Bone marrow: the workhorse organ.

Bone marrow accounts for 3% to 6% of body weight and is dispersed throughout bone. In a healthy adult, bone marrow produces approximately 500 billion new blood cells daily to maintain steady-state levels in the peripheral circulation. Its output is measured using the complete blood count (CBC), which is arguably the most frequently ordered laboratory test in the clinical milieu. Hematopoiesis starts with hematopoietic stem cells that differentiate and mature into any of the three different types of mature blood cells. A simple CBC can rule out, confirm, or direct attention to anemia, cancer, infection, acute hemorrhage states, toxin exposure, allergies, immunodeficiencies, or adverse drug reactions. Interpreting CBCs depends on reference values from the laboratory that processes the blood sample. Since reference intervals are formulated based on a younger, much healthier population, data specific to elderly people are limited.