Fig. 4.

Turbidity assessment of clinical saline gargle samples. Error bars represent standard error. (A) Photographs of negative and positive clinical saline gargle samples, obtained from human subjects. The normalized turbidity was determined by comparing the pixel intensities of the sample tubes against the black background. Red boxes indicate samples that were determined to be turbid using the procedure described in part B. (B) Using the normalized (to empty tube) turbidity, all clinical samples were classified into two categories, turbid and clear, using the threshold value of 1.41. Note: while all samples were classified in this manner, some could not undergo all subsequent testing due to low sample volume. (C) Surface tension measurements showed a decreasing trend with increased turbidity. (D) Total protein concentration of samples according to the Bradford assay. Turbid and clear samples showed no difference in total protein concentration, but SARS-CoV-2 positive samples had a higher (not significant) total protein concentration than negative samples. (n = 5 for negative clear, n = 3 for negative turbid, n = 6 for positive clear, and n = 4 for positive turbid). (E) Samples with a last oral intake (LOI) of 10–30 min prior to sample acquisition (n = 6) had higher turbidity than samples with a longer time since LOI (60+ min; n = 10), and the difference was statistically significant (p < 0.05). Average values are shown in the bar chart. (F) The time to constant velocity (n = 2) and surface tension of no toothpaste vs. toothpaste-added (10 mg/mL) NC samples, along with photos of the samples. Surface tension was measured at 0, 2, 4, 6, 8, and 10 s and the stabilized final value was chosen (hence no error bar). The accuracy of surface tension measurement is less than 1 mN/mm.