Is it permissible to masturbate by hand to perform a semen analysis to confirm the recurrence of prostatitis, in order to avoid the side effects of medication and to avoid a digital rectal exam of the prostate?

Masturbation is forbidden unless there is a legitimate need. If you are unable to marry and your doctor informs you of the necessity of masturbation for diagnosing your condition, then there is no harm in it. Sheikh Al-Uthaymeen mentioned that the needs that permit masturbation are of two types: a religious need, such as the fear of committing zina (fornication/adultery), and a physical need, such as the fear of harm to the body if this excess is not expelled. If one performs it without a need, then he is to be disciplined.