Parents often ask nervously when they bring in the test results: Why are there so many arrows on my child’s blood test? We know that routine blood tests are for blood cells, i.e. red blood cells, white blood cells and platelets. Red blood cells are described in the section on anemia, and the reference indicators are different for different age groups, so I won’t repeat them here. The normal value of platelets in children is the same as that in adults, ranging from 100 to 300×109/L. The white blood cells are further divided into neutrophils, lymphocytes, monocytes, eosinophils, and basophils, which are mainly composed of the first two. The leukocyte classification during a child’s blood test is to see what percentage of these different types of leukocytes are present. The percentage of white blood cells in normal children varies with age. At birth, granulocytes are in the majority (>60%), then after 6 days the proportion of granulocytes decreases and the proportion of lymphocytes is in the majority. For parents, it is easy to remember: for children under 6 years old, the percentage of lymphocytes is 50-70% and the percentage of granulocytes is 30-50%. For children over 6 years of age, it is the other way around: 30-50% lymphocytes and 50-70% granulocytes.