Hematologic Core Concepts

A complete blood count is not a collection of unrelated numbers. It is a map of oxygen delivery, host defence, and hemostasis. Red blood cells carry hemoglobin, the protein that transports oxygen. Anemia reduces oxygen-carrying capacity; the patient may compensate with tachycardia, increased cardiac output, and exertional dyspnea. A slow fall in hemoglobin can be tolerated surprisingly well, whereas an acute hemorrhage may cause shock before the hemoglobin concentration has fallen enough to reflect the true blood loss. White blood cells are the visible part of immune defence, but the total leukocyte count is less useful than the differential. Neutrophils are the immediate antibacterial responders. A patient with a normal total white count can still be profoundly neutropenic if neutrophils are absent. Platelets provide the initial plug at a damaged vessel wall. Coagulation factors then create fibrin strands that stabilize that plug. Platelet disorders usually cause mucocutaneous bleeding—petechiae, purpura, epistaxis, gingival bleeding, and heavy menstrual bleeding. Factor deficiencies or anticoagulant effects more often produce deep bruising, hemarthrosis, muscle hematoma, or delayed bleeding after trauma. When more than one cell line...