They are not only for diagnosing sexually transmitted infections: in premenstrual syndrome they can be the key.
A swab from the cervix or vagina, or a urine sample, identifies bacteria, fungi and other microorganisms, and antibiotic or antifungal susceptibility testing shows which treatment will work. As well as sexually transmitted infections (chlamydia, gonorrhoea, mycoplasmas, trichomonas…), I look for low-grade infections that cause few obvious symptoms.
That is why cultures can be the key to diagnosing and treating PMS: chronic inflammation or infection of the cervix and genital tract can be the source of inflammatory mediators that make every cycle worse. Finding it means we can treat the origin, not just the symptoms.