Photoacoustic technologies for exhaled-breath analysis: A systematic review of analytical performance and reporting quality

Abstract Photoacoustic breath sensing is usually assessed mainly on the basis of its detection limit, even though the review of the literature presented here shows that analytical sensitivity is no longer the only factor that affects the comparison of studies across different sources. This review, which is aligned with PRISMA and focuses on the instrument, comprised 131 independent study families providing a total of 209 study-analyte entries. When operating under conventional real-time conditions, the lowest detection limits recorded were 1 ppbv for ammonia, 2.1 ppbv for methane, 6 ppbv for ethylene, and 1.5 ppbv for carbon monoxide. Systems that used adsorbents or enrichment reported even lower values, such as 0.465 ppbv for nitric oxide and 0.0235 ppbv for acetone, but these measurements cover the entire detection cycle and were therefore kept apart from the uninterrupted real-time measurements. Reporting completeness was much less consistent: in 59 of the 212 entries the averaging time was given, humidity control was mentioned in 54, and end-tidal sampling was reported in only 9; just 48 entries satisfied the criterion of having comparable units with a stated time base. By applying a five-domain framework for assessing reporting quality with predefined scoring rules, 66% of the study families were rated as having a High concern regarding analytical validity and 65% had a High concern about sampling control, while only one family provided all of the environmental conditions at the measurement cell. The evidence also divided into three different operational regimes, each with its own figure of merit, so that a single ranking according to detection limit is not appropriate. We therefore suggest an eight-item minimum reporting set and identify normalized noise-equivalent absorption, Allan-deviation reporting, and inter-laboratory round-robin studies as the most practical priorities for improving comparability in future breath-sensing studies.