Dry Mouth by Report or by Measure? Comparison of Fox and Xerostomia Inventory Scores with Sialometry

BACKGROUND: Despite its high prevalence, xerostomia remains frequently misunderstood and often incorrectly assumed to be synonymous with reduced salivary flow. In reality, these two terms- xerostomia and hyposalivation - are not interchangeable. We aimed to quantify how Fox questionnaire and the Xerostomia Inventory (XI) relate to unstimulated salivary flow, identify the most informative items, and evaluate whether total scores and item‑based models discriminate clinically relevant hyposalivation thresholds and treatment‑related deterioration. METHODS: We analyzed 101 adults with overactive bladder contributing 202 paired assessments (fixed order: off‑treatment baseline, on‑treatment follow‑up). Xerostomia was assessed using the Fox questionnaire (0-10) and Xerostomia Inventory (XI; 11-55). Unstimulated salivary flow (mL/min) was measured with the 5‑min spitting method. Associations between questionnaire scores and salivary flow were summarized using Pearson correlations. Discrimination for hyposalivation thresholds (<0.10, <0.066, <0.033 mL/min) was evaluated using ROC/AUC with cluster bootstrap 95% CIs; item‑based screening was explored using LASSO with patient‑level cross‑validation. RESULTS: Continuous associations: Fox scores were more strongly associated with salivary flow than XI (r = -0.356 vs -0.160), while Fox and XI totals were strongly correlated (r = 0.71). Threshold discrimination: for hyposalivation <0.10 mL/min, AUC was 0.704 for Fox and 0.586 for XI. A sparse XI item model (six items) improved discrimination compared with the XI total score (CV‑AUC 0.709 vs 0.580). During treatment, 29/101 participants met the deterioration endpoint (≥50% reduction in flow and/or incident hyposalivation). CONCLUSIONS: Questionnaire scores showed limited criterion validity for objective hypofunction. Fox performed better than XI for identifying hyposalivation, and item‑focused models were promising but exploratory. We propose a pragmatic diagnostic approach in which questionnaires quantify symptom burden and triage risk, while sialometry confirms hyposalivation when clinically relevant.