Diagnosis is suspected based on typical symptoms but must be confirmed by biopsy, usually via pelvic laparoscopy but sometimes via laparotomy, vaginal examination, sigmoidoscopy, or cystoscopy. Both endometrial glands and stroma must be present to diagnose endometriosis. Macroscopic appearance (eg, clear, red, brown, black) and size of implants vary during the menstrual cycle; however, typically, areas of endometriosis on the pelvic peritoneum are punctate red, blue, or purplish brown spots that are > 5 mm, often called powder burn lesions.

Other procedures (eg, ultrasonography, barium enema, IV urography, CT, MRI) sometimes show the extent of endometriosis and help monitor the disorder but are not specific or adequate for diagnosis. Investigational serum markers for endometriosis (eg, serum cancer antigen 125 [> 35 units/mL], antiendometrial antibody) may help monitor the disorder but require further refinement before being used routinely. Testing for other infertility disorders may be indicated.

Staging the disorder helps physicians formulate a treatment plan and evaluate response to therapy. According to the American Society for Reproductive Medicine, endometriosis may be classified as stage I (minimal), II (mild), III (moderate), or IV (severe), based on number, location, and depth of implants and presence of filmy or dense adhesions. Another staging system is based primarily on pelvic pain. However, because intraobserver and interobserver variability is high in the staging systems, a more reliable method of staging is being sought.