A recombinase polymerase amplification (RPA) combined with strip visualization method for RNA-based presumptive tests of saliva and vaginal secretion

Identifying the origin of body fluids is a critical step in a forensic investigation. One widely used method to identify human body fluids is based on the color visualization of immune antigen detection strips for detecting hemoglobin in blood and prostate-specific antigen in semen. It is highly imperative to construct an easy-to-perform, mRNA-based method for the point-of-care identification of other human body fluids, such as saliva and vaginal secretion. Here, we established specific strips with the mRNA markers STATH (for saliva) and SPINK5 (for vaginal secretion) via reverse transcription recombinase polymerase amplification (RT-RPA) and lateral flow dipstick (LFD) assays (RT-RPA-LFD). RT-RPA could be accomplished in a single tube at a wide temperature range of 30–42 ℃ within 10–25 min if we do not count time for RNA extraction. The diluted RPA products were added onto the LFD strip pad to visually observe the color change of the Control/Test line. The tissue specificity and detection limit of the assays were evaluated using the optimized reaction conditions of RPA at 37 ℃ for 15 min. The positive signals of STATH were observed both in saliva and nasal secretions. SPINK5 was positive in a template-dependent manner in 4 out of 30 female urine samples in addition to vaginal secretion and menstrual blood samples. Cross-reactions were not detected in semen, skin swabs, sweat, or male urine. Both assays were capable of detecting aged samples, which were stored for 180 days (saliva) or 300 days (vaginal secretion) at room temperature. Moreover, saliva or vaginal secretion was successfully detected in all kinds of mixtures made from various body fluids. Overall, the rapid strip test method by the RT-RPA-LFD assay is simple, time-saving and highly sensitive for estimating the tissue origin of saliva and vaginal secretion. This method for the rapid RNA-based presumptive tests of the tissue type of body fluids is easy to perform prior to a multiplex mRNA analysis, which can demonstrate more reliable saliva or vaginal secretion identification.