What does a complete blood count (CBC) measure, what are the normal adult reference ranges, and how are abnormal results interpreted?

Complete Blood Count (CBC): Components, Reference Ranges, and Interpretation

What a CBC Measures

A complete blood count evaluates the total numbers and characteristics of all cellular components in the blood, including red blood cells, white blood cells, and platelets, along with their respective indices and differential counts. 1, 2

The CBC comprises:

• Red blood cell (RBC) parameters: RBC count, hemoglobin (Hb), hematocrit (Hct), mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH), mean corpuscular hemoglobin concentration (MCHC), and red cell distribution width (RDW) 2, 3
• White blood cell (WBC) parameters: Total WBC count and differential (percentages and absolute counts of neutrophils, lymphocytes, monocytes, eosinophils, and basophils) 2, 3
• Platelet parameters: Platelet count and mean platelet volume (MPV) 2, 3
• Reticulocyte count: Often included to assess bone marrow response to anemia 2

Normal Adult Reference Ranges

Red Blood Cell Parameters

• Hemoglobin: Men 13–17 g/dL; Women 12–16 g/dL 4, 5
• Hematocrit: Men 40–52%; Women 36–48% 4
• MCV: 80–100 fL (defines normocytic; <80 fL microcytic; >100 fL macrocytic) 6, 5
• MCHC: 32–36 g/dL (low values indicate hypochromia) 6
• RDW: <14.0% (elevated suggests mixed cell populations or iron deficiency) 6, 5

White Blood Cell Parameters

• Total WBC count: 4,000–11,000 cells/mm³ 4
• Neutrophils: 40–70% (absolute count 1,500–7,000 cells/mm³) 4
• Lymphocytes: 20–40% 4
• Band forms: <6% or <1,500 cells/mm³ (elevated indicates "left shift" suggesting bacterial infection) 4

Platelet Parameters

• Platelet count: 150,000–400,000 cells/mm³ 4

Interpreting Abnormal Results

Anemia (Low Hemoglobin/Hematocrit)

The first step in evaluating anemia is determining the MCV to classify it as microcytic, normocytic, or macrocytic, followed by reticulocyte count to distinguish decreased production from increased destruction or loss. 5, 1

Microcytic Anemia (MCV <80 fL)

• Iron deficiency anemia is the most common cause, characterized by low MCHC, low ferritin (<30 µg/L without inflammation; <100 µg/L with inflammation), and low transferrin saturation (<16%) 6, 5
• Thalassemia minor produces microcytosis with normal or low RDW (<14.0%), distinguishing it from iron deficiency which typically elevates RDW 6
• Obtain iron studies (ferritin, transferrin saturation, serum iron, TIBC) and inflammatory markers (CRP) for all microcytic anemias 6, 5

Normocytic Anemia (MCV 80–100 fL)

• Low reticulocyte count (<1.0–2.0%) indicates decreased RBC production from anemia of chronic inflammation, early chronic kidney disease (GFR <20–30 mL/min), early nutritional deficiencies, medication-induced bone marrow suppression, or bone marrow failure 5
• High reticulocyte count (>2.0%) indicates acute hemorrhage or hemolytic anemia requiring investigation with indirect/direct bilirubin, haptoglobin, LDH, and direct antiglobulin test 5
• Obtain comprehensive workup including iron studies, renal function (creatinine, GFR), inflammatory markers (CRP, ESR), vitamin B12, and folate levels 5
• Anemia of chronic disease shows ferritin >100 µg/L with transferrin saturation <20%, reflecting iron sequestration by hepcidin 5
• Functional iron deficiency in chronic kidney disease affects 25–37.5% of patients despite normocytic appearance, because EPO deficiency prevents microcytic phenotype development 5

Macrocytic Anemia (MCV >100 fL)

• Most commonly caused by vitamin B12 or folate deficiency, alcoholism, certain medications (methotrexate, hydroxyurea), or myelodysplastic syndrome 5
• Measure vitamin B12, folate, and thyroid function; consider bone marrow biopsy if unexplained 5, 1

Leukocytosis (Elevated WBC Count)

• WBC count ≥14,000 cells/mm³ or left shift (bands ≥6% or ≥1,500 cells/mm³) warrants careful assessment for bacterial infection in any patient with suspected infection, with or without fever 4
• Neutrophil-predominant leukocytosis suggests bacterial infection, while lymphocyte-predominant suggests viral infection or lymphoproliferative disorder 1, 7

Leukopenia (Low WBC Count)

• Severe lymphopenia in infants suggests severe combined immunodeficiency (SCID), requiring urgent evaluation with lymphocyte subset enumeration, T-cell proliferation assays, and immunoglobulin levels 4
• In adults, obtain differential to determine which cell line is affected; neutropenia (<1,500 cells/mm³) increases infection risk 1

Thrombocytopenia (Low Platelet Count)

• Platelet count <150,000 cells/mm³ requires evaluation for pseudothrombocytopenia (EDTA-induced clumping), medication effect, immune thrombocytopenia, or bone marrow failure 1
• When accompanied by anemia, elevated LDH, decreased haptoglobin, or schistocytes on smear, consider microangiopathic hemolytic anemia or disseminated intravascular coagulation requiring urgent hematology consultation 8, 5

Thrombocytosis (Elevated Platelet Count)

• Reactive thrombocytosis (platelets 400,000–700,000 cells/mm³) occurs with inflammation, iron deficiency, or malignancy 1, 7
• Platelets >700,000 cells/mm³ raise concern for myeloproliferative neoplasm requiring JAK2 mutation testing and hematology referral 1

When to Obtain Urgent Hematology Consultation

Immediate subspecialty referral is mandatory when CBC abnormalities are accompanied by:

• Unexplained pancytopenia or abnormalities in two or more cell lines 4, 5
• Hemoglobin <10 g/dL with elevated reticulocyte count, elevated LDH, decreased haptoglobin, acute kidney injury, or neurological symptoms 8
• Severe lymphopenia in infants with recurrent infections or failure to thrive (concern for SCID) 4
• Dysplastic features, blasts, or unexplained abnormalities on peripheral blood smear 5
• Progressive anemia despite treatment of identified underlying causes 5
• Transfusion-dependent anemia without obvious etiology 8

Common Clinical Pitfalls

• Do not assume anemia of chronic disease without measuring iron studies, as 25–37.5% of patients with chronic kidney disease have concurrent iron deficiency despite normocytic appearance 5
• Ferritin alone cannot rule out iron deficiency in inflammatory conditions because it behaves as an acute-phase reactant; transferrin saturation must be added to the assessment 6, 5
• Elevated RDW in normocytic anemia is a critical clue suggesting underlying iron deficiency or mixed deficiency state that would otherwise be missed 6, 5
• Routine CBC in preoperative settings yields abnormal findings in 2.9–17.6% of cases but leads to clinical management changes in only 2.4% of abnormal results, supporting selective rather than routine ordering 4
• Combined deficiency states (iron plus B12/folate) may produce normal MCV, masking both deficiencies unless comprehensive testing is performed 5