Blood & Lymphoreticular/Immune Systems
Anemia framework
Classify by MCV. Microcytic: iron deficiency (low ferritin, high TIBC, high RDW), thalassemia (normal or high ferritin, target cells, family history), anemia of chronic disease (low TIBC, high ferritin), lead poisoning (basophilic stippling). Macrocytic: B12 versus folate deficiency, distinguishing neurologic findings and methylmalonic acid. Hemolysis labs: elevated LDH and indirect bilirubin, low haptoglobin, reticulocytosis; intravascular hemolysis adds hemoglobinuria and marked haptoglobin depletion.
Coagulation and platelets
PT measures factor VII plus the common pathway; PTT measures XII, XI, IX, VIII. A prolonged PTT that corrects with normal plasma indicates factor deficiency; failure to correct indicates an inhibitor such as lupus anticoagulant. ITP is isolated thrombocytopenia with mucocutaneous bleeding; TTP adds schistocytes, neurologic and renal findings with ADAMTS13 deficiency; HUS follows bloody diarrhea with predominant renal injury; DIC prolongs PT and PTT with low fibrinogen and elevated D-dimer.
Leukemia, lymphoma, and immunology
MPO and Auer rods indicate AML; TdT indicates lymphoblasts; Reed-Sternberg cells are CD15/CD30 positive. MHC class I presents to CD8 cells on all nucleated cells; MHC class II presents to CD4 cells on antigen-presenting cells. Hypersensitivity types I through IV, Bruton agammaglobulinemia (no B cells), DiGeorge syndrome (22q11 deletion, hypocalcemia), SCID, and chronic granulomatous disease (defective NADPH oxidase) are core content.
Traps
Reading hemolysis labs without a reticulocyte count. Assuming a normal PT excludes a bleeding disorder. Confusing PPV-style reasoning about Coombs testing.
Worked rule
Prolonged PTT that corrects with normal plasma equals factor deficiency; PTT that fails to correct equals an inhibitor.