Accuracy of a raw saliva-based COVID-19 RT-LAMP diagnostic assay

The Severe Acute Respiratory Syndrome Coronavirus-2 (SARS-CoV-2) pandemic demanded rapid diagnosis to isolate new COVID-19 cases and prevent disease transmission. Quantitative Real-Time Polymerase Chain Reaction (qRT-PCR) rapidly became the gold standard for diagnosis. However, due to the high cost and delay of the results, other types of diagnosis were implemented, such as COVID-19 Ag Rapid Tests and Reverse Transcription Technique followed by Loop-Mediated isothermal Amplification (RT-LAMP). In this work, we validated the use of RT-LAMP in saliva samples rather than nasopharyngeal swabs, as the collection is more comfortable. First, we selected 5 primer sets based on the limit of detection for SARS-CoV-2 RNA, then validated their sensitivity and specificity in patient samples. A total of 117 samples were analyzed by fluorometric RT-LAMP and compared with qRT-PCR results. Our results show that the use of a high-sensitive primer ORF1-a, together with a low-sensitive primer set Gene E (time to threshold of 22.9 and 36.4 minutes, respectively, using 200 copies of viral RNA), achieved sensitivity in purified RNA from saliva samples of 95.2% (95% CI 76.1‒99.8) with 90.5% specificity (95% CI 69.6‒98.8) (n = 42).As RNA purification increases the turnaround time, we tested the outcome of RT-LAMP utilizing raw saliva samples without purification. The test achieved a sensitivity of 81.8% (95% CI 59.7‒94.8) and a specificity of 90.9% (95% CI 70.8‒98.8). As a result, the accuracy of 92.9% (95% CI 80.5‒98.5) in purified RNA-saliva samples was lowered to an acceptable level of 86.4% (95% CI 72.6‒94.8) in raw saliva. Although mass vaccination has been implemented, new strains and low vaccination progress helped to spread COVID-19. This study shows that it is feasible to track new COVID-19 cases in a large population with the use of raw saliva as sample in RT-LAMP assay which yields accurate results and offers a less invasive test.