What does an elevated red blood cell (RBC) count with low mean corpuscular hemoglobin (MCH), low mean corpuscular hemoglobin concentration (MCHC), and high red cell distribution width (RDW) indicate?

Elevated Red Cell Count with Low MCH, Low MCHC, and High RDW: Diagnostic Implications

An elevated red blood cell count with low MCH, low MCHC, and high RDW most strongly suggests iron deficiency anemia, particularly in the early or developing stages. 1

Laboratory Pattern Interpretation

• The combination of elevated RBC count with low MCH (mean corpuscular hemoglobin) and low MCHC (mean corpuscular hemoglobin concentration) indicates microcytic, hypochromic red blood cells, which is characteristic of iron deficiency 1
• High RDW (red cell distribution width) reflects increased variation in red cell size (anisocytosis), which is an early indicator of iron deficiency, often appearing before changes in MCV become apparent 1, 2
• This pattern distinguishes iron deficiency from other causes of microcytosis like thalassemia, which typically has a normal or only slightly elevated RDW 1, 3

Diagnostic Significance

• Iron deficiency is the most likely explanation when RDW is elevated with microcytic indices, as RDW increases early in iron deficiency when a population of smaller cells begins to appear in circulation 4
• In early or latent stages of iron deficiency, RDW may increase while MCV remains normal, making RDW a more sensitive early indicator (sensitivity 82.3%, specificity 97.4%) 4
• The elevated RBC count with microcytosis creates a distinctive pattern that requires further investigation to distinguish between:
  • Iron deficiency with reactive erythrocytosis 5
  • Thalassemia trait (though this typically has a normal RDW) 3, 5
  • Combined disorders (iron deficiency with another condition) 1

Recommended Diagnostic Workup

• Serum ferritin is the most powerful test for confirming iron deficiency (levels <30 μg/L indicate deficiency in the absence of inflammation) 1
• In the presence of inflammation, ferritin up to 100 μg/L may still be consistent with iron deficiency 1
• Additional recommended tests include:
  • Reticulocyte count (to assess bone marrow response) 1
  • Transferrin saturation (values <30% support iron deficiency) 1
  • CRP (to identify concurrent inflammation that may affect ferritin interpretation) 1

Clinical Considerations

• This pattern may represent early or developing iron deficiency before anemia is fully established 4
• In some cases, microcytosis and macrocytosis can coexist (e.g., combined iron and B12/folate deficiency), resulting in a normal MCV but elevated RDW 1
• Secondary causes of elevated RBC count with iron deficiency include hypoxic conditions, certain malignancies, and smoking 5

Potential Pitfalls

• Relying solely on MCV for diagnosis may miss early iron deficiency, as RDW increases before significant changes in MCV 4
• Approximately half of thalassemia cases may also present with an elevated RDW, so additional testing (hemoglobin electrophoresis) may be needed for definitive diagnosis 3
• Serum ferritin can be falsely elevated in inflammatory conditions, chronic disease, malignancy, or liver disease, potentially masking iron deficiency 1

The combination of these findings warrants a thorough investigation for the underlying cause of iron deficiency, including evaluation for occult gastrointestinal blood loss, particularly in adult men and postmenopausal women 1.