β-galactosidase test: salivary levels of this enzyme were found to be correlated with oral malodour.Although such instrumentation and examinations are widely used in breath clinics, the most important measurement of bad breath (the gold standard) is the actual sniffing and scoring of the level and type of the odour carried out by trained experts ("organoleptic measurements"). The level of odour is usually assessed on a six-point intensity scale. Classification
Two main classification schemes exist for bad breath, although neither are universally accepted.The Miyazaki et al. classification was originally described in 1999 in a Japanese scientific publication,  and has since been adapted to reflect North American society, especially with regards halitophobia. The classification assumes three primary divisions of the halitosis symptom, namely genuine halitosis, pseudohalitosis and halitophobia. This classification has been suggested to be most widely used, but it has been criticized because it is overly simplistic and is largely of use only to dentists rather than other specialties. Genuine halitosis
A. Physiologic halitosis
B. Pathologic halitosis
(i) Oral
(ii) Extra-oral
Pseudohalitosis
HalitophobiaThe Tangerman and Winkel classification was suggested in Europe in 2002. This classification focuses only on those cases where there is genuine halitosis, and has therefore been criticized for being less clinically useful for dentistry when compared to the Miyazaki et al. classification. Intra-oral halitosis
Extra-oral halitosis
A.
This is accomplished by subgingival scaling and root planing and irrigation with an antibiotic mouth rinse. The bacteria that cause gingivitis and periodontal disease (periodontopathogens) are invariably gram negative and capable of producing VSC. Methyl mercaptan is known to be the greatest contributing VSC in halitosis that is caused by periodontal disease and gingivitis. The level of VSC on breath has been shown to positively correlate with the depth of periodontal pocketing, the number of pockets, and whether the pockets bleed when examined with a dental probe. Indeed, VSC may themselves have been shown to contribute to the inflammation and tissue damage that is characteristic of periodontal disease. However, not all patients with periodontal disease have halitosis, and not all patients with halitosis have periodontal disease. Although patients with periodontal disease are more likely to develop halitosis than the general population, the halitosis symptom was shown to be more strongly associated with degree of tongue coating than with the severity of periodontal disease. Another possible symptom of periodontal disease is a bad taste, which does not necessarily accompany a malodour that is detectable by others. Other causes
Other less common reported causes from within the mouth include:
Deep carious lesions (dental decay) – which cause localized food impaction and stagnation
Recent dental extraction sockets – fill with blood clot, and provide an ideal habitat for bacterial proliferation
Interdental food packing – (food getting pushed down between teeth) - this can be caused by missing teeth, tilted, spaced or crowded teeth, or poorly contoured approximal dental fillings.