Table 1 Results from the PLS-DA analysis of spectrum region of 1800–900 cm⁻¹.

K-Fold	Mean AUROC	Min AUROC	Max AUROC	95% CI AUROC
20	0.68	0.67	0.69	[0.68, 0.68]
10	0.68	0.66	0.69	[0.67, 0.68]
5	0.67	0.65	0.69	[0.67, 0.67]

K-Fold	Mean accuracy	Min accuracy	Max accuracy	95% CI accuracy
20	0.63	0.62	0.64	[0.63, 0.63]
10	0.63	0.61	0.65	[0.63, 0.63]
5	0.62	0.60	0.64	[0.62, 0.62]

K-Fold	Mean specifity	Min specifity	Max specifity	95% CI specifity
20	0.64	0.62	0.66	[0.64, 0.65]
10	0.64	0.62	0.67	[0.64, 0.65]
5	0.64	0.59	0.66	[0.63, 0.64]

K-Fold	Mean sensitivity	Min sensitivity	Max sensitivity	95% CI sensitivity
20	0.61	0.59	0.63	[0.61, 0.62]
10	0.61	0.59	0.64	[0.61, 0.62]
5	0.61	0.58	0.65	[0.61, 0.61]

K-Fold	Mean precision	Min precision	Max precision	95% CI precision
20	0.63	0.62	0.65	[0.63, 0.64]
10	0.63	0.62	0.65	[0.63, 0.63]
5	0.63	0.61	0.65	[0.62, 0.63]

Averaged results from the PLS-DA model k-fold cross-validation predictive ability repeated 100 times with k-fold values 20, 10 and 5. For each mean AUROC value the P-value was P < 0.001, and the P-values were calculated by using test 1 according to Ojala and Garriga²². The number of latent variables were set to 32 for k-folds 20 and 10, but for k-fold 5 the latent variables were set to 31, as it was yielding the best performance in predictive ability. There were 558 negative and 558 positive nasopharyngeal swab samples (dissolved into the viral transport medium and inactivated by adding viral lysis buffer liquid) measured with ATR-FTIR spectroscopy. Every sample was measured three times. The measured samples were preprocessed before PLS-DA, by applying spectrum truncation to the spectrum region of 1800–900 cm⁻¹, vector normalization and spectrum averaging to create a one average spectrum for every measured nasopharyngeal swab sample.

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