Laboratory Tests for PCOS Diagnosis
The essential laboratory tests for diagnosing PCOS include hormonal evaluation (total or free testosterone, TSH, prolactin), metabolic screening (2-hour 75g oral glucose tolerance test and fasting lipid profile), and exclusion tests (17-hydroxyprogesterone), combined with clinical assessment and pelvic ultrasound. 1, 2
Core Hormonal Panel
Androgen Assessment:
- Measure total testosterone or free/bioavailable testosterone as the primary test for biochemical hyperandrogenism, preferably using liquid chromatography-tandem mass spectrometry (LC-MS/MS) which demonstrates 92% specificity compared to 78% for direct immunoassays 2
- Obtain sex hormone-binding globulin (SHBG) levels to calculate free androgen index 1
- Check androstenedione if levels exceed 10.0 nmol/L to rule out adrenal or ovarian tumors 3
- Measure DHEAS (age-specific thresholds: >3800 ng/mL for ages 20-29, >2700 ng/mL for ages 30-39) to exclude non-classical congenital adrenal hyperplasia 3
Ovulatory Function:
- Obtain mid-luteal phase progesterone (day 21 of a 28-day cycle or 7 days before expected menses) where levels <6 nmol/L indicate anovulation 3
- Measure LH and FSH between cycle days 3-6, calculated as an average of three measurements taken 20 minutes apart, with LH/FSH ratio >2 suggesting PCOS 3
Exclusion of Other Endocrine Disorders:
- Check TSH to exclude thyroid disease as a cause of menstrual irregularity 1, 2
- Measure prolactin (morning resting levels, not postictal) to exclude hyperprolactinemia, with levels >20 μg/L being abnormal 3, 2
- Obtain 17-hydroxyprogesterone to exclude non-classical congenital adrenal hyperplasia 2
Metabolic Evaluation
Glucose Metabolism:
- Perform a 2-hour oral glucose tolerance test with 75g glucose load in all women with suspected PCOS regardless of BMI, as insulin resistance occurs independently of body weight 1, 2
- Calculate fasting glucose/insulin ratio, where a ratio >4 suggests reduced insulin sensitivity 3
- Fasting glucose >7.8 mmol/L is suggestive of diabetes 3
Lipid Assessment:
- Obtain fasting lipid profile to assess cardiovascular risk, as women with PCOS have increased risk of dyslipidemia and metabolic syndrome 1, 2
Imaging Studies
Pelvic Ultrasound:
- Transvaginal ultrasound is preferred using transducers with frequency ≥8 MHz, looking for ≥20 follicles (2-9mm diameter) per ovary and/or ovarian volume ≥10 mL 1, 2
- Transabdominal ultrasound may be used as an alternative, focusing on ovarian volume with threshold ≥10 mL 1
- MRI pelvis without contrast may be considered when ovaries cannot be adequately visualized by ultrasound 1
Critical Diagnostic Considerations
When to Suspect Alternative Diagnoses:
- Rapid onset or marked virilization requires immediate evaluation for androgen-secreting tumors with testosterone typically >2.5 nmol/L warranting further investigation 3, 4
- Consider dexamethasone suppression test if Cushing's syndrome is suspected based on physical findings (buffalo hump, moon facies, abdominal striae) 1, 2
Important Pitfalls to Avoid:
- Do not use serum AMH levels as a single diagnostic test for PCOS despite emerging evidence 1, 2
- Ultrasound findings alone are insufficient for diagnosis, as polycystic ovarian morphology may be present in up to one-third of reproductive-aged women without PCOS 1
- Avoid using ultrasound for diagnosis in adolescents with gynecological age <8 years post-menarche due to physiologically normal multifollicular ovaries at this life stage 2
- Ensure prolactin measurements are not taken postictally in patients with epilepsy, as levels may be transiently elevated 3
Physical Examination Components
Anthropometric Measurements:
- Calculate BMI and waist-to-hip ratio (WHR >0.9 indicates truncal obesity) to assess for metabolic risk 3, 1, 2
- Examine for acanthosis nigricans on neck, axillae, under breasts, and vulva, which indicates underlying insulin resistance 2
Clinical Hyperandrogenism:
- Assess for hirsutism using Ferriman-Gallwey scoring, acne, male-pattern alopecia, and clitoromegaly 1