Blood serotonin levels correlate poorly with brain levels. As well, the total body level differs from the levels at nerve endings. Also, there are a variety of drugs that can affect the serotonin test.

You can try to ask specific questions about the test here: http://www.labtestsonline.org/understanding/analytes/serotonin/ask.php
Your questions will be answered by a laboratory scientist as part of a voluntary service provided by the American Society of Clinical Laboratory Scientists.

Best regards to you and your friend!