What Is the Normal Salivary Flow Rate in Healthy Adults? A Systematic Review with Meta-Analyses

Area of Science:

• Oral Medicine
• Biostatistics
• Public Health

Background:

• Established reference values for salivary flow rates are decades old and may not reflect current population norms.
• Hyposalivation-associated conditions (HACs) impact oral health, necessitating accurate diagnostic criteria.
• Contemporary populations may exhibit different salivary flow rates than those used in historical reference values.

Purpose of the Study:

• To establish current average unstimulated (USF) and stimulated (SSF) salivary flow rates in healthy adults.
• To compare salivary flow rates in healthy individuals versus those with HACs.
• To provide updated reference values for defining salivary normality.

Main Methods:

• Systematic literature search across 8 databases and gray literature for studies on adult salivary flow rates (18-60 years).
• Meta-analysis of 63 studies (3,167 healthy, 2,470 HAC individuals) to determine average USF and SSF in healthy adults.
• Pairwise meta-analyses to compare salivary flow between healthy controls and individuals with HACs.

Main Results:

• Average USF in healthy adults was 0.82 mL/min (95% CI: 0.63-1.01) and average SSF was 1.48 mL/min (95% CI: 1.31-1.65).
• Individuals with HACs showed significantly lower salivary flow: mean difference of 0.43 mL/min for SSF (P=0.03) and 0.42 mL/min for USF (P=0.004) compared to healthy controls.
• Heterogeneity in results was not explained by latitude, study year, or methodological quality.

Conclusions:

• Current average salivary flow rates, particularly USF, appear higher than previously established reference values.
• The lower limits of the confidence intervals for healthy salivary flow could serve as new thresholds for normality.
• A difference of approximately 0.4 mL/min in salivary flow may be clinically significant when distinguishing between healthy individuals and those with HACs.