4.1 Hematology and Coagulation Reasoning

Build the Hematology Differential From the CBC

Step 1 blood questions usually start with a complete blood count, a smear, and one extra clue. The fastest route is to classify the anemia by mean corpuscular volume (MCV) and then decide whether the marrow response is appropriate. A high reticulocyte count means the marrow is trying to replace red cells, so think blood loss or hemolysis. A low reticulocyte count points toward underproduction from iron restriction, renal erythropoietin deficiency, marrow failure, inflammation, or impaired DNA synthesis.

Microcytic anemia is a comparison problem. Iron deficiency anemia has low ferritin, high total iron-binding capacity, high red cell distribution width, and may show koilonychia or pica. Anemia of chronic disease is driven by interleukin-6 increasing hepcidin, which traps iron in macrophages; ferritin is normal or high and total iron-binding capacity is low. Thalassemia produces very low MCV with target cells and a normal or high red cell count. Beta-thalassemia has increased hemoglobin A2; severe alpha-chain deletion can cause hemoglobin H disease or hydrops fetalis.

Sideroblastic anemia has ring sideroblasts from defective heme synthesis; lead poisoning, alcohol, vitamin B6 deficiency, copper deficiency, and isoniazid are classic triggers.

Macrocytic anemia divides into megaloblastic and nonmegaloblastic patterns. Megaloblastic anemia shows hypersegmented neutrophils from defective DNA synthesis. Vitamin B12 deficiency raises methylmalonic acid and homocysteine and can cause posterior column and lateral corticospinal tract injury. Folate deficiency raises homocysteine alone. Nonmegaloblastic macrocytosis appears with alcohol, liver disease, hypothyroidism, and reticulocytosis because reticulocytes are larger than mature erythrocytes.

Two marrow clues keep borderline items straight. Aplastic anemia has hypocellular marrow with pancytopenia after drugs, viruses, radiation, inherited DNA repair defects, or immune injury. Myelophthisic anemia has marrow replaced by tumor or fibrosis and may show teardrop cells plus immature myeloid and red cell precursors. Multiple myeloma can produce normocytic anemia through marrow crowding and kidney injury; rouleaux reflects high serum protein, not a red cell membrane disorder.

Hemolysis: Localize the Site of Red Cell Destruction

Hemolysis explains anemia with high reticulocytes, indirect bilirubin, lactate dehydrogenase, and low haptoglobin. Extravascular hemolysis occurs in spleen or liver macrophages and often causes jaundice and splenomegaly. Spherocytes suggest hereditary spherocytosis or warm autoimmune hemolytic anemia. Intravascular hemolysis releases hemoglobin into plasma and urine; causes include paroxysmal nocturnal hemoglobinuria, microangiopathic hemolysis, severe G6PD episodes, and incompatible transfusion.

Smear findings are exam shortcuts:

• Schistocytes: mechanical fragmentation from disseminated intravascular coagulation, thrombotic thrombocytopenic purpura, hemolytic uremic syndrome, malignant hypertension, or prosthetic valves.
• Bite cells and Heinz bodies: oxidative injury, especially G6PD deficiency after infection, fava beans, dapsone, primaquine, or sulfonamides.
• Sickle cells and Howell-Jolly bodies: hemoglobin S polymerization with functional asplenia.
• Target cells: thalassemia, liver disease, hemoglobin C disease, or postsplenectomy state.

A direct antiglobulin test detects antibody or complement already attached to patient red cells; it is useful for autoimmune hemolysis and hemolytic transfusion reactions. An indirect antiglobulin test detects antibodies in serum, such as maternal anti-D antibody before fetal red cell exposure.

Coagulation Reasoning: Bleeding Type Plus Lab Pattern

Primary hemostasis is platelet plug formation. Platelet disorders cause mucosal bleeding, petechiae, purpura, epistaxis, and menorrhagia. Secondary hemostasis is fibrin stabilization. Coagulation factor disorders cause deep tissue bleeding, hemarthroses, delayed surgical bleeding, and large hematomas.

Use the lab pattern before memorizing disease names. Prothrombin time (PT) tests the extrinsic and common pathways, so factor VII and warfarin effects appear early. Partial thromboplastin time (PTT) tests intrinsic and common pathways, including factors VIII, IX, XI, and XII. Thrombin time tests conversion of fibrinogen to fibrin. A mixing study that corrects suggests factor deficiency; failure to correct suggests an inhibitor, such as a factor VIII inhibitor or lupus anticoagulant.

Drug mechanisms are tested as physiology. Unfractionated heparin activates antithrombin and prolongs PTT; heparin-induced thrombocytopenia is an antibody response against platelet factor 4 that paradoxically causes thrombosis. Warfarin inhibits vitamin K epoxide reductase, reducing gamma-carboxylation of factors II, VII, IX, X, protein C, and protein S. Because protein C falls quickly, early warfarin effect can produce a transient prothrombotic state. Aspirin irreversibly inhibits platelet cyclooxygenase; P2Y12 inhibitors block adenosine diphosphate-mediated platelet activation.

Transfusion and Bleeding Vignettes

Transfusion reactions are mechanism questions. Acute hemolytic transfusion reaction is usually ABO mismatch: preformed IgM fixes complement, producing fever, flank pain, hypotension, hemoglobinuria, and possible disseminated intravascular coagulation. Delayed hemolytic reaction is usually IgG against minor red cell antigens, with extravascular hemolysis days later. Febrile nonhemolytic reactions come from cytokines or recipient antibodies against donor leukocytes. Urticaria is allergic histamine release. Anaphylaxis after plasma-containing products suggests anti-IgA antibodies in an IgA-deficient recipient.

Transfusion-related acute lung injury is donor anti-leukocyte antibody activation in pulmonary capillaries; transfusion-associated circulatory overload is hydrostatic volume overload.

For Step 1 reasoning, do not stop at the name of a disease. Ask what process is abnormal: red cell production, membrane stability, hemoglobin synthesis, oxidative protection, platelet adhesion, coagulation factor activation, fibrin breakdown, or immune destruction. That mechanism usually predicts the laboratory pattern, the smear, and the adverse effect clue in the same vignette.