Laboratory Results Expected in Iron Deficiency Anemia
Iron deficiency anemia is characterized by low hemoglobin, low mean corpuscular volume (MCV), low serum ferritin (<12-30 μg/L depending on inflammation status), low serum iron, low transferrin saturation (<16-20%), elevated total iron-binding capacity (TIBC), and elevated red cell distribution width (RDW >14%). 1, 2
Decreased Laboratory Parameters
Hemoglobin and Hematocrit
- Hemoglobin falls below normal ranges: <13 g/dL for men, <12 g/dL for non-pregnant women, and <11 g/dL for pregnant women in the 2nd/3rd trimester 1, 2
- Hematocrit decreases proportionally but is a later indicator than hemoglobin 1
- These are late indicators of iron deficiency, falling only after iron stores are depleted 1
Red Blood Cell Indices
- Mean Corpuscular Volume (MCV) is low, falling below the 5th percentile for age, indicating microcytic red blood cells 1, 2
- Mean Cell Hemoglobin (MCH) is reduced and may be more reliable than MCV as it is less dependent on storage conditions 2
- Mean Cell Hemoglobin Concentration (MCHC) decreases only in severe, late-stage iron deficiency 3
- Reticulocyte hemoglobin is low, reflecting insufficient iron for new red blood cell production 2
Iron Studies
- Serum ferritin is the most powerful single test for iron deficiency 1
- Serum iron concentration is reduced, though day-to-day variation limits reliability 2
- Transferrin saturation is low (<16-20%), calculated as (serum iron/TIBC) × 100 1, 2
Elevated Laboratory Parameters
Red Blood Cell Distribution Width (RDW)
- RDW >14.0% indicates increased variation in red blood cell size (anisocytosis) 2
- The combination of low MCV + RDW >14.0% strongly suggests iron deficiency anemia rather than thalassemia 2
- RDW elevation is an early hematological abnormality, occurring before MCV decreases 3
Iron-Binding Capacity
- Total Iron-Binding Capacity (TIBC) is raised, reflecting increased transferrin production as the body attempts to capture more iron 1, 2
- This contrasts with anemia of chronic disease, where TIBC is decreased 4
Other Elevated Markers
- Erythrocyte protoporphyrin is elevated: >30 μg/dL whole blood or >70 μg/dL red blood cells in adults 2
- Serum transferrin receptor (sTfR) concentration is increased, indicating iron-deficient erythropoiesis 2
- Percentage of hypochromic red cells is raised (hemoglobin concentration <280 g/L per cell), reflecting inadequate hemoglobinization 2, 5
- Red cell zinc protoporphyrin is elevated 2
Key Diagnostic Patterns
Distinguishing Iron Deficiency from Other Microcytic Anemias
- In iron deficiency, the percentage of hypochromic cells exceeds the percentage of microcytes, whereas in beta-thalassemia trait the reverse is true 5
- A microcytic-hypochromic ratio <0.9 has 92.4% discriminant efficiency for iron deficiency 5
- Transferrin saturation <16% with elevated TIBC confirms iron deficiency, while low iron with low TIBC suggests anemia of chronic disease 4
Stages of Iron Deficiency
Iron deficiency progresses through three stages with distinct laboratory findings 3:
- Early stage: Anisocytosis (elevated RDW) and increased percentage of microcytic cells, with normal hemoglobin and transferrin saturation <32% 3
- Intermediate stage: MCV and MCH decline, hemoglobin generally subnormal but >9 g/dL, transferrin saturation usually <16% 3
- Advanced stage: Low MCHC, hemoglobin <9 g/dL, transferrin saturation <16% 3
Critical Diagnostic Considerations
- Serum ferritin may be falsely elevated (above 12-15 μg/L) in patients with concurrent chronic inflammation, malignancy, or hepatic disease, though if >100 μg/L, iron deficiency is almost certainly not present 1
- Less than 50% of anemic individuals actually have iron deficiency as the cause, making additional iron studies essential beyond hemoglobin alone 1, 2
- Microcytosis may be absent in combined deficiency (e.g., with folate deficiency), which may be recognized by elevated RDW 1
- Combining ferritin with transferrin saturation improves diagnostic accuracy over either test alone 2