpretations.

Common Causes and Clinical Scenarios of Iron Deficiency

Iron deficiency rarely appears out of nowhere; it signals ongoing blood loss, inadequate intake, malabsorption, or increased demands. In menstruating women, monthly losses average 15-30 mg of iron. Women with heavy menstrual bleeding (menorrhagia) can lose 60-80 mg per cycle, quickly outpacing dietary absorption of 1-2 mg per day. Even borderline heavy periods over years steadily deplete stores, manifesting as fatigue, brittle nails, and restless legs syndrome long before anemia develops. Starting oral contraceptives or a levonorgestrel IUD often reduces bleeding and allows iron stores to recover without supplementation.

Gastrointestinal blood loss is the leading cause in men and postmenopausal women. Chronic NSAID use erodes gastric mucosa, causing occult bleeding that goes unnoticed until anemia appears. Peptic ulcers, gastritis, esophagitis, and colon polyps all leak small amounts of blood daily. Colorectal cancer is the concerning diagnosis that iron deficiency anemia must prompt investigation for, particularly in patients over 50. Gastroscopy and colonoscopy are standard workup for unexplained iron deficiency in this demographic, with cancer detection rates around 6-10% in studies. Finding iron deficiency without obvious cause demands ruling out malignancy.

Malabsorption affects iron uptake in the duodenum where acidic pH keeps iron soluble. Celiac disease damages duodenal villi, reducing absorption surface area; screening for celiac with tissue transglutaminase antibodies is now routine in unexplained iron deficiency. Atrophic gastritis and long-term proton pump inhibitor (PPI) use reduce stomach acid, impairing iron solubilization. Bariatric surgery, particularly Roux-en-Y gastric bypass, bypasses the duodenum entirely, creating lifelong malabsorption requiring high-dose supplementation. H. pylori infection, affecting 50% of the global population, causes chronic gastritis and impairs iron metabolism through poorly understood mechanisms; eradication often improves iron status.

Increased demands arise during pregnancy, when maternal blood volume expands by 50% and the fetus requires 300-400 mg of iron for development. Without supplementation, most pregnant women deplete stores by the third trimester. Toddlers and adolescents experience rapid growth spurts that outpace dietary intake. Athletes, particularly endurance runners, lose iron through foot-strike hemolysis, gastrointestinal microbleeds from repetitive jarring, and sweat losses. Vegetarians and vegans rely on non-heme iron from plants, which absorbs poorly (2-5% bioavailability) compared to heme iron from meat (15-35% bioavailability). These populations need higher intake or supplementation to maintain adequate stores, with ferritin monitoring every 1-2 years.

Treatment Strategies and Monitoring Response to Iron Therapy

Oral iron remains first-line therapy for uncomplicated iron deficiency. Standard dosing is 150-200 mg elemental iron daily, divided into two or three doses for better tolerance. Ferrous sulfate 325 mg contains 65 mg elemental iron; ferrous gluconate and ferrous fumarate offer alternatives with similar efficacy. Taking iron on an empty stomach maximizes absorption, but many patients tolerate it better with food despite a 30-40% reduction in bioavailability. Vitamin C (250 mg) co-administered with each dose enhances absorption by maintaining iron in its ferrous (Fe²⁺) state. Avoid calcium, antacids, tea, and coffee within two hours of dosing, as they chelate iron and block uptake.

Gastrointestinal side effects—nausea, constipation, dark stools—affect 20-30% of patients and are dose-dependent. Strategies to improve tolerance include starting with a single daily dose for one week before escalating, using lower-dose formulations (18-28 mg elemental iron daily), or switching to polysaccharide iron complex, which some patients tolerate better. Newer formulations like ferrous bisglycinate chelate may cause fewer side effects while maintaining absorption. When oral iron fails due to intolerance, malabsorption, or ongoing blood loss exceeding replacement capacity, intravenous iron becomes necessary. Options include iron sucrose, ferric carboxymaltose, and low-molecular-weight iron dextran, all administered in infusion centers with monitoring for rare anaphylaxis.

Response to treatment is predictable and measurable. Reticulocyte count typically rises within 5-7 days as the bone marrow ramps up red cell production, though this is rarely checked in routine practice. Hemoglobin increases by approximately 1 g/dL every 2-3 weeks, with normalization expected in 6-8 weeks if ongoing losses have stopped. Ferritin, however, takes 3-6 months to replenish. Many patients stop therapy once symptoms improve and hemoglobin normalizes, but this leaves stores empty and recurrence likely. Continue iron supplementation until ferritin exceeds 50-100 ng/mL to establish a reserve buffer against future losses.

Monitoring involves repeat complete blood count and iron studies at 4-6 weeks, then every 3 months until ferritin normalizes. Failure to respond suggests ongoing bleeding not yet identified, malabsorption requiring intravenous iron, or a wrong diagnosis—perhaps the anemia is from B12 deficiency, chronic disease, or thalassemia trait rather than iron deficiency. Combined deficiencies are common; someone with celiac disease may have both iron and B12 malabsorption. If ferritin rises but hemoglobin doesn't, consider hemoglobinopathy screening with hemoglobin electrophoresis. The iron deficiency calculator can be run serially to track the trajectory of TSAT and ferritin, providing objective evidence of adherence and absorption.