Non-cotton swab sample collection may not affect salivary melatonin assay results

BACKGROUND: Salivary melatonin levels have been analyzed in many research fields, including physiological anthropology. Although various devices have been utilized for saliva collection, cotton swabs are among the most common. However, previous studies have reported that cotton swabs may interfere with melatonin assay results, whereas synthetic swabs may not. These studies compared only mean melatonin levels between passive and synthetic-polymer swab collection methods but did not evaluate relative and proportional biases. Our study examines the effects of using swabs made of materials other than cotton, such as polypropylene–polyethylene polymer, on salivary melatonin assay results using a Bland–Altman (BA) plot. The effects of the saliva collection method were analyzed using two concentrations of melatonin, lower (< 6 pg/ml) and higher (> 6 pg/ml), because the threshold of dim light melatonin onset was lower than 6 pg/ml in many studies. RESULTS: Differences detected between passive and polypropylene–polyethylene polymer swab methods of saliva collection were not significant in both lower (< 6 pg/ml) and higher (> 6 pg/ml) melatonin levels detected. All correlations between the collection methods were significant, and 95% confidence intervals for differences in melatonin levels in all samples detected using passive and non-cotton swab saliva collection methods included zero in the BA plots. Averages and differences between non-cotton and passive saliva collection obtained from the BA plots were not significantly correlated at lower and higher melatonin levels. CONCLUSIONS: Our findings demonstrate that swabbing methods, including the use of polypropylene–polyethylene polymer, do not affect salivary melatonin assay results. Therefore, the authors suggest that polypropylene–polyethylene polymer swab methods are appropriate for the assessment of dim light melatonin onset and dose response of the circadian system to light.