Rotterdam criteria: how PCOS is actually diagnosed

PCOS diagnosis sits on the 2003 Rotterdam criteria, refined by the 2018 international evidence-based guideline. The criteria are widely cited but inconsistently applied — leading to both over-diagnosis (women labeled with PCOS who actually have other conditions) and under-diagnosis (women with classic PCOS who don't meet ultrasound criteria).

The three Rotterdam features

Diagnosis requires two of the following three: (1) Oligo-ovulation or anovulation — cycles longer than 35 days or fewer than 8 cycles per year; (2) Clinical or biochemical hyperandrogenism — clinical signs (hirsutism, acne, androgenic hair loss) or elevated total/free testosterone, DHEA-S, or androstenedione; (3) Polycystic ovary morphology on ultrasound — defined as 20 or more follicles 2–9mm in at least one ovary, or ovarian volume >10 mL.

What the ovarian appearance does and doesn't mean

"Polycystic ovary morphology" on ultrasound is common in young women without PCOS — up to 30% of healthy 20-somethings have ovaries that meet the morphology criterion. The ultrasound finding alone is not a diagnosis. Diagnosing PCOS based on "your ovaries looked polycystic" without other criteria is incorrect application of the Rotterdam standard.

Conversely, women with PCOS-defining cycle and hormone features may have normal-appearing ovaries on ultrasound — particularly older women, or those with PCOS phenotypes that don't emphasize polycystic morphology. Ultrasound is one of three potential criteria, not a required test.

The required exclusions

Before applying Rotterdam, several conditions that mimic PCOS must be ruled out: thyroid dysfunction (TSH); hyperprolactinemia (prolactin); non-classical congenital adrenal hyperplasia (17-hydroxyprogesterone); Cushing's syndrome (if clinical suspicion — 24-hour urine cortisol, dexamethasone suppression); androgen-secreting tumor (if rapid hirsutism onset — testosterone significantly elevated, DHEA-S in tumor range).

When Rotterdam doesn't apply

Late perimenopause: cycle irregularity is expected and is not PCOS. Polycystic ovary appearance in a 51-year-old with peri-typical symptoms is not PCOS — it's declining ovarian reserve looking different on imaging. Be cautious of new PCOS diagnoses applied in the perimenopausal window.

Adolescence: the 2018 guideline specifically advises against PCOS diagnosis in adolescents within 2 years of menarche, because irregular cycles are common and resolve. Hyperandrogenism alone in early adolescence usually warrants workup but not yet diagnosis.

Currently on hormonal contraception: the pill suppresses ovulation and changes hormone levels, masking PCOS features. Off-pill evaluation is needed for accurate diagnosis.

What labs to expect at workup

Standard PCOS workup: total and free testosterone, DHEA-S, sex hormone-binding globulin (SHBG), TSH and reflex T4, prolactin, 17-hydroxyprogesterone (preferably morning, early follicular if cycling), fasting glucose and insulin, lipid panel, 25-hydroxyvitamin D. Pelvic ultrasound on day 2–5 of cycle if possible.

After diagnosis

Once PCOS is confirmed by Rotterdam criteria with appropriate exclusions, the next step is phenotype identification (see PCOS phenotype article) — because treatment alignment depends on which underlying driver predominates.

Informational only — diagnosis and treatment require a clinician's evaluation.