The increase in platelets can be caused by a cold, but it is usually transient and the degree of platelet increase is not too obvious. The normal range of platelets is (100~300)×10^9/L. If platelets increase when you have a cold, it may be just above 300, and it is unlikely to reach or exceed 400 significantly. When you have a cold, if the platelets are mildly elevated, there is nothing to worry about. If the increase is significantly higher than 400 or even 500, it will indicate a platelet-related disease. If the platelets remain particularly high after the symptoms of the cold have disappeared, it is time to undergo relevant tests. In addition to colds, there may be other minor infections that can cause platelets to increase in response to infections, but as long as the disease is not of the blood system, the platelets will not increase too much.