## 4 Results
### Longitudinal Assessment of Salivary VOCs  
The methodology experiments suggested 500 \(\upmu\)L of passive drool saliva sample yielded optimal results. Ten healthy participants were recruited with a median age of 30 (IQR: 25-32), with seven (70%) females. Saliva (3 mL) was collected via the passive drool method with a mean salivary flow rate of 0.7 mL/min (IQR: 0.5-0.9) and osmolality 77 mOsm (IQR: 57-89).  
_Intraindividual variability:_ For most compounds, there was an acceptable variation in levels of selected VOCs over the five assessed time points.  
Concentrations of salivary VOCs over a three-week period demonstrated SCFA, including hexanoic- and pentanoic acids, were consistent, with an acceptable CV \(<\) 20%. The remaining SCFAs, butyric-, propanoic-, and acetic acids showed a wider range of variability, with CV reaching 50%. A similar trend was observed for aldehydes with acceptable concentrations between days and weeks for saliva; for example, butanal ranged from 3.3 to 4.4 ppbv (CV 12%), and pentanal from 8.3 to 14.0 ppbv (CV 19%). Concentrations of VOCs and the associated coefficient of variation across the 3 weeks are presented in Table 2.  
_Interindividual variability:_ Interindividual variability, assessing daily and weekly variation over 3 weeks, was minimal for all target compounds. Butyric acid had a CV 6.3% (concentrations of 4.2, 4.1, 3.6, 3.8, and 3.9 ppbv; \(p=0.28\)), suggesting adequate reproducibility and limited influence from external factors. Butanal and pentanal also showed a narrow range of variation with a CV of 12.5% (\(p=0.05\)) and 10.1% (\(p=0.3\)), respectively. Five of the 15 compounds were statistically different, although CV% was deemed acceptable (acetic acid CV 9.6%, \(p=0.03\), propanoic acid CV 15.5%, \(p<0.001\), and heptanal CV 12.9%, \(p<0.02\)). Acceptable VOC variability as a cohort is presented in Figure 2.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Compound**} & \multicolumn{6}{c}{**Median Concentration (ppbv)**} & \multirow{2}{*}{**CV\%**} & \multirow{2}{*}{**_p_ Value} \\ \cline{2-2} \cline{5-8}  & **Day 1** & **Day 2** & **Day 3** & **Day 10** & **Day 17** & \multirow{2}{*}{**CV\%**} & \multirow{2}{*}{**_p_ Value} \\ \hline
**Acetone** & 179 & 145 & 184 & 196 & 246 & 19 & 0.06 \\ \hline
**Short chain fatty acids** & & & & & & & \\ \hline Acetic acid & 8.2 & 6.7 & 6.5 & 6.9 & 6.7 & 9.6 & 0.03 \\ \hline Butanoic acid & 4.2 & 4.1 & 3.6 & 3.8 & 3.9 & 6.3 & 0.28 \\ \hline Hexanoic acid & 1.4 & 1.3 & 1.0 & 1.2 & 1.0 & 15 & 0.15 \\ \hline Pentanoic acid & 4.3 & 3.7 & 3.7 & 3.5 & 3.8 & 7.9 & 0.12 \\ \hline Propanoic acid & 5.3 & 4.2 & 3.9 & 3.6 & 4.2 & 16 & \textless{}0.001 \\ \hline
**Aldehydes** & & & & & & & \\ \hline Acetaldehyde & 153 & 175 & 207 & 135 & 210 & 19 & 0.049 \\ \hline Butanal & 3.9 & 3.1 & 3.6 & 3.1 & 2.8 & 13 & 0.05 \\ \hline Decanal & 1.6 & 1.5 & 1.8 & 1.8 & 1.5 & 8.1 & 0.71 \\ \hline Heptanal & 2.4 & 2.3 & 2.6 & 2.5 & 1.8 & 13 & 0.02 \\ \hline Hexanal & 6.7 & 4.8 & 5.6 & 4.3 & 4.5 & 19 & 0.4 \\ \hline Nonanal & 3.2 & 2.9 & 3.4 & 3.1 & 2.9 & 6.7 & 0.03 \\ \hline Octanal & 2.8 & 2.2 & 2.6 & 2.3 & 2.2 & 11 & 0.69 \\ \hline Pentanal & 14.8 & 13.5 & 12.0 & 11.5 & 12.6 & 10 & 0.3 \\ \hline Propanal & 28.2 & 20.0 & 28.5 & 19.9 & 28.3 & 18.4 & 0.69 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Longitudinal variation of the concentrations (ppbv) of target short chain fatty acids and aldehydes in saliva.  
Figure 2: Longitudinal variation of the concentrations (ppbv) of butyric acid and butanal in saliva. Circles and asterisks denote outlying values.