This is a significant increase in platelet count. Mild increases are usually not an issue and usually do not warrant further evaluation, but this is a significant evaluation.
There are several possible causes of an elevated platelet count.
In a 14 year old, the most common cause would be overproduction of platelets by the bone marrow. This overproduction can affect only the platelets, but can also affect other cells produced by the bone marrow. The blood tests do not show any increase in any of the other cellular components, which would suggest that it is only affecting the platelets. It is also possible that the overproduction of platelets can be due to a familial genetic trait.
There also are some causes of a reactive elevation of the platelet counts, such as a response to inflammation or infection or from decreased functioning of the spleen.
There should be an evaluation to look for any conditions that can cause a reactive elevation, and if this is not seen, then an evaluation for overproduction by the bone marrow should be done. A biopsy of the bone marrow is typically done, as well as genetic tests for common mutations that can cause the genetic trait.
If it is confirmed that this is from overproduction, it is a condition in which serious complications can occur. The high platelet count, itself, is not the primary concern, but these patients have an increased risk for forming blood clots and for bleeding. There also is an increased risk of this condition progressing into a type of leukemia, although this only occurs in a small percentage of patients.
If it is found to be a reactive elevation, it will usually resolve with identification and treatment of the underlying cause, so is less of a concern.
If I can provide any additional information, please let me know.