Exhaled volatile organic compounds for diagnosis and monitoring of asthma

Table 1 Studies that have used gas chromatography-mass spectrometry⁴ to detect volatile organic compounds in asthma

Journal	Title	Main results	Ref.
Metabolites	Real Time Breath Analysis Using Portable Gas Chromatography for Adult Asthma Phenotypes	Pilot study, the first one to use a portable GC device (30 min analysis) that was coupled to a MS succeeded to distinguish by alkalin VOCs different subjecst: 30 asthma, 8 atopic non-asthmatic and 35 non-asthma/non-atopic and their subgroups1	Sharma et al[29], 2021
European Respiratory Journal	Exhaled volatile organic compounds as markers for medication use in asthma	This is the inaugural investigation exploring exhaled VOCs in conjunction with drug utilization as detected by urinary metabolites in asthma. The study encompassed 78 adult patients with severe asthma, demonstrating the potential for detecting VOCs to monitor therapy, in this instance salbutamol and OCS2	Brinkman et al[43], 2020
American Journal of Respiratory and Critical Care Medicine	Exhaled Volatile Organic Compounds Are Able to Discriminate between Neutrophilic and Eosinophilic Asthma	The first study to provide surrogate markers for neutrophilic asthma. 521 patients divided into a discovery study group and a replication study group3. They found out that 2 VOCs for eosinophilic asthma and 3 VOCs for neutrophilic asthma had a classification performance comparable to that of blood eosinophilic count and FeNO	Schleich et al[14], 2019
Clinical and Experimental Allergy Journal	Exhaled breath profiles in the monitoring of loss of control and clinical recovery in asthma	This study demonstrates the superiority of e-NOSE over GC-MS in predicting exacerbations after ICS cessation (correct classification between 86% and 95% for e-NOSE and between 68% and 77% for GC-MS) in 22 patients with a mean age of 25 years. However, the VOCs analyzed with GC-MS were found to be correlated with eosinophilic sputum, which e-NOSE was not able to do4	Brinkman et al[49], 2017
Journal of Breath Research	Can exhaled volatile organic compounds predict asthma exacerbations in children?	7 VOCs detected in 32 children who experienced at least one exacerbation, were able to correctly predict the event 14 days earlier in 88% of cases. Sensitivity of the exam decreased in direct proportion to the temporal distance of the exacerbation5	Van Vliet et al[47], 2017
Respiratory Research	Defining adult asthma endotypes by clinical features and patterns of volatile organic compounds in exhaled air	16 VOCs able to distinguish asthmatic patients from healthy patients with a specificity of 91.1%, a sensitivity of 100%, and a correct classification of 98.7%. Moreover, 4 of these 16 VOCs were detected only in asthmatic subjects. The group was also able to identify 7 clusters of patients based on the clinical characteristics, the therapies carried out and the VOCs, demonstrating the hypothesis that a single asthma phenotype could be characterized by multiple inflammatory mechanisms, in fact they detected similar VOCs for clinical characteristics differentiate and vice versa6	Meyer et al[35], 2014
PloS One	Profiling of volatile organic compounds in exhaled breath as a strategy to find early predictive signatures of asthma in children	The first study able to discriminate between asthmatic patients, transient wheezing patients and healthy controls using VOCs analysis. 252 children between 2 and 6 years and 17 VOCs identified with an accuracy of 80% open the door to early diagnosis and treatment of asthma in preschool children5	Smolinska et al[51], 2014
American Journal of Respiratory and Critical Care Medicine	Exhaled biomarkers and gene expression at preschool age improve asthma prediction at 6 yr of age	The association of VOCs, API and gene expression is able to discriminate asthma from preschool wheezing with an AUC 95%, PPV/NPV 90%/89% and P value < 0.0001 in this study of 198 children followed by 2 to 6 yr5	Klaassen et al[56], 2015
Future Science	Metabolomics pilot study to identify volatile organic compound markers of childhood asthma in exhaled breath	Althoug they found out eight distinguish asthma markers with a P value < 0.05, the group highlights the exiguous number of patiens (23 children of whitch 12 healthy for control). The autors stress the importance of the variability of conditions7	Gahleitner et al[30], 2013
European Respiratory Journal	Exhaled volatile organic compounds predict exacerbations of childhood asthma in a 1-yr prospective study	Six or seven VOCs had been identified to be able to predict exacerbation (SVM) with a correct classification rate of 96%, a sensitivity of 100% and specificity of 93%. On the opposite, FeNO and lung function had not been able to give the same result5	Robroeks et al[45], 2013
Thorax	Non-invasive phenotyping using exhaled volatile organic compounds in asthma	A total of 12 VOCs were used to discriminate between asthmatic subjects, subjects with eosinophilic sputum, those with neutrophilic sputum, and those with uncontrolled asthma. The discriminatory accuracy of the VOC groups for the 4 patient groups ranged from 79% (neutrophilic asthma) to 89% (for loss of control asthma)5	Ibrahim et al[39], 2011
Clinical et Experimental Allergy	Volatile organic compounds in exhaled breath as a diagnostic tool for asthma in children	The group identified eight discriminating compounds between an asthmatic patients children group (63 people) and a control healthy group (57 people), with a sensitivity of 89% and a specificity of 95% and a claim of 92% correct classification. They also tested the reproducibility and intra/inter-individual variability8	Dallinga et al[23], 2010

¹Thermo Scientific Single Quadrupole Mass Spectrometry (ISQTM series). Volatile organic compounds (VOCs) were analyzed with ChromeleonTM and identified with NIST 2014 library.

²The group used gas chromatography (GC)-time of flight (TOF)-mass spectrometry (MS) for breath and liquid chromatography coupled with high resolution-MS (LC-HR-MS) for urine.

³The discovery study’s VOCs were analyzed by GC-TOF-MS. The replication study’s VOCs were analyzed by a comprehensive two-dimensional Gas Chromatography (two columns with two different stationary phases) coupled to HR-TOF-MS, GCxGC HRTOFMS. VOCs have been identified with NIST 2014 library.

⁴The only study of the list that made double analyzation, by electronic-Nose (four different platforms) and GC coupled to Quadrupole MS; identification by NIST library.

⁵VOCs were analyzed by GC-TOF-MS and identified with NIST library.

⁶VOCs were analyzed by GC-TOF-MS and compared between the two groups of the study. No library were used.

⁷VOCs were analyzed by GC-MS (after being collected into a Tenax/Carbotrap hydrophobic adsorbent trap) and identified with NIST except two of them.

⁸They used their developed GC-TOF-MS methodology[48], with two absorption tubes and a GC-TOF-MS. The group did not use a library to compare mass spectra in their approach. Instead, they compared the measured mass spectra with each other at the same retention time. This allowed them to determine whether peaks at the same retention time represented the same component or not based on the similarity of the original spectra.

ICS: Inhaled corticosteroids; GC: Gas chromatography; TOF: Time of flight; MS: Mass spectrometry; VOCs: Volatile organic compounds.

Table 2 Discriminative volatile organic compounds between asthmatic and non-asthmatic subjects

Identified compound	Ref.
Acetic acid	[40]
Acetone	[40]
Alkane	[40]
1,2-Dichlorobenzene	[30]
3-(1-methylethyl)-benzene	[23]
Ethyl benzene	[30]
1-isopropyl-3-methylbenzene	[30]
Benzoic acid	[23]
butanoic acid	[23]
(Branched) hydrocarbon (C11H24)	[23]
(Branched) hydrocarbon (C13H28)	[23]
(Branched) hydrocarbon (C13H28)	[23]
Unsaturated hydrocarbon (C15H26)	[23]
2,4,6-Trimethyldecane	[29]
2,6,6-Trimethyldecane	[29]
Octadecyne	[30]
1,3-Dioxolane, 2-(phenylmethyl)	[35]
2,6,11-trimethyl dodecane	[40]
1-Dodecanol 3,7,11-trimethyl	[35]
2,3-dimethyl heptane	[40]
2,4-Dimethylheptane	[29]
2,3,5-Trimethylheptane	[29]
2,2,4-Trimethylheptane	[29]
4-isopropenyl-1-methylcyclohexene	[30]
isoprene	[40]
Isopropanol	[40]
1,7-Dimethylnaphtalene	[30]
4-Methyloctane	[40]
3,3-Dimethyloctane	[29]
2-Octenal	[30]
2-methyl-1-pentene	[29]
1-penten-2-on	[23]
1-(Methylsulfanyl)propane	[30]
Carbon disulphide (CS2)	[23]
toluene	[40]
2,8-Dimethylundecane	[29]
3,7-Dimethyl undecane	[40]
p-Xylene	[23]

Table 3 Discriminative volatile organic compounds of different asthma subtypes and inflammatory disease characteristics

Inflammatory characteristics	Identified compound	Ref.
Eosinophilic asthma vs paucigranulocytic asthma	Hexane 2-hexanenone	[14]
Neutrophilic asthma vs eosinophilic asthma	3,7-dimethylnonane; Nonanol 1-propanol	[14]
Neutrophilic asthma vs paucigranulocytic asthma	3-tetradecane pentadecane nonane undecane (isomer of 3,7-dimethylnonane)	[14]
Neutrophilic asthma vs paucigranulocytic and eosinophilic asthma	Nonanal propanol hexane	[14]
Neutrophilic sputum cell ≥ 40%	Cyclopentene, 1,3-dimethyl-2-(1-methylethyl) C10H18 Naphthalene, 2,7-dimethyl	[39]
Eosinophilic sputum cell ≥ 2%	Camphene (7a-Isopropenyl-4,5-dimethyloctahydroinden-4-yl) methanol Bicyclo[4.1.0]hept-2-ene, 3,7,7-trimethyl Cyclohexene-4-methylene	[39]
Blood eosinophils level	2-octene; 3,3-Dimethyloctane; 1-Fluorododecane; 2,6,6-Trimethyldecane	[29]

Table 4 Discriminating volatile organic compounds in asthmatic exacerbations and uncontrolled asthma

Identified compound	Ref.
Acetonitrile	[49]
Benzene	[39,45]
1.2-methyl-4H-1,3-benzoxathiine	[45]
2-ethyl-1,3-butadiene	[45]
2-Butanone, 3-methyl/butanal, 2-methyl	[39]
1-phenyl-1-butene	[45]
6,10-dimethyl-5,9-undecadien-2-one	[47]
2(or 3)-methylfuran	[47]
2-ethylhexanal	[47]
Cyclohexane	[45]
1,2-dimethylcyclohexane	[47]
Bicyclo[3.1.0]hex-2-ene, 4-methylene-1-(1-methylethyl)	[39]
Methanol	[49]
Nonanal	[47]
Pentadecane, 1-methoxy-13-methyl	[39]
3-methylpentane	[45]
2-ethyl-4-methyl-1-pentanol	[45]
(1E)-1-(methylsulphanyl)1-propene	[39]
Octanal	[47]
2-octen-1-ol	[45]
4-methyl-Bicyclo[2.2.2]octan-1-ol	[49]
2,2,4,4-Tetramethyloctane	[39]
4,6,9-nonadecatriene	[45]
p-xylene	[45]
O-xylene	[39]

Table 5 Discriminatory volatile organic compounds in asthma therapies

	Salbutamol	OCS	ICS
Methyl-acetate	[43]
Butanal	[43]
3-Methyl-butanal	[43]
Butyrolactone	[43]
Carene		[43]
Carvone	[43]
Chloroacetic acid odecyl ester			[29]
1-Butyl-1-methyl-2-propyl- cyclopropane			[29]
2,6,6-Trimethyldecane			[29]
Glycolic acid		[43]
Lysine		[43]
Octanal		[43]
1-Propanol	[43]
Methyl propionate	[43]
3,6-Dimethylundecane			[29]

OCS: Oral corticosteroids; ICS: Inhaled corticosteroids.

Table 6 Discriminatory volatile organic compounds between asthmatic subjects and subjects with transient wheezing

Identified compound	Ref.
Acetone	[51,56]
Biphenyl	[51]
1-methyl-4-(1-methylethenyl) cyclohexene	[51]
2,6,10-trimethyldodecane	[51,56]
2,4-dimethylheptane	[51,56]
2,2,4-trimethylheptane	[51]
2-methylhexane	[51,56]
2-ethenylnaphthalene	[51]
2-methylpentane	[51,56]
2,4-dimethylpentane	[51,56]
octane	[51,56]
2,3,6-trimethyloctane	[51,56]
2-undecenal	[51,56]

Table 7 Discriminative volatile organic compounds in asthmatic/non asthmatic subjects with comorbidities

	Asthmatics with obesity	Asthmatics with upper respiratory illness	Asthmatics vs non-asthmatics/atopics	Non-asthmatics/non-atopics vs non-asthmatics/atopics
2-Methylbutane		[29]
1-Cyclopropane ethanol		[29]
2,5,9-Trimethyldecane			[29]
2,4-Dimethylheptane				[29]
2,3,6-Trimethylheptane	[29]			[29]
2,2,4-Trimethylheptane				[29]
n-Hexane	[29]	[29]
Isoprene		[29]
2-Methyloctane	[29]
2,6-Dimethyl (S,E)-4-octene	[29]
2-Methylpentane			[29]
2,3,4-Trimethylpentane	[29]
2-methyl-1-pentene				[29]
4-Methyl-1-pentene		[29]