How is PCOS actually diagnosed?

PCOS is diagnosed on a pattern, not on one test. The 2023 International Evidence-based Guideline keeps the Rotterdam criteria: two of three features are required, and conditions that mimic PCOS have to be excluded. The three features are ovulatory dysfunction, high androgens on examination or on blood testing, and polycystic ovarian morphology on ultrasound or a raised anti-Mullerian hormone level.

The three Rotterdam criteria

The Rotterdam criteria for PCOS are irregular or absent ovulation, hyperandrogenism, and polycystic ovarian morphology. Any two of the three make the diagnosis in an adult. The 2023 guideline adds a shortcut that changes practice: when a woman has both irregular cycles and hyperandrogenism, an ovarian ultrasound adds nothing and is not needed. Adolescents are held to a stricter standard. Both irregular cycles and hyperandrogenism are required, and neither ultrasound nor AMH is used, because normal puberty produces ovaries that look multifollicular.

What counts as an irregular cycle

The guideline defines irregular cycles by time since the first period. In the first year after menarche, irregular cycles are normal. From one to three years after menarche, cycles shorter than 21 days or longer than 45 days are irregular. From three years after menarche to perimenopause, the thresholds are cycles shorter than 21 days, longer than 35 days, or fewer than eight cycles in a year. After the first year, any single cycle longer than 90 days is abnormal.

Ultrasound thresholds and the AMH alternative

For polycystic ovarian morphology, the 2023 guideline sets the adult threshold at a follicle number per ovary of 20 or more in at least one ovary. Where image quality limits assessment, an ovarian volume of 10 mL or more, or 10 or more follicles per cross section, can be used instead. Serum AMH is now accepted as an alternative way of defining polycystic ovarian morphology in adults. The guideline is blunt about the limits: AMH "should not be used as a single test for the diagnosis of PCOS", and ultrasound and AMH should not both be done, to limit over-diagnosis.

What has to be excluded first

A short list of conditions produces the same picture as PCOS and has to be cleared. Thyroid function, prolactin, and an early morning 17-hydroxyprogesterone for nonclassic congenital adrenal hyperplasia are checked in most women. Pregnancy is excluded. Where symptoms came on quickly or include voice change or clitoral enlargement, Cushing syndrome and an androgen-producing tumour are considered.

Ask for named tests rather than "a hormone panel": total and free testosterone by mass spectrometry, early morning 17-hydroxyprogesterone in the follicular phase, prolactin, TSH, and a 75 g oral glucose tolerance test. One naming note: in May 2026 an international consensus renamed the condition polyendocrine metabolic ovarian syndrome, or PMOS, over a three year transition. The diagnostic criteria themselves did not change.