Is RDW Routinely Checked in a CBC?
Yes, RDW (red cell distribution width) is automatically calculated and reported as a standard component of every complete blood count (CBC) performed on modern automated hematology analyzers. 1, 2
Routine Inclusion in CBC Testing
- RDW is calculated automatically by dividing the standard deviation of red blood cell volume by the MCV and multiplying by 100 to express the result as a percentage 1, 2
- No special request is needed - RDW appears on standard CBC reports alongside other red cell indices including hemoglobin, hematocrit, MCV, MCH, and MCHC 3
- The calculation is instantaneous and requires no additional blood sample, cost, or processing time beyond the standard CBC 2
Clinical Relevance in Your Patient's Context
For a 54-year-old man with microcytic, hypochromic anemia, the RDW value on his CBC is particularly valuable:
Diagnostic Utility
- Elevated RDW (>14.0%) combined with low MCV strongly suggests iron deficiency anemia rather than thalassemia trait 1, 2
- Normal or minimally elevated RDW (≤14.0%) with low MCV points toward thalassemia minor as the more likely diagnosis 2, 4
- This distinction is critical because iron deficiency anemia in an adult man requires investigation for gastrointestinal blood loss, including potential malignancy 1, 4
Expected Findings
- Iron deficiency typically shows RDW >14.0% (often 20% or higher) due to heterogeneous red cell populations with varying degrees of iron-depleted cells 1, 5
- Thalassemia trait typically shows RDW ≤16% because the microcytic cells are relatively uniform in size 2, 6
Important Caveats
- RDW alone cannot definitively distinguish causes - it must be interpreted alongside MCV, ferritin, transferrin saturation, and hemoglobin electrophoresis 1, 6, 7
- Overlap exists between conditions - approximately 48-72% of thalassemia minor patients may have elevated RDW, particularly if anemic 6, 7
- Combined deficiencies complicate interpretation - concurrent iron deficiency and thalassemia trait will show elevated RDW similar to isolated iron deficiency 4, 5
Next Steps for This Patient
The minimum workup should include: 1
- Complete blood count with MCV and RDW (already obtained)
- Reticulocyte count
- Serum ferritin (<30 μg/L suggests iron deficiency; <100 μg/L with inflammation)
- Transferrin saturation (<20% supports iron deficiency)
- CRP to assess for inflammation
If iron deficiency is confirmed in this adult man, both upper and lower gastrointestinal endoscopy must be performed to exclude malignancy, regardless of symptom presence 1, 4