The effects of storage conditions on the stability of salivary melatonin in synthetic fiber swabs for home sampling

Home sampling of saliva is noninvasive, easy, and convenient, especially for multiple sampling. Such samples are therefore appropriate for the measurement of melatonin, a biomarker for circadian dysregulation. However, home sampling is restricted by access to appropriate storage conditions. This study, therefore, evaluated the effect of common storage conditions on the stability of melatonin in synthetic fiber swabs employed for home sampling. Saliva was provided by healthy volunteers during daytime, pooled and subsequently divided into fractions, each spiked with different amounts of melatonin. Synthetic fiber swabs were allowed to accumulate saliva from these fractions followed by storage at room temperature, 4 °C or -20 °C for 24, 48 or 72 h. The melatonin levels were analyzed employing a commercial ELISA assay. Differences in concentrations at each storage condition were evaluated with a two-way repeated measures ANOVA followed by a Tukey multiple comparison test. Samples were significantly more stable at -20 °C compared to room temperature and 4 °C, irrespective of the storage duration. However, no significant decrease from baseline was observed for samples stored at either 4 °C or -20 °C after 72 h. In comparison, a significant decrease was observed at room temperature after just 24 h. In conclusion, storage at -20 °C may be considered the gold standard for synthetic fiber swabs for quantification of salivary melatonin. However, storage at 4 °C ensures stability for 72 h while also ensuring convenience for home sampling. It is therefore our recommendation that such home samples are refrigerated, transported cold and centrifuged within 72 h of collection.