###Human: What causes high platelet count in a baby? ###Assistant: Hello. Your child's liver, spleen and lymph nodes should be palpated to check for any enlargement, if present. The thrombocytosis is usually considered reactive when decreasing with the time. Your child's platelet reduces in three days so mostly it will be reactive secondary high platelet for which no specific treatment needed. But you need to repeat your child's CBC again after one week. If after one week it remains high, then you should consult hematologist or pediatrician for detailed evaluation as per my opinion.