Repurposing Water Analysis Nitrate and Nitrite Test Strips for Salivary Diagnostics

Salivary nitrate (NO₃^-) and nitrite (NO₂^-) are key components of the entero-salivary nitrate-nitrite-nitric oxide pathway and have been associated with cardiovascular, metabolic, and oral health. A reliable method that is reproducible in clinical environments (and potentially suitable for self-use) would democratize monitoring of nitric oxide-related pathways. However, despite advances in this field, commercially available test strips are not widely accessible. This study aimed to evaluate the feasibility of adapting colorimetric reagent strips originally developed for water analysis to assess NO₃^- and NO₂^- concentrations in human saliva. Artificial saliva containing sodium nitrate and sodium nitrite was used for standardization. Measurements obtained with the adapted strips were compared with a spectrophotometric reference method and with nitric oxide-specific salivary test strips in samples from systemically healthy donors. Agreement and reproducibility were assessed using intraclass correlation coefficients (ICC) and Bland-Altman analysis, while associations were evaluated using Spearman's correlation. The adapted strips demonstrated high agreement and strong correlation with the reference method for nitrate (ICC = 0.926; rho = 0.868; p = 0.001) and good performance for nitrite (ICC = 0.886; rho = 0.814; p = 0.004), with minimal mean bias in Bland-Altman analysis. Inter-rater reliability was moderate. Positive correlations were observed between salivary NO₃^- and blood pressure, with weaker associations for NO₂^-. Overall, the findings support the initial feasibility of adapting commercially available reagent strips for salivary NO₃^- and NO₂^- assessment, particularly for nitrate, offering a practical and low-cost alternative for clinical and research applications.