Interpret the CBC results showing hemoglobin 12.7 g/dL (low), hematocrit 40.4 %, MCV 80 fL, MCH 25.2 pg (low), MCHC 31.4 g/dL (low), and RDW 17.7 % (high).

Iron Deficiency Anemia with Elevated RDW

This CBC demonstrates iron deficiency anemia (IDA) with microcytic, hypochromic red cells and elevated RDW, requiring immediate serum ferritin measurement and investigation for underlying blood loss, particularly gastrointestinal sources.

Key Findings Analysis

The pattern is classic for iron deficiency:

• Hemoglobin 12.7 g/dL: Mild anemia by WHO criteria (normal >13.0 g/dL for women) 1
• Low MCH (25.2 pg): Hypochromia—MCH is actually more reliable than MCV for detecting iron deficiency as it's less dependent on the analyzer and is reduced in both absolute and functional iron deficiency 1
• Borderline low-normal MCV (80 fL): Early microcytosis at the lower limit of normal
• Low MCHC (31.4 g/dL): Confirms hypochromia
• Elevated RDW (17.7%): This is the most significant finding—high RDW is the most sensitive marker for iron deficiency with 72% sensitivity 2, and indicates anisocytosis (variation in red cell size) typical of evolving iron deficiency

Diagnostic Approach

Immediate Next Steps

1. Measure serum ferritin - This is the single most useful marker of IDA 1

   • <15 μg/L = highly specific for iron deficiency (99% specificity)
   • <30 μg/L = generally indicates low iron stores
   • <45 μg/L = optimal cutoff balancing sensitivity/specificity (92% specificity)
   • Note: Ferritin is an acute phase reactant—if inflammation suspected and ferritin 15-150 μg/L, iron deficiency may still be present 1

2. Consider transferrin saturation if ferritin is equivocal (15-150 μg/L range with suspected inflammation) 1

Investigation for Blood Loss

Given any level of anemia with iron deficiency, investigation should be considered, though the case is stronger with more severe anemia 1. The British Society of Gastroenterology guidelines emphasize:

• Gastrointestinal evaluation is warranted even at this mild anemia level, particularly if ferritin confirms iron deficiency
• Men and non-menstruating women require more aggressive investigation
• For menstruating women, consider gynecologic causes but don't assume menstruation is the sole cause without confirming adequate dietary intake

Clinical Significance of RDW

The markedly elevated RDW (17.7% vs normal <15.4%) is particularly informative 3, 2:

• Indicates a mixed population of red cells (older normocytic cells and newer microcytic cells)
• Suggests ongoing iron deficiency affecting erythropoiesis
• Combined with low hemoglobin, this pattern has 93% sensitivity for IDA when using cutoffs of Hb ≤10.3 g/dL and RDW ≥16.4% 3

Common Pitfalls to Avoid

1. Don't assume thalassemia trait without testing: If ferritin is normal, obtain hemoglobin electrophoresis, especially with appropriate ethnic background 1

2. Don't miss functional iron deficiency: In chronic inflammatory conditions, ferritin may be falsely normal—values <150 μg/L don't exclude iron deficiency in inflammation 1

3. Don't delay investigation: Even mild anemia with iron deficiency can indicate serious GI pathology including colorectal cancer 1

4. Response to iron therapy is diagnostic: A hemoglobin rise ≥10 g/L within 2 weeks of iron supplementation is highly suggestive of absolute iron deficiency, even if iron studies are equivocal 1

Pattern Recognition

This CBC shows the early-to-moderate stage of iron deficiency where:

• Hemoglobin has dropped but not severely
• Red cell indices are becoming abnormal (low MCH, borderline MCV)
• RDW is elevated, reflecting the transition from normal to iron-deficient erythropoiesis
• The body is depleting iron stores and cannot maintain normal hemoglobin synthesis