Salivary Flow Rate in Patients with Kidney Failure on Hemodialysis—A Systematic Review and Meta-Analysis

Background/Objectives: During kidney failure, chronic hemodialysis therapy (HD) is required to replace lost renal function, and patients on regular HD frequently report xerostomia. This systematic review and meta-analysis aims to compare salivary flow rates between patients with kidney failure on HD and healthy controls and to evaluate acute changes in salivary secretion before and after a dialysis session. Methods: A systematic review was conducted in accordance with PRISMA guidelines. PubMed, Web of Science, and Embase were searched for observational studies quantifying salivary flow rates in adult patients with kidney failure on chronic hemodialysis versus healthy controls or pre- versus post-dialysis. Data on salivary flow rates were extracted and stratified by subtype (whole or gland-specific) and condition (stimulated or unstimulated), along with key study characteristics including participant demographics, saliva collection methods, and dialysis duration. Study quality was appraised using NHLBI tools and categorized as poor, fair, or good. Where ≥2 homogeneous datasets existed, random-effects meta-analyses (α = 0.05) were performed to estimate mean differences (95% CI) for each salivary parameter; heterogeneity was evaluated via I2. Results: A total of 20 studies (13 cross-sectional, 7 before-after) met inclusion, of which 17 studies (with a total of 1224 HD patients and 548 controls) were meta-analyzed. Compared with controls, HD patients showed lower secretion rates of unstimulated whole saliva (UWS: MD −0.11 mL/min; 95% CI −0.20 to −0.02; I2 = 94%) and stimulated whole saliva (SWS: MD −0.77 mL/min; 95% CI −0.94 to −0.60; I2 = 92%), whereas stimulated parotid saliva (SPS) did not differ significantly (MD −0.08 mL/min; 95% CI −0.77 to 0.60; I2 = 96%). In before-after analyses, both UWS (MD +0.15 mL/min; 95% CI 0.02–0.28; I2 = 90%) and SWS (MD +0.20 mL/min; 95% CI 0.14–0.26; I2 = 0%) increased immediately post-HD. Conclusions: Despite methodological challenges and population heterogeneity, the evidence indicates salivary hypofunction in HD patients and improvement after hemodialysis. The magnitude of these effects seems influenced by underlying comorbidities (notably diabetes), HD duration, and methodological factors. Since saliva is of major importance to maintaining good oral health, recognizing and managing dry mouth should therefore be part of the comprehensive care of patients with kidney failure.