What is the step‑by‑step approach to diagnosing anemia?

Diagnosing Anemia: A Step-by-Step Approach

Begin by confirming anemia with hemoglobin thresholds: <13.0 g/dL in men and <12.0 g/dL in non-pregnant women, then immediately order a complete blood count with MCV, reticulocyte count, iron studies (ferritin and transferrin saturation), and CRP to classify the anemia and guide targeted investigation. 1, 2

Step 1: Confirm Anemia with Hemoglobin Measurement

• Use WHO-defined thresholds: Hemoglobin <13.0 g/dL (130 g/L) in adult men and <12.0 g/dL (120 g/L) in non-pregnant adult women 1, 2
• For pregnant women (2nd and 3rd trimester), use <11.0 g/dL (110 g/L) 1, 2
• For children, age-specific cutoffs apply: 11.0 g/dL for ages 0.5-5 years, 11.5 g/dL for ages 5-11 years, and 12.0 g/dL for ages 12-13 years 1

Critical caveat: Standard definitions may not apply to elderly patients (≥70 years in men), pregnant/menstruating women, high-altitude residents, smokers, non-Caucasian populations, or those with chronic lung disease or hemoglobinopathy 1, 2

Step 2: Order Minimum Initial Workup

The minimum laboratory panel must include 1, 3:

• Complete blood count with red cell indices (MCV, MCH, RDW)
• Reticulocyte count (absolute or index)
• Serum ferritin
• Transferrin saturation (TfS)
• C-reactive protein (CRP)
• Differential blood cell count

This panel allows classification by MCV and assessment of bone marrow response, iron stores, and inflammatory state 1, 3

Step 3: Classify by Mean Corpuscular Volume (MCV)

Microcytic Anemia (MCV <80 fL)

First, evaluate iron studies 1:

• Without inflammation (normal CRP): Ferritin <30 μg/L indicates iron deficiency 1
• With inflammation (elevated CRP): Ferritin up to 100 μg/L may still represent iron deficiency 1
• Low transferrin saturation (<20%) supports iron deficiency 1
• High RDW is an additional indicator of iron deficiency 1

If iron studies are normal, consider:

• Thalassemia (requires hemoglobin electrophoresis) 1
• Anemia of chronic disease (may present with microcytosis) 1

Normocytic Anemia (MCV 80-100 fL)

Evaluate reticulocyte count to determine bone marrow response 1, 3:

Low or normal reticulocytes (inadequate response):

• Check ferritin and transferrin saturation for functional iron deficiency 1
• Assess kidney function (creatinine, urea) for chronic kidney disease 1
• Consider anemia of chronic disease if CRP elevated with normal/high ferritin 1, 4
• Evaluate for primary bone marrow disease if other cell lines affected 1

Elevated reticulocytes (appropriate response):

• Indicates hemolysis or acute blood loss 1
• Order haptoglobin, lactate dehydrogenase, and bilirubin 1
• Perform direct Coombs test and peripheral blood smear 3

Macrocytic Anemia (MCV >100 fL)

Immediately check vitamin B12 and folate levels 1, 3:

• Low B12 or folate confirms nutritional deficiency 1
• Never start folic acid before excluding B12 deficiency to prevent irreversible neurological complications 3

If B12 and folate are normal, consider:

• Medication effects (thiopurines, antiretrovirals, hydroxyurea) 1
• Alcohol abuse 1
• Hypothyroidism 1
• Myelodysplastic syndrome (especially if other cytopenias present) 1
• Reticulocytosis (check reticulocyte count) 1

Step 4: Extended Workup When Diagnosis Remains Unclear

Order additional tests based on clinical context 1, 3:

• Soluble transferrin receptor (distinguishes iron deficiency from anemia of chronic disease) 1
• Percentage of hypochromic red cells or reticulocyte hemoglobin (functional iron deficiency) 1
• Haptoglobin, LDH, indirect bilirubin (hemolysis) 1
• Creatinine and urea (renal disease) 1
• Bone marrow examination if cytopenias in multiple cell lines or suspected primary marrow disorder 1

Consult hematology if the cause remains unclear after extended workup 1

Critical Pitfalls to Avoid

• Ferritin is an acute phase reactant: It may be falsely elevated in inflammatory states despite true iron deficiency, requiring ferritin up to 100 μg/L as the cutoff when inflammation is present 1, 3, 4
• Coexisting microcytosis and macrocytosis can neutralize each other, resulting in normal MCV; high RDW helps identify this situation 1
• Low reticulocyte count does not always mean deficiency: In CKD patients with adequate iron, B12, and folate, it likely indicates insufficient erythropoietin production or inflammation 1
• Any level of anemia warrants investigation when iron deficiency is present, not just severe anemia 2
• Hemoglobin is preferred over hematocrit for diagnosis due to better reproducibility and lack of interference from storage time or glucose levels 1