The primary parameter used to differentiate between normal salivary secretion and xerostomia is the salivary flow rate.

- Salivary flow rate measures the volume of saliva produced per unit time, typically expressed in mL/min.
- In a healthy individual, the unstimulated salivary flow rate usually ranges between 0.3 to 0.4 mL/min.
- A flow rate of less than 0.1 mL/min is considered indicative of hyposalivation, which is often associated with xerostomia (the subjective feeling of dry mouth).
- Other parameters such as pH, amylase concentration, or electrolyte composition can provide additional information on saliva quality but are not reliable primary indicators for distinguishing normal secretion from xerostomia.

Thus, the quantitative measurement of salivary flow rate remains the most objective and clinically relevant method to assess salivary gland function and diagnose xerostomia.

Reference: Textbook of Oral Physiology, Volume 2, Chapter 10, Page 154