How to Diagnose Iron Deficiency Anemia
Iron deficiency anemia is diagnosed by demonstrating both anemia (hemoglobin below normal for age and sex) and iron deficiency through laboratory testing, with the minimum essential workup including complete blood count with red cell indices, serum ferritin, transferrin saturation, and inflammatory markers (CRP). 1
Step 1: Confirm Anemia
Begin by measuring hemoglobin to establish if anemia is present using WHO criteria 1:
- Men: Hemoglobin <13.0 g/dL
- Non-pregnant women: Hemoglobin <12.0 g/dL
- Pregnant women: Hemoglobin <11.0 g/dL
- Children 5-11 years: Hemoglobin <11.5 g/dL
Hemoglobin is preferred over hematocrit because it has better reproducibility across laboratories and is not affected by storage time or patient variables like serum glucose 1.
Step 2: Obtain Minimum Laboratory Workup
The essential initial tests are 1:
- Complete blood count with red cell indices (MCV, MCH, RDW)
- Reticulocyte count
- Serum ferritin
- Transferrin saturation (TfS)
- C-reactive protein (CRP)
This combination allows differentiation between iron deficiency anemia, anemia of chronic disease, and mixed forms 1.
Step 3: Interpret Red Cell Indices
Mean corpuscular volume (MCV) provides the first clue 1:
- Microcytosis (low MCV): Suggests iron deficiency, but can also occur in thalassemia or anemia of chronic disease
- Normal MCV: Does not exclude iron deficiency, especially when combined with inflammation or vitamin deficiencies
- High RDW (red cell distribution width): Indicates iron deficiency even when MCV appears normal due to coexisting microcytosis and macrocytosis 1
Reticulocyte count determines bone marrow response 1:
- Low or normal reticulocytes: Indicates deficiency states (iron, B12, folate) or bone marrow disease
- Elevated reticulocytes: Excludes deficiency and suggests hemolysis or blood loss with adequate marrow response
Step 4: Assess Iron Status Based on Inflammatory State
Critical distinction: Ferritin interpretation depends entirely on whether inflammation is present 1.
Without Inflammation (Normal CRP, No Clinical/Endoscopic Evidence)
- Serum ferritin <30 μg/L: Diagnostic of iron deficiency 1
- Transferrin saturation <16%: Supports iron deficiency 1
With Inflammation (Elevated CRP or Active Disease)
Because ferritin is an acute-phase reactant, higher thresholds are required 1:
- Serum ferritin <100 μg/L: May still indicate iron deficiency 1
- Serum ferritin 30-100 μg/L with TfS <16%: Suggests combined iron deficiency and anemia of chronic disease 1
- Serum ferritin >100 μg/L with TfS <16%: Indicates anemia of chronic disease (functional iron deficiency) 1
- Serum ferritin >100 μg/L: Makes true iron deficiency unlikely 1
Note that in chronic kidney disease patients, ferritin <25 ng/mL (males) or <11 ng/mL (females) predicts insufficient iron stores 1.
Step 5: Extended Workup When Diagnosis Remains Unclear
If the cause of anemia is not evident after initial testing, obtain 1:
- Vitamin B12 and folic acid levels
- Haptoglobin, lactate dehydrogenase (for hemolysis)
- Differential white blood cell count
- Percentage of hypochromic red cells or reticulocyte hemoglobin content (if available)
- Soluble transferrin receptor (sTfR) - elevated in iron deficiency, normal/low in anemia of chronic disease 1
Consult hematology if the diagnosis remains unclear after extended workup 1.
Common Pitfalls to Avoid
Do not rely on hemoglobin and hematocrit alone - these decrease only with severe iron depletion and can miss early iron deficiency 1, 2. Many individuals with normal CBC parameters have reduced serum iron and ferritin 2.
Do not assume dietary insufficiency explains the anemia - in adult men and postmenopausal women, gastrointestinal blood loss (including occult malignancy) is the most common cause and requires investigation 1, 3, 4.
Do not misinterpret ferritin in inflammatory states - a "normal" ferritin of 50 μg/L may represent iron deficiency if inflammation is present 1.
Do not confuse iron deficiency with anemia - these terms are not synonymous. Iron deficiency can exist without anemia (iron deficiency without anemia), and anemia can exist without iron deficiency 1, 3.
Recognize pseudoanemia in athletes - expanded plasma volume can create laboratory appearance of anemia despite adequate iron stores 1.
Special Populations
Premenopausal women: Heavy menstrual bleeding is the most common cause; bidirectional endoscopy may not be required in women <40 years without alarm symptoms 3, 4.
Pregnant women: Up to 84% develop iron deficiency by third trimester; screening is recommended 1, 3.
Patients with chronic inflammatory conditions (IBD, CKD, heart failure, cancer): Use higher ferritin thresholds (<100 μg/L) for diagnosis and consider functional iron deficiency 1, 3.