Anemia
Anemia is a reduction in red blood cells or hemoglobin that lowers the blood's oxygen-carrying capacity. It is classified by red cell size (MCV) into microcytic, normocytic, and macrocytic types, each with distinct causes.
Anemia is a decrease in the number of circulating red blood cells or in hemoglobin concentration, reducing the blood's ability to deliver oxygen to tissues. It is a laboratory finding with an underlying cause, not a final diagnosis — the clinical task is always to determine why the red cell mass is low. Common symptoms include fatigue, weakness, pallor, exertional dyspnea, and tachycardia, though mild anemia is often asymptomatic.
The standard classification uses mean corpuscular volume (MCV), the average red cell size. Microcytic anemias (low MCV) include iron deficiency — the most common anemia worldwide — thalassemia, anemia of chronic disease, and sideroblastic anemia. Normocytic anemias include acute blood loss, hemolysis, and anemia of chronic kidney disease. Macrocytic anemias (high MCV) include vitamin B12 and folate deficiency, in which impaired DNA synthesis produces large, immature red cells. Beyond MCV, labs such as reticulocyte count, iron studies, haptoglobin, and the peripheral smear narrow the differential.
Anemia matters clinically because it is frequently a marker of another disease — occult gastrointestinal bleeding, malignancy, renal failure, chronic inflammation, or nutritional deficiency — and because severe anemia stresses the cardiovascular system.
Anemia is a high-yield topic across health exams. USMLE Step 1 tests the mechanism-based classification in detail, from thalassemia genetics to B12 pathways; the CCMA exam covers red blood cell analysis and recognizing abnormal values; and the NPTE-PTA includes anemia among hematologic disorders that affect a patient's activity tolerance during rehabilitation.
Key takeaways
- Anemia is reduced red cell mass or hemoglobin, lowering oxygen delivery to tissues.
- Classification by MCV divides anemias into microcytic, normocytic, and macrocytic groups.
- Iron deficiency is the most common anemia worldwide and is microcytic; B12 and folate deficiency cause macrocytic anemia.
- Anemia is a finding, not a diagnosis — the underlying cause must always be identified.
- USMLE Step 1, the CCMA, and the NPTE-PTA all test anemia, from mechanisms to lab values to rehab implications.