Detecting COVID-19 from Breath: A Game Changer for a Big Challenge
https://pubs.acs.org/doi/10.1021/acssensors.1c00312
COVID-19 Detection from Exhaled Breath
The analysis of exhaled breath could be a less invasive method of analysis for COVID-19 screening.
(11,12,79,16) Unfortunately, to date it has been extremely challenging to detect SARS-CoV-2 from exhaled breath. SARS-CoV-2 can be detected in air
(80−83) and objects that could affect the air around them (e.g., ventilation fans
(84) and on hospital floors
(84)), mainly because the virus remains viable in the air for up to 3 h.
(68,84) Of special importance, parts of these studies
(80) show that COVID-19 patients exhaled millions of severe acute respiratory syndrome coronavirus RNA copies per hour. Experimental analyses show that exhaled breath had a higher positive rate (26.9%) than surface (5.4%) and air (3.8%) samples. Again, this emphasized the importance of aerosol transmission in virus spread. However, in order to detect the virus directly from exhaled breath, it was necessary to collect the sample for a long time with a specific method and technology called exhaled breath condensate (EBC). As demonstrated in recent papers, collecting and analyzing breath’s liquid phase (exhaled breath condensate or aerosol, EBC, and EBA, respectively), nonvolatile molecules such as RNA, DNA, microorganisms, and viruses can be directly detected (typically by means of successive PCR-based methods) and visualized.
(85) The use of EBC is related to the very low viral load in the breath. However, the viral load of SARS-CoV-2 in aerosol samples is several orders of magnitude below those in nasopharyngeal swabs, making the detection of the virus from the air in close contact with positive/acute patients more challenging.
(86) The use of EBC
(87) solves this challenge by preconcentrating the virus and its metabolic byproducts in exhaled breath, as well as large droplets or small aerosol particles from the epithelial lining fluid to the level of detectable concentrations. Importantly, even nonvolatile markers are released in the breath as large droplets or small aerosol particles from the epithelial lining fluid, and can be assessed in the exhaled breath.
(88,78) An EBC device can efficiently collect different particles in relation to two parameters: (1) the number of collected particles compared to the total amount of particles in the air; or (2) the fraction of virus that remains viable after collection. Apart from chilling tubes (called R-tubes), isolating particles from the breath can be achieved by specifically designed filters for aerosols, with an electrostatic concentrator, etc. Challenges associated with this approach is that the collected aerosol sample is usually ∼1 mL,
(89) and the results are affected by the breathing protocol (e.g., how deep the breath is, etc.). Since the viral load is very low, sample collection from 10 to 1500 mL/breath should be carried for a long time (30 min), or the patient should be asked to cough rather than simply breathing.
(18) Studies on exhaled breath showed that infection leads to a variation of the microbial flora in the lungs and, as a consequence, to a variation of exhaled metabolites. The variation of VOCs could be used to diagnose COVID-19 infection.
(85,90) In ref
(60), for instance, the authors designed a method for direct detection of the virus, as well as related C-reactive protein and IgG and IgM markers, which, respectively, indicate the severity and immune response of the disease. While the detection of SARS-CoV-2 in saliva could be advantageous in terms of sample collection compared to nasopharyngeal sampling, the signals obtained are close to blank signal (sample/blank signal ratio 2.8–16). Grassin-Delyle et al.
(9) measured very specific VOCs in exhaled breath from mechanically ventilated adults with COVID-19 and compared that signature to ventilated patients with non-COVID acute respiratory distress syndrome. VOC-based breath signatures of COVID-19 could be distinguished from control cases with high accuracy. With this in mind, we think that the analysis of VOCs in breath has the potential to detect ketogenesis and other hematologic conditions related to SARS-CoV-2 infection, ensuring rapid detection and noninvasive sample collection. The rationale behind this approach relies on findings showing that viral agents and/or the body response (e.g., immune system) to the infectious/viral agent emit VOCs into the exhaled breath.
(11,12) The presence of VOCs in breath occurs in the early stages of the infection, thus serving for immediate detection of the COVID-19. The four most prominent VOCs in COVID-19 are methylpent-2-enal, 2,4-octadiene 1-chloroheptane, and nonanal, with typical concentrations of 10 to 250 ppb. Comprehensive reviews regarding the potential of VOCs as chemical biomarkers for disease diagnostics have been published.
(12,11,91)