Diagnosis: Iron Deficiency Anemia
The combination of low MCH, low MCHC, and high RDW is highly suggestive of iron deficiency anemia (IDA), and you should immediately check serum ferritin to confirm the diagnosis and then investigate for gastrointestinal blood loss as the underlying cause. 1, 2
Understanding the Laboratory Pattern
High RDW with low MCH/MCHC indicates iron deficiency rather than thalassemia. The elevated RDW (>14%) reflects anisocytosis—variation in red cell size—which is characteristic of iron deficiency as the body produces progressively smaller red cells over time 1, 2
Low MCHC specifically points toward iron deficiency, as this parameter is typically reduced in IDA but not in other microcytic anemias 3
This pattern distinguishes IDA from thalassemia trait, where RDW is typically normal or only mildly elevated despite microcytosis 1, 4
Diagnostic Workup
Confirm iron deficiency with serum ferritin:
- Ferritin <30 μg/L confirms iron deficiency in the absence of inflammation 1, 2
- In the presence of inflammation (elevated CRP), ferritin up to 100 μg/L may still indicate iron deficiency 1
- Ferritin <12 μg/L is diagnostic of iron deficiency regardless of inflammatory status 1
Additional useful tests:
- Transferrin saturation <30% supports the diagnosis 1
- Check reticulocyte count to assess bone marrow response 1
- Measure CRP to determine if inflammation is present 1
Investigation for Underlying Cause
For adult men and post-menopausal women, gastrointestinal evaluation is mandatory:
- Upper GI endoscopy with small bowel biopsies (to exclude coeliac disease, present in 2-3% of IDA cases) 1
- Lower GI endoscopy (colonoscopy) to exclude colorectal cancer, even if upper endoscopy reveals a lesion, as dual pathology occurs in ~10% of cases 1
- Common GI causes include NSAID use, colorectal cancer/polyps, gastric cancer, angiodysplasia, and coeliac disease 1
Document medication history:
- NSAIDs and aspirin should be stopped whenever possible 1
- Anticoagulant use should not deter investigation 1
Dietary assessment:
- Poor dietary intake alone should not be presumed as the sole cause; full GI investigation is still required 1
Management
Iron replacement therapy:
- Initiate oral or intravenous iron supplementation once diagnosis is confirmed 2
- Response to three weeks of oral iron can confirm true iron deficiency if diagnostic uncertainty remains 1
Critical Pitfalls to Avoid
- Do not assume thalassemia based on ethnicity alone—high RDW strongly argues against thalassemia trait and favors iron deficiency 1, 2
- Do not skip GI investigation in adult men and post-menopausal women—occult malignancy must be excluded 1, 2
- Do not overlook combined deficiencies—iron deficiency can coexist with folate or B12 deficiency, which may normalize MCV but maintain elevated RDW 1, 2
- Do not attribute IDA to dietary causes without investigation—GI blood loss is the most common cause in adult men and post-menopausal women 1