Background: Skin color may influence the presence of acanthosis nigricans (AN) as predictor of insulin resistance (IR). The objective of this study was to determine how skin color influences the diagnostic utility of AN as a marker of IR.
Methods: We designed a cross-sectional study. Subjects were classified according to their skin color (phototypes I-VI of Fitzpatrick), presence, and severity of AN (Burke quantitative scale). Blood samples were taken for measurement of glucose and insulin to calculate the Matsuda insulin sensitivity index (ISI).
Results: Sixty two obese subjects with BMI of 39.6 +/- 8 were included. Males were more prone to AN (9/12, 75% vs. 24/50, 48%, p <0.085) than females. The neck was the most frequent affected body region (53/62, 85.5%), followed by the elbows (15/62, 24.2%), knuckles (6/62, 9.7%), and knees (4/62, 6.5%). No concordance was found among them. AN was more frequent in those with skin phototype IV (18/23, 78.3%, p = 0.001) than those with phototype III (12/23, 52.2%) and phototype II (3/16, 18.8%). The utility of AN on the neck to predict ISI <3.5 (surrogate of insulin resistance) was sensitivity, 66.7 (95% CI 40.9-79.5); specificity, 82.5 (95% CI 56-95); positive predictive value, 91%; negative predictive value, 48% (p <0.001 for all values).
Conclusions: People with skin phototype IV have a high frequency of AN on the neck, compared with those with phototypes II and III. Those with AN and skin phototype II showed high specificity and positive predictive values for insulin resistance. Thus, AN is a good marker for IR only in lower phototypes.