Invasive, labor-intensive, and potentially risky – a list of characteristics that don’t sound desirable in a standard diagnostic tool. Yet tissue biopsy, with all of these characteristics, is the global gold standard in disease detection and diagnosis – and other well-established biopsy methods have their own limitations. It’s clear that we have an urgent need for new approaches that are affordable, accessible, reliable, and safe to enable effective early detection of disease and advance precision medicine. Breath biopsy – the detection, identification, and precise quantification of chemicals in breath – has the potential to transform clinical pathology.
Exhaled breath is a valuable source of prospective disease biomarkers, containing over 1,000 volatile organic compounds (VOCs) in addition to respiratory droplets, which carry non-volatile compounds, proteins, lipids, nucleotides, bacteria, and viral particles. Together, these provide a rich source of information on metabolism, environmental factors, and disease processes. Breath collection is completely noninvasive and increasing sampling time allows detection of VOCs that may be present at very low levels in the earliest stages of disease. Such capabilities make breath a unique sampling option.
VOCs in breath can arise from external sources or from within the body itself. As such, they can reflect biochemical and metabolic activity, diet, prescription drugs, and environment. Many VOCs from all parts of the body are readily transported to the lungs via the blood, making breath samples compatible with whole-body disease sampling. Furthermore, because endogenous VOCs link directly to metabolic activity in the body, changes in their levels can be characteristic of specific disease processes from the earliest stages.
Respiratory droplets generated in the deep airways of the lungs have been the center of attention recently because of their role in transmitting respiratory infections. With the right collection approaches, biomarkers relevant to various diseases can be captured from respiratory droplets and analyzed using well-established techniques, such as ELISA and PCR.
Discovering VOC biomarkers in a complex sample like breath requires both highly reproducible tools for collection and advanced chemical analysis to resolve and identify compounds. In the past, technical limitations and a lack of standardized analytical techniques have hindered the development of clinically relevant breath tests. In recent years, though, experts in the field have developed advanced collection technologies that offer consistent breath sampling. At the same time, though many techniques have been applied to breath analysis, gas chromatography-mass spectrometry (GC-MS) has emerged as the gold standard for VOC biomarker discovery. The latest high-resolution GC-MS platforms excel in the identification and quantification of biomarkers within the complexity of a breath sample, providing vital biological insight across the full range of exhaled VOCs. Together, these advances enable the development of novel breath tests in areas of high clinical need.
The range of potential applications for breath biomarkers in both research and clinical settings is expansive. Test development programs are under way in areas as diverse as respiratory disease, liver disease, cancer, and environmental exposure.
The early detection of cancer, particularly lung cancer, is a key area of interest in breath research. Despite being the most common cancer worldwide, lung cancer has one of the lowest five-year survival rates. Why? Because early diagnosis is costly and inefficient. Multiple studies have suggested that lung cancer could be diagnosed by the presence of certain carbonyls in a patient’s breath (1). The benefits of a low-cost, noninvasive test that can be deployed in screening programs are clear.
Liver disease is rapidly growing as a cause of global morbidity and mortality. Existing liver function tests largely assess liver damage rather than current function and struggle to determine the stage of liver disease. Limonene, however, shows excellent potential as a biomarker of both cirrhosis and broader liver health (2,3). Originating from diet and detected noninvasively on breath, limonene abundance increases due to metabolic shifts linked to cirrhosis. Work is now ongoing to understand whether limonene and other breath VOCs can also be used to monitor liver disease, which is increasingly widespread due to high-fat diets.
Over half a billion people worldwide suffer from chronic inflammatory airway diseases including asthma, chronic obstructive pulmonary disease, and idiopathic pulmonary fibrosis. A lack of reliable diagnostic tools means that treatment often depends on trial and error – increasing costs, prolonging periods of poor disease control, and raising the risk of exacerbations. Breath analysis could provide rapid, noninvasive patient stratification to identify steroid responders and enable targeted therapies. Recent research is already exploring the identification and validation of breath biomarkers with the potential to differentiate inflammatory phenotypes in asthma (4), and there is hope for tools that could predict oncoming exacerbations.
Taken together, these three examples highlight the huge untapped potential for breath biomarkers to revolutionize early disease detection and precision medicine. And with the unprecedented attention COVID-19 has brought to breath research, the next few years promise to be an incredibly exciting time of development for the field, with the possibility of disruptive new healthcare technologies just around the corner.
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References
- Y Saalberg, M Wolff, “VOC breath biomarkers in lung cancer,” Clin Chim Acta, 459, 5 (2016). PMID: 27221203.
- R Fernández del Río et al., “Volatile biomarkers in breath associated with liver cirrhosis – comparisons of pre- and post-liver transplant breath samples,” EBioMedicine, 2, 1243 (2015). PMID: 26501124.
- ME O’Hara et al., “Limonene in exhaled breath is elevated in hepatic encephalopathy,” J Breath Res, 10, 046010 (2016). PMID: 27869108.
- FN Schleich et al., “Exhaled volatile organic compounds are able to discriminate between neutrophilic and eosinophilic asthma,” Am J Respir Crit Care Med, 200, 444 (2019). PMID: 30973757.
