Interpretation of Complete Blood Count Results

Red Blood Cell Parameters

The American College of Physicians recommends evaluating hemoglobin and hematocrit first to assess oxygen-carrying capacity, with low values indicating anemia and high values suggesting polycythemia or dehydration 1, 2
For cancer patients, the National Comprehensive Cancer Network recommends an Hb level of 11 g/dL or below should prompt evaluation for anemia, or a decrease of 2 g/dL or more from baseline regardless of absolute value 3
The Alzheimer's Association suggests laboratory testing should include CBC alongside thyroid-stimulating hormone, B12, calcium, electrolytes, creatinine, alanine transaminase, lipid panel, and hemoglobin A1c when investigating potential contributing factors 4
The National Comprehensive Cancer Network recommends using MCV to classify anemia type as microcytic, normocytic, or macrocytic 3, 1, 2
Microcytic anemia most commonly results from iron deficiency, but also consider thalassemia, anemia of chronic disease, and sideroblastic anemia 3
Normocytic anemia may be caused by hemorrhage, hemolysis, bone marrow failure, anemia of chronic inflammation, or renal insufficiency—the reticulocyte count becomes the key follow-up test 3
Macrocytic anemia is most often megaloblastic, indicating vitamin B12 or folate deficiency from insufficient uptake or inadequate absorption 3
A low reticulocyte index indicates decreased red blood cell production, while a high index suggests blood loss or hemolysis 2

The American College of Physicians recommends assessing total white blood cell count for leukocytosis or leukopenia as the initial step 2
Neutrophilia with left shift strongly indicates bacterial infection, with a likelihood ratio of 14.5 when band count exceeds 1,500 cells/mm³ 2
Lymphocytosis may indicate viral infection, while eosinophilia suggests allergic or parasitic conditions 2
When reporting immunophenotyping results, correct data for lymphocyte purity of the gate 5

The American College of Physicians recommends assessing platelet count for thrombocytopenia or thrombocytosis, with low counts increasing bleeding risk and high counts potentially indicating inflammation or myeloproliferative disorders 1, 2
Consider potential causes of thrombocytopenia including bone marrow suppression, immune destruction, or consumption 2

The Centers for Disease Control and Prevention recommends reporting both percentages and absolute counts when available, particularly for lymphocyte subsets 5, 2
Calculate absolute values by multiplying the lymphocyte subset percentage by the absolute number of lymphocytes from the WBC differential 5
Include both percentage and absolute number when reporting CD4+ T-cell counts 2
Ensure blood is drawn at the same time for both CBC and immunophenotyping when absolute counts are needed 5

The National Comprehensive Cancer Network recommends always performing a detailed history and physical examination focusing on duration and onset of symptoms, comorbidities, family history, and exposure to antineoplastic drugs or radiation 3
Look for specific signs: syncope, exercise dyspnea, headache, vertigo, chest pain, fatigue, pallor, jaundice, splenic enlargement, neurologic symptoms, blood in stool, petechiae, or heart murmur 3
Visual review of the peripheral blood smear is critical to confirm size, shape, and color of RBCs beyond automated parameters 3