complete blood count

**Components and typical adult reference ranges (vary by lab/sex):** - **Hemoglobin:** men ~13.5-17.5 g/dL; women ~12-16 g/dL. - **Hematocrit:** men ~41-53%; women ~36-46%. - **RBC count:** ~4.3-5.9 (men) and 3.5-5.5 (women) ×10^6/µL. - **WBC:** ~4,500-11,000/µL. - **Platelets:** ~150,000-400,000/µL. - **RBC indices:** MCV (~80-100 fL), MCH, MCHC, and RDW (~11.5-14.5%). - **Differential:** neutrophils 54-62%, lymphocytes 25-33%, monocytes 3-7%, eosinophils 1-3%, basophils 0-0.75%. Absolute counts are more clinically useful than percentages. - **Reticulocyte count** is not part of a standard CBC but is often added (normal ~0.5-1.5%).

**Interpreting RBC parameters:** - **Microcytic anemia (MCV <80):** iron deficiency (high RDW, low ferritin, high TIBC), thalassemia (normal or high RBC count, normal RDW, Mentzer index <13), anemia of chronic disease (can be normocytic or microcytic; high ferritin, low TIBC), and sideroblastic anemia, with lead poisoning as a related cause. - **Normocytic anemia (MCV 80-100):** acute blood loss, hemolysis, aplastic anemia, CKD, and anemia of chronic disease. Use the reticulocyte count to separate hypoproliferative from hemolytic or blood-loss causes. - **Macrocytic anemia (MCV >100):** - Megaloblastic causes are B12 or folate deficiency and drugs that impair DNA synthesis (methotrexate, hydroxyurea, zidovudine). Findings include hypersegmented neutrophils. - Non-megaloblastic causes are alcohol use, liver disease, hypothyroidism, and reticulocytosis. - **High RDW** indicates anisocytosis (e.g., early iron deficiency). - **High MCHC** suggests spherocytosis or cold agglutinins. - **Polycythemia:** high Hgb/Hct. Causes include polycythemia vera (low EPO, JAK2 mutation), secondary causes (high EPO from hypoxia, COPD, EPO-secreting tumors such as RCC), and relative causes (dehydration).

**Interpreting WBC parameters:** - **Neutrophilia:** bacterial infection, stress, steroids. A left shift (bands) suggests acute infection. - **Neutropenia:** ANC <1500 (severe <500 → risk of febrile neutropenia). Causes include chemotherapy, drugs (clozapine, carbamazepine), and autoimmune disease. - **Lymphocytosis:** viral infection (EBV smear shows atypical lymphocytes), pertussis, and CLL (smudge cells). - **Eosinophilia:** parasites, allergy/atopy, drug reaction (DRESS), adrenal insufficiency, Churg-Strauss (EGPA), and neoplasia (Hodgkin lymphoma). - **Leukemoid reaction vs CML:** a leukemoid reaction has a high LAP score and no BCR-ABL; CML has a low LAP score and BCR-ABL t(9;22).

**Interpreting platelets:** - **Thrombocytopenia:** ITP, TTP/HUS, DIC, HIT, hypersplenism, marrow failure, and pseudothrombocytopenia (EDTA clumping; repeat in citrate or check the smear). - **Thrombocytosis:** reactive (infection, inflammation, iron deficiency, post-splenectomy) versus essential thrombocythemia (JAK2, CALR, or MPL mutation).

**Pancytopenia:** aplastic anemia, marrow infiltration (leukemia, myelofibrosis), megaloblastic anemia, hypersplenism, and drugs or radiation.

**Clinical pearls:** - The CBC is automated (impedance or flow cytometry), and a peripheral smear is the follow-up when abnormalities are flagged. Examples are schistocytes (MAHA), target cells (thalassemia, liver disease), sickle cells, spherocytes, and Howell-Jolly bodies (asplenia). - Hgb/Hct can be falsely normal early in acute hemorrhage because of equilibrium lag. Hemodilution in pregnancy causes physiologic anemia. - Hgb transfusion thresholds are generally <7 g/dL in stable patients (<8 with cardiovascular disease). - Pregnancy, altitude, smoking, and neonatal age change normal values. Neonates have higher Hgb and MCV (HbF). - Mnemonic for WBC differential abundance: "Never Let Monkeys Eat Bananas" (neutrophils, lymphocytes, monocytes, eosinophils, basophils).