It is reasonable to associate the feeling of oral dryness with a lack of saliva. Yet, this relationship is not exact. Many patients who complain of oral dryness, do indeed, demonstrate reduced salivary flow. Others, do not. The converse is also true. Many patients, with significant reductions in the flow rate of saliva do, as one might expect, complain of dry mouth. But some do not. Since oral dryness is a subjective complaint, it is not surprising that there is a huge variation in the patient's threshold of discomfort. Salivary flow rates are rarely determined in clinical practice, so baseline rates of flow of saliva for individual patients are generally not known. As a consequence, it is not possible to determine if there has been a reduction in their salivary flow.

However, results obtained from a number of clinical studies show that there is a 25 to 50% chance that a patient who claims that her or his mouth is dry, will have a rate of flow of whole saliva which is below that which is generally accepted as normal. This likelihood increases to about 80% if about 3-4 symptoms of desiccation are concomitantly present. Since oral dryness and salivary gland hypofunction are indicators of systemic diseases, it is important that "symptom-surveys" and sialometric tests should be performed on all patients who complain of xerostomia.

The flow rate of saliva varies widely from person to person. However, when the conditions under which salivary samples are obtained are uniform, the flow is remarkably stable for each individual. High salivary flow rates are generally associated with oral health; low rates of flow, with oral disease. The normal and reference values are shown below (Table 5). The values cited for "Abnormal Flow Rates" should not be viewed as absolute or fixed. Rather, they should "raise suspicion" about the presence of disease.