Salivary attenuated total reflectance–Fourier transform infrared spectroscopy in systemic diseases: a scoping review of clinical applications, spectral findings, and methodological challenges

INTRODUCTION: Salivary attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) has emerged as a rapid, minimally invasive approach for assessing systemic diseases through global biochemical spectral profiles. This scoping review mapped and critically synthesized the clinical applications, spectral findings, and methodological characteristics of salivary ATR-FTIR in systemic diseases. AREAS COVERED: The review followed established methodological and reporting guidelines for scoping reviews, with a prospectively registered protocol (DOI: 10.17605/OSF.IO/N36HP). Searches were conducted in MEDLINE (PubMed), Embase, The Cochrane Library, LIVIVO, LILACS, Web of Science, Scopus, Epistemonikos, and grey literature sources. Twenty-nine studies published from 2016 to 2026 were included, based on searches last updated on 5 June 2026. Investigated conditions comprised infectious, metabolic/endocrine, neoplastic, renal, and autoimmune/inflammatory diseases. Reported spectral alterations mainly involved proteins, lipids, carbohydrates, nucleic acids, and phosphates. EXPERT OPINION: Salivary ATR-FTIR shows promise for screening, diagnostic support, disease monitoring, and prognostic assessment, particularly when combined with chemometric and machine-learning methods. However, substantial heterogeneity in saliva collection, sample processing, spectral acquisition, preprocessing, and validation, together with the absence of true external validation, currently limits clinical translation.