Table 1.

Diagnostic performance of droplet digital PCR (ddPCR) compared to traditional testing methods in the included studies.

First Author (Year)	Number of Patients (Samples)		Sources of Samples	Summary of Results
	Patients (Samples)	Controls
Abasiyanik et al. (2020) [48]	166	N/A	Nasopharyngeal swabs Saliva	ddPCR showed higher sensitivity than RT-qPCR, especially in low viral load samples. ddPCR detected eight more positive cases than RT-qPCR.
Alteri et al. (2020) [34]	55 (100)	N/A	Nasopharyngeal swabs	ddPCR was positive in 19/55 cases tested negative with RT-qPCR, all of which had a low viral load.
Cassinari et al. (2021) [35]	130	N/A	Nasopharyngeal swabs Saliva	ddPCR was more sensitive than RT-qPCR (85% and 62%, respectively) with saliva testing, whereas both modalities showed concordance with nasopharyngeal testing.
Cento et al. (2021) [36]	960 (960)	N/A	Nasopharyngeal swabs	ddPCR was positive in 50 samples with a low viral load deemed negative by the LumiraDx rapid antigen test.
Chen et al. (2021) [20]	52 (87)	N/A	Blood Oropharyngeal swabs	ddPCR exhibited higher sensitivity and specificity than RT-qPCR, especially in blood samples with a low viral load.
Dang et al. (2020) [21]	30 (117)	61	Pharyngeal swabs Sputum	Positive detection rate of ddPCR was 100%, compared with 93.3% of RT-qPCR. Three samples negative with RT-qPCR were tested positive with ddPCR. 17 samples of a low viral load were tested positive with ddPCR, yet only 9/17 were positive with RT-qPCR.
de Kock et al. (2020) [38]	5 samples	5 samples	Nasopharyngeal swabs	Sensitivity of RT-qPCR decreased in the presence of background nucleic acids, in contrast to ddPCR’s.
Dong et al. (2021) [22]	103 (196)	N/A	Pharyngeal swabs	From the 103 febrile patients included in this study, ddPCR was positive in 19/25 negative and 42/49 equivocal RT-qPCR results; sensitivity was improved from 28.2% to 87.4%. From 29 close contacts, ddPCR detected SARS-CoV-2 in 16 equivocal and 1 negative RT-qPCR results.
Duong et al. (2021) [56]	20 (60)	12	Nasopharyngeal swabs	ddPCR showed higher sensitivity and precision with a lower LoD compared to RT-qPCR.
Falzone et al. (2020) [39]	1 (multiple dilutions)	1 (multiple dilutions)	Nasopharyngeal swabs	ddPCR showed enhanced detection ability than RT-qPCR in diluted samples with a low viral load.
Gniazdowski et al. (2020) [50]	185	N/A	Nasopharyngeal swabs	ddPCR showed enhanced SARS-CoV-2 detection ability compared to RT-qPCR. ddPCR assay was positive in low viral load samples tested negative with RT-qPCR.
Jiang et al. (2020) [24]	10 (32)	N/A	Nasopharyngeal swabs Oropharyngeal swabs Blood	ddPCR showed enhanced SARS-CoV-2 detection ability compared to RT-qPCR. Whereas all RT-qPCR positive samples were also ddPCR positive, three RT-qPCR negative clinical samples were tested positive with ddPCR.
Kim et al. (2021) [32]	366 samples	N/A	Nasopharyngeal swabs Sputum samples Blood samples	63 samples negative with RT-qPCR were positive with ddPCR. These 63 samples had low copy numbers, while only 55% of them were from symptomatic patients.
Lee et al. (2021) [33]	20	20	Pharyngeal swabs Sputum samples	ddPCR required a lower sample concentration compared to RT-qPCR to detect SARS-CoV-2.
Liu et al. (2020) [25]	43 (74)	N/A	Stool samples Sputum samples Throat swabs	Whereas RT-qPCR was negative in 18 samples received from 9 relapsed patients, ddPCR was positive in 12 of them. ddPCR performed better than RT-qPCR in samples of a low viral load. ddPCR had a positive detection rate of 55.41%, compared to RT-qPCR’s 36.5%.
Liu et al. (2020) [26] *	N/A	N/A	Nasopharyngeal swabs	ddPCR was positive in samples of a low viral load testing negative with RT-qPCR. ddPCR showed higher sensitivity and precision than RT-qPCR.
Marchio et al. (2021) [40]	208 samples	N/A	Nasopharyngeal swabs	8.6% of the negative RT-qPCR results were deemed positive with ddPCR. All positive samples detected by RT-qPCR were confirmed by ddPCR. ddPCR was positive in two samples with ambiguous RT-qPCR results.
Martin et al. (2021) [41]	448 samples	N/A	Nasopharyngeal swabs	ddPCR’s detection performance in group testing was comparable with RT-qPCR’s, yet the former showed a lower LoD.
Mio et al. (2021) [42]	90	N/A	Nasopharyngeal swabs	ddPCR’s detection performance was comparable with RT-qPCR’s, yet the former performed better at low concentrations.
Nyaruaba et al. (2020) [27]	94 samples	N/A	Oropharyngeal swabs	ddPCR was more sensitive than RT-qPCR (96.3% versus 92.6%).
Park et al. (2021) [19]	5 (8)	N/A	Nasopharyngeal swabs Oropharyngeal swabs	ddPCR’s detection ability was at least equivalent to RT-qPCR’s; yet, unlike RT-qPCR, ddPCR’s performance was not affected by the primer-probe sets’ sequences.
Poggio et al. (2021) [51]	64	N/A	Nasopharyngeal swabs	Out of the 18 RT-qPCR negative patients, 11 tested positive with ddPCR.
Ramirez-Rosas et al. (2021) [55]	404 samples	N/A	Nasopharyngeal swabs Colostrum samples	Samples from asymptomatic mothers and their neonates were tested. ddPCR detected 25 more positive samples (41/133) than RT-qPCR (16/133) from the mothers’ nasopharyngeal swabs tested. ddPCR detected 13 more positive samples (24/131) than RT-qPCR (11/131) from the neonates’ nasopharyngeal swabs tested. ddPCR detected 17 more positive samples (20/140) than RT-qPCR (3/140) from the colostrum samples tested.
Savela et al. (2021) [53]	7 (105)	N/A	Saliva Nasopharyngeal swabs	Saliva was superior to nasopharyngeal sampling for the early detection of SARS-CoV-2. ddPCR performed better than RT-qPCR in the saliva testing.
Scutari et al. (2020) [44]	2	N/A	Nasopharyngeal swabs Rectal swabs Urine Bile Plasma	ddPCR allows SARS-CoV-2 quantification in multiple sample types and even many days after the onset of symptoms In one patient and unlike RT-qPCR, ddPCR on nasopharyngeal swabs detected the SARS-CoV-2 RNA at different time points during hospitalization.
Sun et al. (2021) [28]	21	6	Throat swabs Sputum swabs Anal swabs	ddPCR showed higher sensitivity compared to RT-qPCR, especially at low viral load samples.
Suo et al. (2020) [12]	77	N/A	Throat swabs	ddPCR showed higher sensitivity [94% (95% CI: 83–99%), vs. 40% (95% CI: 27–55%)] and NPV [63% (95% CI: 33–83%) vs. 16% (95% CI: 13–19%)] compared to RT-qPCR.
Szwebel et al. (2021) [45]	1	N/A	Nasopharyngeal swabs BAL Plasma	ddPCR displayed increased tolerance to PCR inhibitors, showing high potential for viral diagnostics.
Tedim et al. (2021) [46]	90	N/A	Nasopharyngeal swabs Plasma	ddPCR detected the viral RNA in the plasma of 36 patients, whereas RT-qPCR was positive in 34 (94.4%) of them.
Xu et al. (2021) [54]	30 samples	30 samples	Nasopharyngeal swabs	ddPCR had higher sensitivity and specificity compared to RT-qPCR, especially in samples with low viral load. ddPCR had a PPV of 97.9%.
Yin et al. (2021) [29]	6	3	Throat swabs	Authors developed a rapid ddPCR which yielded robust results within 15 min By testing serial diluted samples, rapid ddPCR was accurate at both positive and negative reference samples; it was also more consistent than RT-qPCR at low-viral-load sample testing.
Yu et al. (2020) [30]	76 (323)	N/A	Nasal swabs Throat swabs Sputum Blood Urine	Of the 161 negative samples reported by RT-qPCR, 4 (4/161) were positive with ddPCR.
Zhang et al. (2020) [31]	24 (34)	N/A	Throat swabs Anal swabs Sputum Blood	ddPCR was more accurate than RT-qPCR in detecting positive samples with low viral load. Positive rates were higher in ddPCR (67.7%) than in RT-qPCR (58.8%).

* This study does not specify the number of samples tested from patients and healthy controls. Abbreviations: ddPCR, droplet digital polymerase chain reaction; dPCR, digital polymerase chain reaction; RT-qPCR, reverse transcription quantitative polymerase chain reaction; SARS-CoV-2, severe acute respiratory syndrome coronavirus 2; NPV, negative predictive value; PPV, positive predictive value; RNA, ribonucleic acid; BAL, bronchoalveolar lavage; LoD, limit of detection, N/A: not available, CI: confidence interval.