A complete blood count (CBC) is a blood test used to evaluate your overall health and detect a wide range of disorders, including anemia, infection and leukemia.A complete blood count test measures several components and features of your blood, including:Red blood cells, which carry oxygenWhite blood cells, which fight infectionHemoglobin, the oxygen-carrying protein in red blood cellsHematocrit, the proportion of red blood cells to the fluid component, or plasma, in your bloodPlatelets, which help with blood clottingAbnormal increases or decreases in cell counts as revealed in a complete blood count may indicate that you have an underlying medical condition that calls for further evaluation.