Laboratory Tests for PCOS Diagnosis
To diagnose PCOS in a reproductive-age woman, order TSH, prolactin, and 17-hydroxyprogesterone (17-OHP) to exclude mimicking disorders, then measure total testosterone (TT) and free testosterone (FT) using mass spectrometry as first-line androgen tests. 12
Essential Exclusionary Labs (Required for All Patients)
Before diagnosing PCOS, you must exclude conditions that mimic it 1:
- TSH - excludes thyroid disease causing menstrual irregularity
- Prolactin - excludes hyperprolactinemia (can present with amenorrhea or hirsutism)
- 17-hydroxyprogesterone (17-OHP) - excludes nonclassic congenital adrenal hyperplasia (found in 1.5-6.8% of patients with androgen excess)
First-Line Androgen Testing
The most accurate biochemical markers are 2:
- Total testosterone (TT) - sensitivity 74%, specificity 86%, AUC 0.87
- Free testosterone (FT) - sensitivity 89%, specificity 83%, AUC 0.85
Critical technical point: Mass spectrometry (LC-MS/MS) is superior to immunoassays for measuring androgens, providing better sensitivity and specificity 2. Where mass spectrometry is unavailable, the Free Androgen Index (FAI) can substitute (sensitivity 78%, specificity 85%, AUC 0.87) 2.
Total testosterone was the most frequently abnormal marker in PCOS (70% of cases), making it the best single hormonal marker 3.
Second-Line Androgen Testing
If TT or FT are not elevated but clinical suspicion remains high, consider 2:
- Androstenedione (A4) - sensitivity 75%, specificity 71%
- DHEAS - sensitivity 75%, specificity 67%
Important caveat: A4 and DHEAS have poorer specificity, meaning more false positives 2.
What NOT to Use for Diagnosis
Do NOT use these tests for diagnosis 12:
- Polycystic ovarian morphology on ultrasound - not required if two other Rotterdam criteria are met clinically
- Anti-Müllerian hormone (AMH) - can be used as alternative to ultrasound in adults only, but not necessary if clinical criteria already met
- LH/FSH ratio - abandoned as diagnostic criterion due to low sensitivity (only 35-44% abnormal in PCOS patients) 3
Additional Testing for Select Patients
In women with amenorrhea, virilization, or atypical features (proximal muscle weakness, frontal bossing), consider broader evaluation to exclude 1:
- Cushing's syndrome
- Acromegaly
- Androgen-secreting tumors
- 24-hour urinary free cortisol
Ovulation Assessment
For women seeking fertility, assess ovulatory status using 1:
- Menstrual history (primary screening)
- Midluteal serum progesterone (if eumenorrheic but anovulation suspected - some PCOS women with regular cycles still don't ovulate)
Diagnostic Algorithm
- Document clinical features: irregular menses (oligomenorrhea/amenorrhea), clinical hyperandrogenism (hirsutism, acne, alopecia)
- Order exclusionary labs: TSH, prolactin, 17-OHP
- Measure androgens: TT and FT by mass spectrometry (or FAI if unavailable)
- Apply Rotterdam criteria: Diagnose PCOS if 2 of 3 present:
- Androgen excess (clinical OR biochemical)
- Ovulatory dysfunction
- Polycystic ovaries (optional if other two criteria met)
The diagnosis can be made clinically without universal androgen screening or ultrasound if the patient already meets two criteria based on history and physical examination 1. However, biochemical confirmation strengthens diagnostic certainty and helps risk-stratify patients, as hyperandrogenism correlates more strongly with metabolic abnormalities 1.