Hematologic System
Core concepts
Start anemia with mean corpuscular volume and the reticulocyte count. A low reticulocyte count means a production problem; a high one means loss or destruction. Microcytic anemia: iron deficiency shows low ferritin, low serum iron, and high total iron-binding capacity; anemia of chronic disease shows low iron with low binding capacity and normal or high ferritin; thalassemia shows microcytosis with normal iron studies.
Macrocytic anemia: vitamin B12 deficiency causes neurologic findings with elevated methylmalonic acid, while folate deficiency does not. Hemolysis shows elevated lactate dehydrogenase, low haptoglobin, elevated indirect bilirubin, and a high reticulocyte count; a positive direct Coombs test indicates autoimmune hemolysis.
Bleeding disorders: von Willebrand disease is the most common inherited bleeding disorder; hemophilia gives a prolonged partial thromboplastin time with normal platelets. Thrombotic thrombocytopenic purpura shows schistocytes and the classic pentad, is treated with plasma exchange, and platelet transfusion is avoided. Disseminated intravascular coagulation shows prolonged clotting times, low fibrinogen, and elevated D-dimer.
Neutropenic fever, defined by an absolute neutrophil count below 500 with fever, needs empiric antipseudomonal beta-lactam therapy within an hour.
Traps
- Ordering ferritin and calling it iron deficiency in an inflamed patient without interpreting the pattern.
- Giving platelets to a patient with suspected thrombotic thrombocytopenic purpura.
- Missing B12 deficiency because the anemia is mild, while neurologic damage progresses.
Rule of thumb
Anemia with a high reticulocyte count means the marrow is responding to loss or destruction; anemia with a low reticulocyte count means the marrow is failing or lacks building blocks.