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Halitosis is chronic, endogenous malador that is etiologically classified from type 0 to 5—physiologic, oral, airway, gastroesophageal, bloodborne, and subjective, respectively.1 Halitosis measurement methods should cover each of the three malodor sources: the mouth, nose cavity, and breath.
The diagnostic tests currently used in halitosis examination can be divided into direct and indirect (Figure 1). Direct tests include self assesment of the odor or other people’s assesment, and the determination of odorous substances by halitometry or gas chromatography. Indirect tests determine the presence of some bacterial species, metabolic products generated by microorganisms in vitro or their enzymes.
Patients become concerned when either they themselves or others detect the odor at a socially unacceptable level. For this reason self assessment or assessment by other people are prominent among all diagnostic testing.
Halitometers should be used for confirmation of halitosis, comparing similar cases, and monitorizing the therapy effect. This test gives a quantification of the gases but not of the halitosis. Organoleptic examination is highly subjective and has not been standardized. There are many methods and different protocols to examine halitosis, however which among them is most predictable has not been clearly determined.
Self or Other People’s Assessments
Asking individual patients about the level of their own halitosis is the simplest way to gauge how much the patient is affected by the condition. However, there is no statistically significant correlation between self assessment and measurable halitosis levels by objective methods.2,3 This is due to the fact that patients with halitosis may not be aware of the situation, or they become inured to their own malodor over time,4 because olfactory desensitization in objective halitosis patients can cause “false-negative” results in self assesing halitosis.5,6 It has been observed that psychopathological factors can lead to misdiagnosis (false positive) of halitosis.7
Although there are some conflicting findings in the literature, subjective opinion generally correlated well with objective evaluation of halitosis.8-13 Significant associations between self-reported oral malodor, socio-demographic or medical history, and oral hygiene variables were also found.9 In actuality, self assessment is the primary diagnostic criterion for halitosis and is the main reason patients seek treatment for the condition. Their initial visit commonly stems from a complaint by another person or their own suspicion. Therefore, despite the general reluctance of people to mention the halitosis of another out of politeness,14 the most descriptive question of the medical history and the most convincing tool to determine the result of a halitosis treatment is self or other people’s assesment that covers the three sources of malodor. Those patients who self assess their halitosis are usually at least type 5 halitosis patients.
Organoleptic Tests
Studies argue that organoleptic assessments (sniffing the patient’s breath and scoring the level of malodor by an examiner such as a general practitioner, periodontist, or dental hygienist) are regarded as the gold standard for measuring halitosis and are significantly related to volatile sulfur compounds (VSCs)15-17 and amines.18,19 However, volatiles, or odorous gases, that can be detected by the human olfactory system are limited. For instance, the human nose is less sensitive to ammonia,20 though breath that has the smell of ammonia is considered halitosis.14,21 The latter nonstandard preparation and test protocols for organoleptic measurement have been used, but even the scales used for scoring of malodor are often changed.
Before performing organoleptic measurement, antibiotics should be ceased at least 3 weeks prior22; others proscribe them for 4 to 8 weeks.2,23 Some investigators forbid patients to drink, eat, chew, rinse, or smoke for at least 2 hours,24-26 although some extend it to 4 hours, or prohibit odorous foods for 24 or 48 hours before the appointment. Some allow restricted oral hygiene (tooth brushing without using toothpaste) 2 to 3 hours prior to the patient’s appointment, while some prohibit even drinking water for an hour. Some practitioners prefer to examine their patients in the morning, while others favor the afternoon or evening.27 A pre-measurement protocol is not truly standardized thus far.
It is a challenge to ask individuals to fast for hours and not clean their mouth to cause (or exacerbate) halitosis, which is a previously described mechanism for inducing morning breath.1 Even people with no halitosis will have malodor in their mouth under those conditions.
Organoleptic measurement protocols available in the literature also vary. Patients may be asked to exhale either with or without a tube inserted into the mouth,2,23,24 or to count aloud to 10 while their breath is evaluated by the examiner.14 Lips are kept closed for 30 seconds,26 60 seconds,28 2 minutes,27 or 3 minutes,29 after which the oral air is assessed by the examiner from 5 cm to 15 cm,29 5 cm to 10 cm,27 or 10 cm.30,31 Another method is having patients hold their breath for a while before exhaling with their mouth 20 cm from the examiner (a pipette could be used).21
Modified methods of organoleptic assessments have been widely applied and include the following:
• Spoon test—A spoon is sniffed 5 cm away 5 seconds after scraping the dorsum of the tongue.25
• Floss test—A piece of unwaxed floss is sniffed 3 cm away after flossing through the interdental regions of teeth.14
• Salivary odor test—The subject spits out 1 mL to 2 mL of saliva into a glass tube32 or 0.7 mL to 0.8 mL of saliva into a petri dish,33 after which it may or may not be incubated at 37°C for 5 minutes, then evaluated from a distance of 4 cm.34
• Glass rod test—A glass rod (15 cm x 0.5 cm) test consists of sniffing the saliva from a distance of 2.5 cm away after the saliva sample has been inserted and stirred three times.34
These four aforementioned tests are categorized as organoleptic.35 Other such assessments include:
• Wrist-licking test—This consists of sniffing the wrist from a distance 3 cm after licking it and waiting for 524 or 1014 seconds.
• Tongue-coating test—A 2 cm x 2 cm piece of gauze—some use a scraper, periodontal probe, or dental floss—is applied to the dorso-posterior surface of the tongue and drawn anteriorly for 2 cm to 3 cm, then immediately evaluated.29
• Prosthesis test—If the patient wears a removable denture, the prosthesis odor can be scored.14
• Tonsil test—This involves subjectively assessing the odor of the tonsils.36
It is known that some odorants (eg, indole, methylamine, and cadaverine) do not increase malodor when added to bulk saliva24 because they are resolved in the dental plaque37; however, they become detectable when saliva is dried or agitated.15,38 Some organoleptic tests (eg, floss, spoon, gauze, wrist licking) are often found to be false-positive, even in patients with no pathologic halitosis, so their diagnostic values are questionable.
The severity of the odor sniffed by the examiner is usually numerically scored from less strong to extremely strong. Some investigators use a 3-point,28 4-point,26,34,39 5-point,23,40 6-point,15 or 10-point15 scale; even half-scores are used occasionally. Nonstandardized scaling of organoleptic scoring will obviously cause misinterpretation of findings.
That 50% of patients feel embarrassed or otherwise dislike the testing process1 may also be a disadvantage of organoleptic testing (n = 283). To minimize this issue, indiviudals can instead be asked to breathe inside a plastic bag, which the examiner sniffs afterwards.21 Sometimes, a privacy screen is used to hide the direct-sniffing contact from the patients, who assume that they have undergone a specific malodor examination instead.23 Some examiners place a non-transparent wall with a hole to separate patients.41 A syringe method is recommended to obtain a higher degree of privacy for the patient.22,42 A questionnaire by the authors asked dental practioners (n = 151) this question: “Do you smell the breath of your halitosis patients?” A total of 133 practitioners (88%) declined to smell patients’ breath because they found it unpleasant.
Variable factors (eg, age, gender, individual odor memory of examiner, time of day, temperature and humidity of room air, etc.) affect organoleptic scoring, which makes the organoleptic assessment unreliable and, therefore, not reproducible.15,43 Further, assessment is extremely subjective, emotional, instinctive, learnable, intuitive, and is also subject to the socioeconomic background or examiner’s experiences; also, there is not yet international calibration44 and standardization.
Chemical and Enzymatic Tests
Some studies45,46 hypothesized a potential relationship between halitosis and particular bacterial species, but others found no clear association.47 The authors consider merely labeling bacteria as either odorigenic or not to be an oversimplification, given that every bacterium is odorigenic.48,49 Nevertheless, there are tests that colorimetrically indicate oral bacteria or their by-products in the mouth.
Beta-galactosidase test—β-galactosidase is an enzyme that catalyzes the hydrolysis of lactose to monosaccharides. It is only synthesized by lactose-positive bacterial species (eg, Peptostreptococcus productus IIa or Actinomyces denticolens). The β-galactosidase test detects an enzyme that may be related to odorigenic bacteria present in the medium. Although β-galactosidase activity has been correlated with malodor,50 it can be associated with physiologic halitosis, which is not necessarily associated with oral problems,51 nor truly reflects halitosis level.52
Indole test—Indole, ammonia, and pyruvate are the result of the deaminating process of tryptophan by tryptophanase. Indole is methylated to skatole. Each of these components have a bad odor. The indole test exhibits the presence of indole, which is one of the odorous compounds in halitosis,34 but no clear correlation was found between odor concentrations and the indole amounts.38
Ninhydrin test—Low molecular-weight amines and amino-acid levels may provide information on halitosis caused from bacterial putrefaction. The ninhydrin test is a simple, rapid enzymatic test that can provide information on halitosis-related bacterial putrefaction.8 It can be used qualitatively (eg, for chromatographic visualization) or quantitatively (eg, for peptide sequencing). Typically, α-amino acids give a blue-purple product, whereas proline, a secondary amine, gives a yellow-orange product.
Lead acetate test—The lead acetate test is also used to detect sulfurs present in the medium. For this test, saliva taken from a patient is incubated overnight26 or 30 minutes29 in an agar plate containing Pb-acetate (0.02%); afterwards, a black color indicates sulfur content. If there were a method to instantly quantify sulfur content, it would make it possible to more accurately estimate the VSC content of saliva.
BANA test—A hydrolase enzyme of hydrolyzing benzoyl-DL-arginine-naphthylamide (BANA) is present on commercially available test strips. BANA strips turn blue, indicating a positive result, if the three particular bacteria—Porphyromonas gingivalis, Treponema denticola, and Tannerella forsythia—are present in the medium with this hydrolase.53,54 The specificity and sensitivity of the BANA test is over 80%, and its predictability for periodontal disease is greater than 83%.55 However, these three bacteria are not the unique oral species that have the BANA hydrolase enzyme. Bacteria identification software,56 using its library, was able to find 37 more bacteria that can hydrolyze arginine, showing the BANA test to be unreliable.
It is important to note that the above chemical and enzymatic tests are used to diagnose only malodor of oral origin by searching for the presence of bacteria or their products, but cannot be used to diagnose nasal or alveolar malodor.
There are other enzymatic methods for diagnosing halitosis that principally use base peptidase enzymes of the three aforementioned bacteria and of some Capnocytophaga strains.57 Other methods utilize colorimetric hydrogen sulfide sensors, engineered both as an optical fiber, capable of measuring reflectance change of an immobilized reagent,58 or as thin reactive films of chromophores.59
A bioelectronic nose, capable of detecting the oxygen consumption induced by an enzymatic reaction with methyl sulfide, has also been developed.60 Detected levels are in the range of 0.2 µg/l to 0.4 µg/l.61
Stress or unsuitable environmental conditions compel bacteria to unpredictable metabolic pathways. Therefore, all of the above chemical and enzymatic test results may vary from ancestral expectation of those bacteria. This is another reason why chemical and enzymatic tests may not be descriptive enough for halitosis diagnosis.
Halitometry
Gas chromatography (GC) is highly sensitive for VSCs but impractical.62 A GC-based portable device (OralChroma™, FIS Inc.) is capable of quantifying sulfur family gases. Other portable halitometers include: Halimeter® (Interscan), Breathtron® (New Cosmos Electric Co., LTD.), the Twin Breasor™ (GC Co.), Probe/Perio (Diamond General Development Corporation), and B/B Checker® (Taiyo).
Some authors63 found 3,481 VOCs in the mouth or alveolar air of healthy subjects, while others64 found 400 to 700. Sulphide detectors cannot cover all of these gases. There is a need for new halitometers to detect sulfur, nitrogen, and organic-based gases (at least H2S, NH3, VOC, H2). There are industrial portable gas detectors capable of detecting more than four gas groups that could potentially be utilized.1 Sensor systems for monitoring the simple gases with a breath test kit (BreathTracker, QuinTron), electronic noses like FF-2A (Shimadzu) and Cyranose® 320 (Sensigent), and a bioelectronic gas sensor to detect sulfur family and trimethyl amine have been developed to measure halitosis.65 Some of them use chemical sensor arrays for the detection of the odorant profile (halitoprint) based on an algorithm,66 which covers many gases.
Halitometers are generally used to detect orally originated malodor. They are only occasionally used for alveolar or nasal malodors, depending on the measurement method used. The halitometers mentioned can detect gases but not halitosis, and should be used only for confirmation, comparing similar cases, and monitoring the therapy of halitosis; they should not be used for diagnostic purposes alone because halitometric readings are meaningless in the absense of a halitosis complaint.
Conclusion
Patients typically consult a healthcare professional for halitosis when the malodor is detected by themselves or they received feedback from others in their social environment. Organoleptic test methods, including directly sniffing oral air or indirectly sniffing a sample, such as floss or a spoon, are subjective and not reproducible. Chemical and enzymatic methods briefly estimate presence of bacteria or their enzymes but do not prove halitosis. Halitometric assesment, especially multigas detecting systems, are very objective and reproducible and can detect odorous gases in a wide spectrum; however, if the patient does not complain about halitosis, then halitometeric readings are of little value since there seems to be no problem. Therefore, halitometers should be used for confirmation of halitosis, comparing similar cases, and monitorizing the therapy, but not for diagnositic purposes alone.
Diagnosing and resolving this condition is important to patients’ self esteem and quality of life. Therefore, it is important to give strong consideration to the patient’s self assessment, which may include feedback from others in their social environment, rather than solely rely on objective examination methods such as halitometer readings or testing the enzymatic activity of the saliva.
Disclosure
The authors had no disclosures to report.
About the Authors
Murat Aydin, DDS, PhD
Doctor of Philosophy in Microbiology
Halitorium International Halitosis Research Group
Turkey
Private Practice
Adana, Turkey
Curd M.L. Bollen, DDS, PhD, MSc
Director, Mondcentrum Parimplant
Department of Periodontology USA
BioCore, Richmond, Virginia
Murat Eren Özen, MD
Psychiatrist
Private Adana Hospital
Adana, Turkey
Halitorium International Halitosis Research Group
Turkey
Queries to the author regarding this course may be submitted to authorqueries@aegiscomm.com.
References
1. Aydin M, Harvey-Woodworth CN. Halitosis: a new definition and classification. Br Dent J. 2014;217(1):E1.
2. Brunner F, Kurmann M, Filippi A. The correlation of organoleptic and instrumental halitosis measurements. Schweiz Monatsschr Zahnmed. 2010;120(5):402-408.
3. Bornstein MM, Kislig K, Hoti BB, et al. Prevalence of halitosis in the population of the city of Bern, Switzerland: a study comparing self-reported and clinical data. Eur J Oral Sci. 2009,117(3):261-267.
4. Iwakura M, Yasuno Y, Shimura M, Sakamoto S. Clinical characteristics of halitosis: differences in two patient groups with primary and secondary complaints of halitosis. J Dent Res. 1994;73(9):1568-1574.
5. Eli I, Baht R, Koriat H, Rosenberg M. Self-perception of breath odor. J Am Dent Assoc. 2001;132(5):621-626.
6. Altundag A, Cayonu M, Kayabasoglu G, et al. The evaluation of olfactory function in individuals with chronic halitosis. Chem Senses. 2015; 40(1):47-51.
7. Suzuki N, Yoneda M, Naito T, et al. Association between oral malodour and psychological characteristics in subjects with neurotic tendencies complaining of halitosis. Int Dent J. 2011;61(2):57-62.
8. Iwanicka-Grzegorek E, Michalik J, Kepa J, et al. Subjective patients’ opinion and evaluation of halitosis using halimeter and organoleptic scores. Oral Dis. 2005;11 suppl 1:86-88.
9. Al-Ansari JM, Boodai H, Al-Sumait N, et al. Factors associated with self-reported halitosis in Kuwaiti patients. J Dent. 2006;34(7):444-449.
10. Rosenberg M, Kozlovsky A, Wind Y, Mindel E. Self-assessment of oral malodor 1 year following initial consultation. Quintessence Int. 1999;30(5):324-327.
11. Özen ME, Aydin M. Subjective halitosis: definition and classification. J N J Dent Assoc. 2015;86(4):20-24.
12. Greenstein RB, Goldberg S, Marku-Cohen S, et al. Reduction of oral malodor by oxidizing lozenges. J Periodontol. 1997;68(12):1176-1181.
13. Pham TA, Ueno M, Shinada K, Kawaguchi Y. Comparison between self-perceived and clinical oral malodor. Oral Surg Oral Med Oral Pathol Oral Radiol. 2012;113(1):70-80.
14. Bollen CM, Beikler T. Halitosis: the multidisciplinary approach. Int J Oral Sci. 2012;4(2):55-63.
15. Rosenberg M, McCulloch CA. Measurement of oral malodor: current methods and future prospects. J Periodontol. 1992;63(9):776-782.
16. Tanaka M, Anguri H, Nishida N, et al. Reliability of clinical parameters for predicting the outcome of oral malodor treatment. J Dent Res. 2003;82(7):518-522.
17. Chen X, Ye W, Feng XP. [The relationship between two halitosis diagnostic methods: organoleptic test and VSCs measurement by a portable sulfide detector]. Shanghai Kou Qiang Yi Xue. 2006;15(6):575-577.
18. Van den Velde S, van Steenberghe D, Van Hee P, Quirynen M. Detection of odorous compounds in breath. J Dent Res. 2009;88(3):285-289.
19. Dadamio J, van Tornout M, Van den Velde S, et al. A novel and visual test for oral malodour: first observations. J Breath Res. 2011;5(4):046003.
20. Amano A, Yoshida Y, Oho T, Koga T. Monitoring ammonia to assess halitosis. Oral Surg Oral Med Oral Pathol Oral Radiol Endod. 2002;94(4):692-696.
21. Aylıkcı BU, Çolak H. Halitosis: From diagnosis to management. J Nat Sci Biol Med. 2013;4(1):14-23.
22. Seemann R, Conceicao MD, Filippi A, et al. Halitosis management by the general dental practitioner—results of an international consensus workshop. J Breath Res. 2014;8(1):017101.
23. Miyazaki H, Arao M, Okamura K, et al. Tentative classification of halitosis and its treatment needs. Niigata Dent J. 1999;32:7-11.
24. Goldberg S, Kozlovsky A, Gordon D, et al. Cadaverine as a putative component of oral malodor. J Dent Res. 1994;73(6):1168-1172.
25. Rosenberg M. Clinical assessment of bad breath: current concepts. J Am Dent Assoc. 1996;127(4):475-482.
26. Washio J, Sato T, Koseki T, Takahashi N. Hydrogen sulfide-producing bacteria in tongue biofilm and their relationship with oral malodour. J Med Microbiol. 2005;54(pt 9):889-895.
27. Greenman J, Lenton P, Seemann R, Nachnani S. Organoleptic assessment of halitosis for dental professionals—general recommendations. J Breath Res. 2014;8(1):017102.
28. Nachnani S. Efficacy of zinc chloride based mouth rinse (BreathRx) compared to chlorine dioxide based mouth rinse (Oxyfresh). Discus Dental–Clinical Data and Safety. 2000. http://www.halitosis-research.com/Zinc_Chloride_Compared_to_Chlorine_Dioxide_Mouthrinse.html. Accessed Januray 26, 2015.
29. Richter JL. Diagnosis and treatment of halitosis. Compend Contin Educ Dent. 1996;17(4):370-376.
30. Rosenberg M, Kulkarni V, Bosy A, McCulloch CA. Reproducibility and sensitivity of oral malodor measurements with a portable sulphide monitor. J Dent Res. 1991;70(11):1436-1440.
31. Rosenberg M, Septon I, Eli I, et al. Halitosis measurement by an industrial sulphide monitor. J Periodontol. 1991;62(8):487-489.
32. Yaegaki K, Sanada K. Biochemical and clinical factors influencing oral malodor in periodontal patients. J Periodontol. 1992;63(9):783-789.
33. Weinberg M. Halitosis: the ‘bad breath’ syndrome. US Pharmacist. 2001;26(3):46-57.
34. Copidilly DP, Kaufman HW, Kleinberg I. Use of a novel group of oral malodor measurements to evaluate an anti-oral malodor mouthrinse (TriOral™) in humans. J Clin Dent. 2004;15(4):98-104.
35. Ongole R, Shenoy N. Halitosis: much beyond oral malodor. Kathmandu Univ Med J. 2010;8(30):269-275.
36. Ferguson M, Aydin M, Mickel J. Halitosis and the tonsils: a review of management. Otolaryngol Head Neck Surg. 2014;151(4):567-574.
37. Hayes ML, Hyatt AT. The decarboxylation of amino acids by bacteria derived from human dental plaque. Arch Oral Biol. 1974;19(5):361-369.
38. Tonzetich J. Oral malodour: an indicator of health status and oral cleanliness. Int Dent J. 1978;28(3):309-319.
39. Schmidt NF, Missan SR, Tarbet WJ. The correlation between organoleptic mouth-odor ratings and levels of volatile sulfur compounds. Oral Surg Oral Med Oral Pathol. 1978;45(4):560-567.
40. Frascella J, Gilbert RD, Fernandez P, Hendler J. Efficacy of a chlorine dioxide-containing mouthrinse in oral malodor. Compend Contin Educ Dent. 2000;21(3):241-248.
41. Wigger-Alberti W, Gysen K, Axmann EM, Wilhelm KP. Efficacy of a new mouthrinse formulation on the reduction of oral malodour in vivo. A randomized, double-blind, placebo-controlled, 3 week clinical study. J Breath Res. 2010;4(1):017102.
42. Kim DJ, Lee JY, Kho HS, et al. A new organoleptic testing method for evaluating halitosis. J Periodontol. 2009;80(1):93-97.
43. Mantini A, Di Natale C, Macagnano A, et al. Biomedical application of an electronic nose. Crit Rev Biomed Eng. 2000;28(3-4):481-485.
44. Quirynen M, Zhao H, Avontroodt P, et al. A salivary incubation test for evaluation of oral malodor: a pilot study. J Periodontol. 2003;74(7):937-944.
45. Awano S, Gohara K, Kurihara E, et al. The relationship between the presence of periodontopathogenic bacteria in saliva and halitosis. Int Dent J. 2002;52 suppl 3:212-216.
46. Kazor CE, Mitchell PM, Lee AM, et al. Diversity of bacterial populations on the tongue dorsa of patients with halitosis and healthy patients. J Clin Microbiol. 2003;41(2):558-563.
47. Donaldson AC, McKenzie D, Riggio MP, et al. Microbiological culture analysis of the tongue anaerobic microflora in subjects with and without halitosis. Oral Dis. 2005;11 suppl 1:61-63.
48. Aydin M. Halitosis. In: Aydin M, Mısırlıgil A, eds. Oral Microbiology. Ankara, Turkey: MN Medical & Nobel; 2012:97-104.
49. Inglis TJ, Hahne DR, Merritt AJ, Clarke MW. Volatile-sulfur-compound profile distinguishes Burkholderia pseudomallei from Burkholderia thailandensis. J Clin Microbiol. 2015;53(3):1009-1011.
50. Sterer N, Shaharabany M, Rosenberg M. β-Galactosidase activity and H2S production in an experimental oral biofilm. J Breath Res. 2009;3(1):016006.
51. Yoneda M, Masuo Y, Suzuki N, et al. Relationship between the β-galactosidase activity in saliva and parameters associated with oral malodor. J Breath Res. 2010;4(1):017108.
52. Aydin M. Odorigenic bacteria. In: Halitosis. Istanbul, Turkey: Nobel Medical; 2008:65-82.
53. Loesche WJ, Lopatin DE, Giordino J, et al. Comparison of the benzoyl-DL-arginine-naphthylamide (BANA) test, DNA probes, and immunological reagents for ability to detect anaerobic periodontal infections due to Porphyromonas gingivalis, Treponema denticola, and Bacteroides forsythus. J Clin Microbiol. 1992;30(2);427-433.
54. Bretz WA, Lopatin DE, Loesche WJ. Benzoyl-arginine naphthylamide (BANA) hydrolysis by Treponema denticola and/or Bacteroides gingivalis in periodontal plaques. Oral Microbiol Immunol. 1990;5(5):275-279.
55. Schmidt EF, BretzWA, Hutchinson RA, Loesche WJ. Correlation of the hydrolysis of benzoyl-arginine naphthylamide (BANA) by plaque with clinical parameters and subgingival levels of spirochetes in periodontal patients. J Dent Res. 1988;67(12):1505-1509.
56. Aydin M, Günay İ, Köksal F, Serin MS. [Taxometry and computerization in bacterial identification]. Mikrobiyol Bült. 1996;30:281-287.
57. Seida K, Saito A, Yamada S, et al. A sensitive enzymatic method (SK-013) for detection of Treponema denticola, Porphyromonas gingivals and Bacteroides forsythus in subgingival plaque samples. J Periodontal Res. 1992;27(2):86-91.
58. Rodriguez-Fernández J, Pereiro R, Sanz-Medel A. Optical fibre sensor for hydrogen sulphide monitoring in mouth air. Anal Chim Acta. 2002;471(1):13-23.
59. Wallace KJ, Cordero SR, Tan CP, et al. A colorimetric response to hydrogen sulfide. Sensors and Actuators B: Chemical. 2007;120(2):362-367.
60. Mitsubayashi K, Minamide T, Otsuka K, et al. Optical bio-sniffer for methyl mercaptan in halitosis. Anal Chim Acta. 2006;573-574:75-80.
61. Alagirisamy N, Hardas SS, Jayaraman S. Novel colorimetric sensor for oral malodour. Anal Chim Acta. 2010;661(1):97-102.
62. Ciaffoni L, Peverall R, Ritchie GA. Laser spectroscopy on volatile sulfur compounds: possibilities for breath analysis. J Breath Res. 2011;5(2):024002.
63. Phillips M, Herrera J, Krishnan S, et al. Variation in volatile organic compounds in the breath of normal humans. J Chromatogr B Biomed Sci Appl. 1999;729(1-2):75-88.
64. Spielman AI. Chemosensory function and dysfunction. Crit Rev Oral Biol Med. 1998;9(3):267-291.
65. Minamide T, Mitsubayashi K, Jaffrezic-Renault N, et al. Bioelectronic detector with monoamine oxidase for halitosis monitoring. Analyst. 2005;130(11):1490-1494.
66. Shykhon ME, Morgan DW, Dutta R, et al. Clinical evaluation of the electronic nose in the diagnosis of ear, nose and throat infection: a preliminary study. J Laryngol Otol. 2004;118(9):706-709.
