What investigation to do when a peripheral smear shows microcytic hypochromic anemia?

Diagnostic Approach for Microcytic Hypochromic Anemia

When peripheral smear shows microcytic hypochromic anemia, a systematic diagnostic workup should begin with iron studies including serum ferritin, transferrin saturation (TSAT), and complete blood count parameters to differentiate between the most common causes. 1

Initial Laboratory Evaluation

• Complete Blood Count (CBC) - Evaluate MCV, RDW, and RBC count
• Iron Studies:
  • Serum ferritin
  • Serum iron
  • Total iron binding capacity (TIBC)
  • Transferrin saturation (TSAT)
• Free erythrocyte protoporphyrin - Helps distinguish between different causes 2

Differential Diagnosis Parameters

Parameter	Iron Deficiency	Thalassemia Trait	Anemia of Chronic Disease
MCV	Low	Very low (<70 fl)	Low/Normal
RDW	High (>14%)	Normal (≤14%)	Normal/Slightly elevated
Ferritin	Low (<30 μg/L)	Normal	Normal/High
TSAT	Low	Normal	Low
RBC count	Normal/Low	Normal/High	Normal/Low

Diagnostic Algorithm

1. First-line test: Serum ferritin

   • If low (<30 μg/L): Consistent with iron deficiency anemia
   • If normal or elevated: Proceed to next tests 2

2. Second-line tests (if ferritin normal or elevated):

   • Serum iron level and TIBC
   • Free erythrocyte protoporphyrin
   • Hemoglobin electrophoresis (to rule out hemoglobinopathies)

3. Interpretation of second-line tests:

   • Low serum iron + elevated protoporphyrin + normal/high ferritin: Anemia of chronic disease
   • Normal/high serum iron + normal protoporphyrin: Consider thalassemia or hemoglobinopathies
   • Low serum iron + elevated protoporphyrin + elevated lead levels: Consider lead toxicity 2

4. Additional tests based on suspected diagnosis:

   • For suspected genetic disorders of iron metabolism or heme synthesis:
     • ALAS2 gene testing (for X-linked sideroblastic anemia)
     • GLRX5 testing (for sideroblastic anemia with iron loading)
     • Bone marrow examination for ring sideroblasts 3

Special Considerations

• Inflammatory conditions: In patients with chronic inflammation, ferritin may be falsely normal or elevated despite iron deficiency. Consider measuring soluble transferrin receptor (sTfR) or sTfR/ferritin ratio in these cases 4

• Sideroblastic anemia: If iron parameters suggest iron loading with microcytic anemia, consider bone marrow examination to look for ring sideroblasts 3

• Rare genetic disorders: In cases where common causes are excluded, consider:

  • Iron-refractory iron deficiency anemia (IRIDA)
  • Defects in mitochondrial iron utilization
  • Defects in iron recycling 5

Common Pitfalls to Avoid

• Accepting a positive dietary history as the sole cause of iron deficiency anemia without further investigation

• Misinterpreting ferritin levels in the presence of inflammation

• Inadequate duration of iron therapy before reassessment

• Deferring iron replacement while awaiting investigations 1

• Failure to consider genetic causes of microcytic anemia when standard treatments fail 6

Remember that the definitive test for iron deficiency remains the absence of iron stores in bone marrow examination, but this is usually reserved for cases where non-invasive testing is inconclusive 7.