What does a Complete Blood Count (CBC) Red Blood Cell (RBC) test tell you?

What a Complete Blood Count (CBC) Red Blood Cell (RBC) Test Tells You

A CBC with RBC parameters provides essential information about red blood cell quantity, size, hemoglobin content, and variation—allowing you to detect anemia, classify its type, and guide targeted diagnostic workup.

Core RBC Parameters and Their Meaning

Primary Measurements

• Hemoglobin concentration is the preferred measure of anemia (rather than hematocrit) because it has better reproducibility across laboratories and is not affected by sample storage time or patient glucose levels 1
• RBC count indicates the absolute number of red blood cells per volume of blood, which when combined with hemoglobin helps assess the severity of anemia 2
• Hematocrit represents the percentage of blood volume occupied by red cells but is less reliable than hemoglobin due to higher variability 1

Red Cell Indices: The Morphologic Approach

• Mean Corpuscular Volume (MCV) categorizes anemia into three critical types 1, 2:

  • Microcytic (MCV < 80 fL): Most commonly indicates iron deficiency; also consider thalassemia, anemia of chronic disease, or sideroblastic anemia
  • Normocytic (MCV 80-100 fL): Suggests hemorrhage, hemolysis, bone marrow failure, anemia of chronic inflammation, or renal insufficiency
  • Macrocytic (MCV > 100 fL): Indicates vitamin B12 or folate deficiency (megaloblastic), or may result from alcoholism, myelodysplastic syndromes, or certain drugs like hydroxyurea

• Mean Corpuscular Hemoglobin (MCH) is a more reliable marker of iron deficiency than MCHC because it is less dependent on storage conditions and counting equipment 2

• Mean Corpuscular Hemoglobin Concentration (MCHC) measures hemoglobin concentration within red cells; low values suggest hypochromia, often accompanying iron deficiency even when MCV remains normal 2, 3

• Red Cell Distribution Width (RDW) quantifies variation in red cell size 2, 3:

  • Elevated RDW indicates increased size variation, seen in early iron deficiency, mixed nutritional deficiencies, or anemia of chronic inflammation
  • Normal RDW with abnormal MCV suggests a more uniform population of abnormal cells (e.g., thalassemia trait)

What the CBC Tells You About Anemia Mechanism

The Kinetic Approach: Production vs. Destruction

• Reticulocyte count (though not always part of standard CBC) is essential for determining the underlying mechanism 1, 2:
  • Low reticulocyte index (< 2.0): Indicates decreased RBC production, suggesting iron deficiency, vitamin B12/folate deficiency, aplastic anemia, or bone marrow dysfunction from cancer or chemotherapy
  • High reticulocyte index (> 2.0): Indicates normal or increased RBC production, suggesting blood loss or hemolysis despite anemia

Bone Marrow Function Assessment

• The complete blood count (including white blood cells and platelets alongside RBC parameters) assesses overall bone marrow function 1
• Abnormalities in two or more cell lines warrant careful evaluation and likely hematology consultation, as this suggests bone marrow pathology rather than isolated anemia 1

Critical Clinical Context

When RBC Parameters Are Misleading

• Anemia from blood loss may be proportional to bleeding duration and can result in iron deficiency over time 1
• Conditions shortening RBC survival (recovery from acute blood loss, hemolytic anemia) can affect certain measurements regardless of assay method 1
• Age-related changes: Population data show HbA1c (a glycated protein marker) increases approximately 0.1% per decade after age 30, and similar considerations apply to RBC parameters 1

Essential Follow-Up Based on CBC Results

For microcytic anemia (low MCV):

• Obtain iron studies: serum ferritin (< 30 ng/mL confirms iron deficiency without inflammation), transferrin saturation (< 15% supports iron deficiency), and total iron binding capacity 1, 2
• If iron studies are normal, consider hemoglobinopathies like thalassemia trait and obtain hemoglobin electrophoresis 2

For normocytic anemia (normal MCV):

• Check reticulocyte count to distinguish production defects from hemolysis/blood loss 1
• Assess for chronic kidney disease: glomerular filtration rate and erythropoietin level 1
• Consider inflammatory markers like C-reactive protein 2, 3

For macrocytic anemia (high MCV):

• Measure vitamin B12 and folate levels 1, 2
• Consider bone marrow biopsy if myelodysplastic syndrome is suspected 1

For elevated RDW with normal or near-normal MCV:

• This pattern suggests early iron deficiency or mixed deficiencies before MCV becomes abnormal 2, 3
• Obtain iron studies and reticulocyte count immediately 2, 3

Common Pitfalls to Avoid

• Do not rely on hematocrit alone for anemia diagnosis—hemoglobin is more accurate and reproducible 1
• Do not assume normocytic anemia is always "anemia of chronic disease"—check reticulocyte count to rule out hemolysis or occult bleeding 1
• Do not overlook elevated RDW—this often indicates mixed deficiencies or early iron deficiency even when MCV appears normal 2, 3
• Do not skip peripheral blood smear review by a qualified hematologist when CBC shows unexplained abnormalities—it can reveal schistocytes (thrombotic thrombocytopenic purpura), giant platelets (inherited thrombocytopenia), or other critical findings 1
• In patients with CKD, do not assume anemia is solely from erythropoietin deficiency—iron deficiency should prompt evaluation for gastrointestinal bleeding 1