0898 Title: Test - Retest Reliability of the Multiple Sleep Latency Test in Non-cataplectic Central Hypersomnia Disorders

Abstract Introduction Differentiation of narcolepsy type II (NT2) and idiopathic hypersomnia (IH) is challenging in clinical practice. Diagnostic criteria rely on the multiple sleep latency test (MSLT), previously shown to have poor reproducibility these disorders. We leveraged a large polysomnographic database study test-retest reliability non-cataplectic central Methods Patients evaluated at Cleveland Clinic Sleep Disorders Center from 2008-2021 with suspected disorders excluding I >1 PSG/MSLT were included. Studies not meeting validity (< 7hr time PSG, < 5 MSLT trials, use REM suppressants/alerting medications within 14 days) excluded. groupings included NT2, IH Normal/Indeterminate based ICSD-3. Reliability correlation between tests was performed using intra-class Spearman’s correlations, respectively. Data presented as mean+SD or median [25th,75th percentiles] for continuous variables N(%) categorical variables. Results A total 58 patients (62.1% female, age 31.9 yr [18.5, 48.6]) Interval MSLTs 3.6 + 3.18 (same diagnosis both tests: 3.24 3.15; changed diagnosis: 3.98 3.22). Diagnosis 1 NT2 8(13.8%), 20(34.5%) 30(51.7%). Overall, 33(56.9%) remained same diagnostic category including 3(5.2%) 12(20.7%) 18(31.0 %) Normal/Indeterminate. Of 25(43.1%) cases changing diagnosis, 2(3.4%) Normal/Indeterminate; 4(6.9%) 5(8.6%) 7(12.1%) IH. Median MSL 2 10.8 [4.6,12.9] 9.0 [4.5,13.1], Correlation moderate [0.59 (0.39, 0.73), p< 0.001] fair SOREMPs [0.32 (0.07, 0.53), p=0.015]. Intra-class showed [0.56 (0.36, 0.72)] [0.54 (0.33, 0.70)]. Conclusion hypersomnias tertiary care clinic population. While more favorable than prior studies, findings underscore challenges that may contribute misdiagnosis hypersomnia. Support (if any)