Sensitivity assessment of droplet digital PCR for SARS-CoV-2 detection.

ABSTRACT

Reverse transcription­quantitative polymerase chain reaction (RT­qPCR) is the gold standard method for the diagnosis of COVID­19 infection. Due to pre­analytical and technical limitations, samples with low viral load are often misdiagnosed as false­negative samples. Therefore, it is important to evaluate other strategies able to overcome the limits of RT­qPCR. Blinded swab samples from two individuals diagnosed positive and negative for COVID­19 were analyzed by droplet digital PCR (ddPCR) and RT­qPCR in order to assess the sensitivity of both methods. Intercalation chemistries and a World Health Organization (WHO)/Center for Disease Control and Prevention (CDC)­approved probe for the SARS­CoV­2 N gene were used. SYBR­Green RT­qPCR is not able to diagnose as positive samples with low viral load, while, TaqMan Probe RT­qPCR gave positive signals at very late Ct values. On the contrary, ddPCR showed higher sensitivity rate compared to RT­qPCR and both EvaGreen and probe ddPCR were able to recognize the sample with low viral load as positive even at 10­fold diluted concentration. In conclusion, ddPCR shows higher sensitivity and specificity compared to RT­qPCR for the diagnosis of COVID­19 infection in false­negative samples with low viral load. Therefore, ddPCR is strongly recommended in clinical practice for the diagnosis of COVID­19 and the follow­up of positive patients until complete remission.