How PCOS is diagnosed — and the blood tests worth asking for.

Many people wait years for a PCOS diagnosis, often because tests were ordered in the wrong order, at the wrong time, or not at all. Knowing the standard work-up — set out in the 2023 international guideline — helps you have a productive conversation.

Step 1: Cycle history

Your cycle history is itself a diagnostic test. In adults, irregular cycles mean cycles shorter than 21 days or longer than 35 days, or fewer than eight periods a year. Bring a record of your last several period start dates if you can; the cycle calculator and a notes app both work.

If your cycles look regular but PCOS is suspected, a mid-luteal progesterone blood test (about a week before your expected period) can show whether you ovulated.

Step 2: Androgen tests

If you clearly have hirsutism or other signs of androgen excess, a blood test isn’t strictly needed to meet that criterion — but labs are still useful as a baseline.

Step 3: Ovarian assessment — ultrasound or AMH

If you have both irregular cycles and high androgens, you can be diagnosed without imaging. Otherwise, a transvaginal ultrasound counts follicles and measures ovarian volume. New in 2023, anti-Müllerian hormone (AMH) can be used instead of ultrasound in adults, because AMH is made by the small follicles that accumulate in PCOS. AMH isn’t used to diagnose PCOS in adolescents, and ultrasound isn’t recommended for diagnosis within eight years of a first period.

Step 4: Ruling out lookalikes

PCOS is a diagnosis of exclusion, so these are standard:

• TSH — thyroid disorders can disrupt cycles.
• Prolactin — high prolactin stops ovulation and can cause irregular periods.
• 17-hydroxyprogesterone — screens for non-classic congenital adrenal hyperplasia, which can look almost identical to PCOS.
• FSH and estradiol — when absent periods could reflect low ovarian function or hypothalamic causes (for example, under-eating or over-training).
• Pregnancy test, always, when a period is missed.

LH and the LH:FSH ratio are often elevated in PCOS but are not diagnostic criteria.

Step 5: Metabolic screening after diagnosis

Because PCOS raises metabolic risk, the guideline recommends checking glycaemic status at diagnosis and periodically afterwards:

• 75 g oral glucose tolerance test (OGTT) — the most accurate test for prediabetes and type 2 diabetes in PCOS. HbA1c or fasting glucose are alternatives when an OGTT isn’t feasible.
• Lipid profile — cholesterol and triglycerides.
• Blood pressure, weight and waist — routinely.

What about insulin tests? Fasting insulin and HOMA-IR are popular online, but the guideline advises against using them clinically to assess insulin resistance in PCOS, because the assays are poorly standardised and results don’t reliably change management. Insulin resistance is common in PCOS regardless; treat the pattern, not a single number. Our insulin resistance guide explains more.

Questions to bring to your appointment

• Which of the three criteria do I meet, and which phenotype does that make me?
• Have thyroid, prolactin and 17-OH-progesterone been checked?
• Was my testosterone measured with a high-quality assay, and what were my SHBG and free androgen index?
• When should I have an oral glucose tolerance test, and how often should it be repeated?
• Do I need endometrial protection given how often I bleed?
• Can you screen me for depression, anxiety and sleep apnoea?

Not sure your symptoms warrant testing? The symptom quiz helps you organise them first, and our symptoms guide covers the red flags.