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Why would an 18-year-old male have a low white blood cell count?

The rest of his CBC and CMP are within normal limits.No family or social history of drug abuse or HIV.
There are many potential causes for a low white blood cell count. If you, or someone you know well, has this problem, then the first step would be to seek a full evaluation from a doctor. It is best to start with your primary care doctor although, if the low white blood cell count persists, it may be necessary to involve a specialist, such as a hematologist. By far the most common cause of a low white blood cell count is a transient decrease in the production of white blood cells by the bone marrow. This most commonly occurs in the setting of a viral infection. Most of the time, there is no need to do much in these cases except check serial blood counts to make sure the issue resolves. Low white blood cells counts are also often associated with various medications which can induce suppression of the bone marrow. It is therefore important to review with the doctor closely all medications that have been or have recently been taken. Most doctors will also work up low white blood cell count with basic tests for other common causes, most notably an HIV test and also a blood smear to look for abnormal cells (which might be concerning for a blood cancer).
This answer is for general informational purposes only and is not a substitute for professional medical advice.
If you think you may have a medical emergency, call your doctor or (in the United States) 911 immediately. Always seek the advice of your doctor before starting or changing treatment. Medical professionals who provide responses to health-related questions are intended third party beneficiaries with certain rights under Zocdoc’s Terms of Service.