The complete blood count: physiologic basis and clinical usage.

The complete blood count is one of the most frequently ordered laboratory tests in medicine. The components are the red blood cell count, hemoglobin, hematocrit, red blood cell indices (including the mean cell volume, mean cell hemoglobin, mean cell hemoglobin concentration, and red cell distribution width), reticulocyte count, white blood cell count and differential, and platelet count. To aid in understanding the multiple uses of the complete blood count, the article discusses the function, life cycle, and physiology of the various cell components. In addition, the characteristics of the various tests, including their strengths and limitations, are presented. This information can be helpful in difficult diagnoses and in monitoring treatments for hematologic disorders and other medical problems.