A normal adult is considered to have low white blood cells if they are less than 4.0×10^9/L. A normal adult has a white blood cell range of (4.0-10.0)x10^9/L; a normal child has a white blood cell range of (5.0-12.0)x10^9/L; and a normal newborn has a white blood cell range of (15.0-20.0)x10^9/L. A white blood cell that is lower than the lowest of the normal ranges in each category is considered low. The causes of low white blood cells may be related to medications, hematologic disorders, rheumatologic and immunologic disorders, and infections. Therefore, when low white blood cells appear in the examination, it is recommended to cooperate with the doctor for further examination to find out the cause before treatment.