SUN-643 Pelvic Magnetic Resonance Imaging as a Diagnostic Tool for Familial Partial Lipodystrophy: Validation of Gluteal Fat Thickness and Pubic/Gluteal Fat Ratio Thresholds in a Large Cohort of Females

Abstract Disclosure: M. Celik Guler: None. E. Avci: None. D. Guler: None. K. Sinci: None. C. Ozturk: None. T. Caliskan: None. B. Ulucan: None. G. Bayram: None. Y. Celik: None. C. Kavi: None. B. Ergin: None. N. Huseynli: None. E. Bayraktar: None. M. Unal: None. U. Cavdar: None. E. Guler: None. A. Koc: None. E. Aydin: None. M. Demirpence: None. I. Yildirim Simsir: None. C. Altay: None. E.A. Oral: Regeneron Pharmaceuticals, Ionis Pharmaceuticals Inc., Chiesi Farmaceutici, Morphic Medical, Rhythm Pharmaceuticals, Third Rock Ventures, Rejuvenate Bio, Novo Nordisk, Fractyl. B. Akinci: AstraZeneca, Chiesi Farmaceutici, Alnylam, Regeneron Pharmaceuticals, Novo Nordisk, Boehringer Ingelheim, Sanofi, Bilim Ilac, ARIS, Servier, Third Rock Ventures. Background: We recently developed a method that utilizes pelvic magnetic resonance imaging (MRI), a widely accessible imaging modality, to diagnose familial partial lipodystrophy (FPLD). This study aimed to: i) validate our previously established diagnostic thresholds in a larger cohort of FPLD patients and random subjects; ii) identify and analyze cases with false-negative and false-positive results; and iii) explore potential adjustments to the diagnostic thresholds. Methods: We measured gluteal and pubic fat thickness on pelvic MRI scans from females over 14 years old, including 41 patients with confirmed FPLD and 2336 random female controls who underwent pelvic MRI for various medical reasons. Data are shown as median, 25-75 percentiles. Results: Patients with FPLD were younger (44 years, 31-53 vs. 50 years, 40-63, p = 0.001), had significantly lower gluteal fat thickness (7.0 mm, 4.0-9.5 vs. 29.5 mm, 24.0-35.5; p < 0.001) and higher pubic/gluteal fat ratios (4.5, 3.3-8.9 vs. 0.9, 0.7-1.3; p < 0.001). ROC analysis showed an area under the curve (AUC) of 0.996 (95% CI: 0.992-1.000; p < 0.001) for gluteal fat thickness and 0.980 (95% CI: 0.962-0.999; p < 0.001) for the pubic/gluteal fat ratio in diagnosing FPLD. A gluteal fat thickness of ≤ 13 mm alone identified FPLD with 95.1% sensitivity, 98.6% specificity, a positive predictive value (PPV) of 54.2%, and a negative predictive value (NPV) of 99.9%. Combining gluteal fat thickness ≤ 13 mm with a pubic/gluteal fat ratio ≥ 2.5 improved specificity to 99.4% but reduced sensitivity to 87.8% (PPV: 73.5%, NPV: 99.8%). Within the FPLD cohort, five false-negative cases were identified, while 13 false positives were observed among controls, most of whom had conditions associated with reduced adiposity (e.g., malignancies, organ transplantation, steroid use, immunosuppressive therapy, or cachexia). The Youden’s index reaffirmed ≤ 13 mm as an effective predictor, though alternative thresholds were explored: ≤ 8 mm improved specificity to 99.9% but reduced sensitivity to 65.9%, while ≤ 17.5 mm increased sensitivity to 97.5% with a specificity of 93.5%. The pubic/gluteal fat ratio threshold was revisited, as some typical FPLD cases had a ratio < 2.5 but still displayed preserved pubic fat. Lowering the threshold to ≥ 1.5 combined with gluteal fat thickness ≤ 13 mm yielded 95.1% sensitivity and 99.1% specificity (PPV: 65.0% and NPV: 99.9%). Combining a pubic/gluteal fat ratio ≥ 1.5 with gluteal thickness thresholds ≤ 8 mm or ≤ 17.5 mm also showed slight improvements in performance. Conclusion: Our findings validate the diagnostic utility of pelvic MRI for FPLD and reaffirm gluteal fat thickness ≤ 13 mm as an effective threshold. More specific (≤ 8 mm) or more sensitive (≤ 17.5 mm) thresholds may offer added clinical value. Combining gluteal fat thickness measurement with a pubic/gluteal fat ratio further enhances specificity. Presentation: Sunday, July 13, 2025