Comprehensive Evaluation of Night-to-Night Variability in PSG Metrics and AHI–Based Diagnostic Reclassification

BACKGROUND: Night-to-night variability (NtNV) in polysomnography (PSG) contributes to diagnostic uncertainty in obstructive sleep apnea (OSA), yet multi-metric evaluations using closely spaced PSG nights-particularly in moderate-to-severe disease-remain limited. The comparative stability of apnea-hypopnea index (AHI) definitions, hypoxic burden (HB), and threshold calibration remains unclear. RESEARCH QUESTION: What is the NtNV across PSG-derived metrics, and how do AHI scoring definitions and threshold calibration influence diagnostic stability in OSA? STUDY DESIGN AND METHODS: We performed a retrospective analysis of a prospective study including 147 participants with prior diagnosis or high pretest likelihood of moderate-to-severe OSA who underwent two PSGs within 10 days. NtNV was quantified across 20 PSG-derived metrics. A normalized NtNV matrix was analyzed using PCA followed by unsupervised k-means clustering to identify data-driven variability-pattern groups. Diagnostic stability was compared across AHI definitions (3%/arousal vs. 4%) and HB risk categories. Statistical calibration models derived AHI 4% thresholds aligned with AHI 3%/arousal severity cutpoints. In the PCA followed by k-means analysis, respiratory event frequency metrics contributed most strongly and separated participants into lower- and higher-respiratory-variability pattern groups. AHI 4% showed higher classification disagreement than AHI 3%/arousal in short-interval (29.9% vs. 21.2% overall; 14.3% vs. 5.4% at the moderate-to-severe threshold) and longitudinal comparisons (45.9% vs. 31.1%; 20.9% vs. 8.2%). HB showed low inter-night disagreement (11.8%). Calibration models aligned AHI 4% thresholds of 6.1-6.9 and 18.4-22.3 events/h with AHI 3%/arousal cutpoints of 15 and 30 events/h. INTERPRETATION: Positional, autonomic, and sleep architecture metrics showed the highest NtNV; respiratory event frequency metrics were intermediate and oxygenation most stable. Greater classification disagreement with AHI 4% was threshold-driven, with implications for hypopnea scoring, and payer policy in OSA diagnosis.