Acanthosis Nigricans with Normal A1c and Glucose
Despite normal fasting glucose and A1c, a patient with acanthosis nigricans requires evaluation for insulin resistance and should undergo oral glucose tolerance testing (OGTT) with insulin levels, as standard screening tests miss a significant proportion of insulin-resistant states that acanthosis nigricans reliably indicates.
Why Standard Tests Are Insufficient
Acanthosis nigricans (AN) is a dermatologic marker of insulin resistance that appears before frank diabetes develops 1, 2. The critical issue is that fasting glucose and A1c detect diabetes, not early insulin resistance—and AN signals the latter 3, 4. Research demonstrates that all patients with AN have elevated fasting insulin levels and exaggerated insulin responses to glucose loads, even when glucose metabolism appears normal by standard criteria 4.
The Pathophysiology
AN develops from hyperinsulinemia stimulating insulin-like growth factor receptors in keratinocytes and dermal fibroblasts, causing the characteristic hyperpigmented, velvety plaques 1. Studies show 63-84% of patients with AN have measurable insulin resistance by more sensitive testing methods, despite normal screening labs 5, 6.
Required Evaluation
Metabolic Assessment
- 75-gram OGTT with insulin levels at 0,30,60,90, and 120 minutes—this is the gold standard that standard screening misses 3, 4
- Calculate HOMA-IR (Homeostatic Model Assessment of Insulin Resistance) or QUICKI (Quantitative Insulin Sensitivity Check Index) 5, 6
- Lipid profile (particularly triglycerides and HDL) 4, 6
Anthropometric Measurements
- Body mass index (BMI)
- Waist circumference and waist-to-hip ratio (WHR)—AN severity correlates with central adiposity 6
Endocrine Screening
Screen for associated conditions that commonly present with AN 4:
- Polycystic ovary syndrome (PCOS) in women—present in 42% of AN cases
- Thyroid function tests (TSH, free T4)
- Prolactin level if menstrual irregularities present
- Consider testosterone and DHEA-S in women with hirsutism
Rule Out Malignancy
In adults over 40, particularly with rapid onset or extensive involvement, consider age-appropriate cancer screening, though malignancy-associated AN is rare and typically has distinct features 1, 2.
Management Strategy
Primary Intervention
Metformin 1500-2550 mg daily is the evidence-based treatment for insulin resistance with AN 5. A study demonstrated:
- 36.3% reduction in insulin resistance (p < 0.01)
- Significant decrease in hyperinsulinemia
- 4.7% reduction in body weight
- 8.95% reduction in body fat mass
Lifestyle Modification
- Weight reduction of 5-10% significantly improves insulin sensitivity
- Focus on reducing waist circumference specifically
- Regular physical activity targeting 150 minutes weekly
Cosmetic Treatment
While treating underlying insulin resistance is primary, topical retinoids, alpha-hydroxy acids, or laser therapy can address cosmetic concerns 2.
Critical Pitfall to Avoid
Do not reassure the patient based solely on normal A1c and fasting glucose. Research consistently shows that 44-63% of patients with impaired glucose tolerance or frank diabetes are detected only through OGTT in AN patients who had normal fasting values 3, 4. The presence of AN indicates the patient is already in a high-risk metabolic state requiring intervention, regardless of screening test results.
Follow-Up Protocol
- Repeat OGTT and insulin studies in 6 months after initiating metformin
- Monitor A1c every 3-6 months—these patients will likely progress to prediabetes/diabetes without intervention
- Annual screening for associated endocrinopathies
- Track improvement in AN severity as a clinical marker of treatment response
The evidence is clear: AN is not just a cosmetic finding but a reliable clinical sign demanding metabolic evaluation beyond standard diabetes screening 4, 7.