Today RT-PCR is considered the gold standard in the diagnosis of SARS-CoV-2- infection but even in trained hands a false negative rate of about 25% and a false positive rate of 2.3–6.9% has to be expected.17 However, false negative results are often caused by low swab and sampling technique quality while in here only well trained staff performed the sampling applying brushed swab from Copan which can be considered as high quality. Hence, we did only notice false negative nasopharyngeal swabs in patients who were either in the pneumonia phase of COVID-19 or were at the detection limit.
Our unpublished experience was that patient at the detection limit usually are at the very end of a COVID-19 and are most likely not infective anymore.18
Currently SARS-CoV-2 infections are rising in several countries causing tremendous cost. In underdeveloped countries neither trained staff nor financial power to afford RT-PCR for mass screening is available.2
To the best of our knowledge our study is the first showing that breath-analysis is able to discriminate SARS-CoV-2 infected patients from controls with respiratory infections and Influenza infection as well. Even one suspected but not clearly PCR-confirmed patient could be assigned to the SARS-CoV-2 group and turned out to be positive later.
We used MCC-IMS because of the ease of application. The STEP-IMS device does not need any pre-analytic procedures or test gases. So, no shortage of swabs, tubes or reagents has to be faced in the scope of a pandemic.
The device draws the breath into the system by an internal pump. This simplifies the sampling compared to other IMS devices where absorption/desorption tubes are needed.9
The only task to be fulfilled by less trained staff is to introduce the foam-cuffed catheter into a nostril of a spontaneously breathing individual and to hit a key to start the measurement.
For our study written consent of the patient was a prerequisite. Therefore, we had to exclude demented, delirious or too severely ill patients not able to consent but this should be no constraint for the method in real life.
It could be argued that IMS only provides peaks according to retention time and drift time while mass spectrometry (MS) is able to chemically describe the VOCs but also in MS not all peaks are clearly assigned to a chemical substance and are therefore also only numbered or characterised by the time of flight.6 Furthermore, even though it is academically interesting to identify the relevant peaks in the breath of COVID-19 patients it is dispensable for SARS-CoV-2 screening. Hence, we think that not knowing exactly the chemical structure of the VOC is no detriment as another attempt to screen for SARS-CoV-2 is by the smell of trained dogs.13,14 Like scent dogs, a fingerprint of peaks should enable the classification of the odour of infected patients.
As the scent of the breath does not rely on the virus itself but on the host response to the infection cross-reactivity of breath analysis with other viral infections has to be expected.10 As MCC-IMS could differentiate between SARS-CoV-2 and Influenza-A infection we assume that different viruses cause at least to a certain extend different host responses and therefore produce different fingerprints of IMS spectra. However, this needs to be addressed in future studies.
Similar to antibody tests an overlap of the VOCs with other corona virus infections has to be anticipated. But this is a constraint every analysis of metabolomics has to face. However, within the current pandemic we detected almost no endemic corona viruses in our adult patients.
Compared to other breath analysis studies we did not require fasting before the sampling. Though fasting state may reduce interferences with other metabolism it will not be feasible for large scale screening.
Another necessity for a further progress of breath analysis in screening for infections is the extension of the study to other ethnicities and civilisations to investigate whether ethnos and life style needs to be considered for the analysis.
One drawback of our study is the limited number of patients. As pointed out already many patients with SARS-CoV-2 were not able to give informed consent. Another point is the weakening of the SARS-CoV-2 wave in Germany end of April as this led to a slowing of accrual.
We therefore assess this study as a proof-of-concept and encourage other researches to further investigate breath analysis by MCC-IMS for the detection of SARS-CoV-2 infections. We are currently developing a point-of-care prototype with an instant analysis of the data as this will be the relevant step for large scale screening.
As MCC-IMS is fast, non-invasive and does not need any reagents or pre-analytical procedures it seems promising for a screening device even in underdeveloped countries or air travel. In conclusion we identified a quick and cheap way of large-scale SARS-CoV-2 testing.