Leukemia is a malignant increase in the number of leukocytes, usually at an immature stage, in the bone marrow. The confirmatory test for leukemia is microscopic examination of bone marrow obtained by bone marrow aspirate and biopsy, which is considered positive if blast cells are present. An altered platelet count occurs as a result of the disease, but also may occur as a result of chemotherapy and does not confirm the diagnosis. The white blood cell count may be normal, high, or low in leukemia. Alumbar puncture may be done to look for blast cells in the spinal fluid that indicate central nervous system disease.