Iron Deficiency Anaemia: The Lab Tests That Diagnose It

Iron deficiency is the most common nutritional deficiency globally, affecting over 1 billion people. It progresses through stages — from depleted stores to iron-deficient erythropoiesis to overt iron deficiency anaemia — and different lab tests detect different stages.

Ferritin: The First to Fall

Ferritin is the main iron storage protein and the earliest marker of iron depletion, falling before haemoglobin drops. Normal adult ranges: males 20–250 ng/mL, females 10–120 ng/mL. A ferritin below 12 ng/mL virtually confirms iron deficiency in the absence of inflammation.

Serum Iron and TIBC

Serum iron measures iron circulating bound to transferrin. Total Iron Binding Capacity (TIBC) measures transferrin's capacity to carry iron — it rises when iron stores are depleted (more transferrin is produced to capture scarce iron). In iron deficiency: serum iron is LOW, TIBC is HIGH, transferrin saturation is LOW (< 16%).

• Serum iron normal: males 65–175 µg/dL, females 50–170 µg/dL
• TIBC normal: 250–370 µg/dL
• Transferrin saturation = (Serum Iron ÷ TIBC) × 100
• Saturation < 16% confirms iron-deficient erythropoiesis

CBC Changes in Iron Deficiency

As iron deficiency progresses, red blood cell production is affected. The CBC shows: low haemoglobin and haematocrit, low MCV (microcytic cells, < 80 fL), low MCH (hypochromic cells), elevated RDW (variable cell sizes, > 14.5%) — one of the earliest CBC changes.

Staging Iron Deficiency

Understanding the stage helps guide treatment:

• Stage 1 — Depletion: Low ferritin only; Hgb normal; no symptoms
• Stage 2 — Iron-deficient erythropoiesis: Low ferritin + high TIBC + low saturation; Hgb still normal
• Stage 3 — Iron deficiency anaemia: Low Hgb + microcytic hypochromic cells; symptomatic

Treatment Response

With oral iron supplementation, reticulocytes rise within 5–10 days, haemoglobin begins to rise within 2–4 weeks, and ferritin normalises after 3–6 months of continued therapy. Always treat the underlying cause — blood loss, poor absorption, or inadequate intake.