A stimulated saliva flow rate less than 0.5 ml per gland in 5 minutes or less than 1 ml per gland in 10 minutes is decreased. The term subjective xerostomia is sometimes used to describe the symptom in the absence of any clinical evidence of dryness. Xerostomia may also result from a change in composition of saliva (from serous to mucous). Salivary gland dysfunction is an umbrella term for the presence of xerostomia, salivary gland hyposalivation, and hypersalivation. Signs and symptoms
Hyposalivation may give the following signs and symptoms:

Dental caries (xerostomia related caries) – Without the buffering effects of saliva, tooth decay becomes a common feature and may progress much more aggressively than it would otherwise ("rampant caries"). It may affect tooth surfaces that are normally spared, e.g., cervical caries and root surface caries. This is often seen in patients who have had radiotherapy involving the major salivary glands, termed radiation-induced caries. Therefore, it is important that any products used in managing dry mouth symptoms are sugar-free, as the presence of sugars in the mouth support the growth of oral bacteria, resulting in acid production and development of dental caries. Acid erosion. Saliva acts as a buffer and helps to prevent demineralization of teeth. Oral candidiasis – A loss of the antimicrobial actions of saliva may also lead to opportunistic infection with Candida species. Ascending (suppurative) sialadenitis – an infection of the major salivary glands (usually the parotid gland) that may be recurrent.
An injury to the face or surgery can cause nerve damage to your head and neck area which can effect the nerves that are associated with the salivary flow.Xerostomia may be a consequence of infection with hepatitis C virus (HCV) and a rare cause of salivary gland dysfunction may be sarcoidosis.Infection with Human Immunodeficiency Virus/Acquired immunodeficiency Syndrome (AIDS) can cause a related salivary gland disease known as Diffuse Infiltrative Lymphocytosis Syndrome (DILS).Similar to taste dysfunction, xerostomia is one of the most prevalent and persistent oral symptoms associated with COVID-19. Despite a close association with COVID-19, xerostomia, dry mouth and hyposalivation tend to be overlooked in COVID-19 patients and survivors, unlike ageusia, dysgeusia and hypogeusia. Diagnostic approach
A diagnosis of hyposalivation is based predominantly on the clinical signs and symptoms. The Challacombe scale maybe used to classify the extent of dryness. The rate of the salivary flow in an individuals mouth can also be measured. There is little correlation between symptoms and objective tests of salivary flow, such as sialometry. This test is simple and noninvasive, and involves measurement of all the saliva a patient can produce during a certain time, achieved by dribbling into a container. Sialometery can yield measures of stimulated salivary flow or unstimulated salivary flow. Stimulated salivary flow rate is calculated using a stimulant such as 10% citric acid dropped onto the tongue, and collection of all the saliva that flows from one of the parotid papillae over five or ten minutes.