Antigen rapid tests, nasopharyngeal PCR and saliva PCR to detect SARS-CoV-2: A prospective comparative clinical trial

BACKGROUND: Nasopharyngeal antigen Rapid Diagnostic Tests (RDTs), saliva RT-PCR and nasopharyngeal (NP) RT-PCR have shown different performance characteristics to detect patients infected by SARS-CoV-2, according to the viral load (VL)—and thus transmissibility. METHODS: In October 2020, we conducted a prospective trial involving patients presenting at testing centres with symptoms of COVID-19. We compared detection rates and performance of RDT, saliva PCR and nasopharyngeal (NP) PCR, according to VL and symptoms duration. RESULTS: Out of 949 patients enrolled, 928 patients had all three tests performed. Detection rates were 35.2% (95%CI 32.2–38.4%) by RDT, 39.8% (36.6–43.0%) by saliva PCR, 40.1% (36.9–43.3%) by NP PCR, and 41.5% (38.3–44.7%) by any test. For those with viral loads (VL) ≥10(6) copies/ml, detection rates were 30.3% (27.3–33.3), 31.4% (28.4–34.5), 31.5% (28.5–34.6), and 31.6% (28.6–34.7%) respectively. Sensitivity of RDT compared to NP PCR was 87.4% (83.6–90.6%) for all positive patients, 94.5% (91.5–96.7%) for those with VL≥10(5) and 96.5% (93.6–98.3%) for those with VL≥10(6). Sensitivity of STANDARD-Q(®), Panbio™ and COVID-VIRO(®) Ag tests were 92.9% (86.4–96.9%), 86.1% (78.6–91.7%) and 84.1% (76.9–89.7%), respectively. For those with VL≥10(6), sensitivity was 96.6% (90.5–99.3%), 97.8% (92.1–99.7%) and 95.3% (89.4–98.5%) respectively. No patient with VL<10(4) was detected by RDT. Specificity of RDT was 100% (99.3–100%) compared to any PCR. RDT sensitivity was similar <4 days (87.8%, 83.5–91.3%) and ≥4 days (85.7%, 75.9–92.6%) after symptoms onset (p = 0.6). Sensitivity of saliva and NP PCR were 95.7% (93.1–97.5%) and 96.5% (94.1–98.1%), respectively, compared to the other PCR. CONCLUSIONS: RDT results allow rapid identification of COVID cases with immediate isolation of most contagious individuals. RDT can thus be a game changer both in ambulatory care and community testing aimed at stopping transmission chains, and even more so in resource-constrained settings thanks to its very low price. When PCR is performed, saliva could replace NP swabbing. TRIAL REGISTRATION: ClinicalTrial.gov Identifier: NCT04613310 (03/11/2020).