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The following is a curated review of key information and literature about this topic. It is not comprehensive of all data related to this subject.
Diagnosis of acute COVID-19 is typically made using standard RT-PCR, which requires specialized materials, equipment, personnel, and transportation to a centralized laboratory. Due to these intricacies, interest in rapid molecular and antigen tests (per IDSA, tests that typically result in an hour or less) tests for SARS-CoV-2 identification has been high since the beginning of the pandemic. There are 2 major categories of rapid tests: SARS-CoV-2 rapid molecular tests (e.g., RT-PCR or RT-LAMP), and SARS-CoV-2 rapid antigen test. The rapid antigen tests are most often antibody-based to capture SARS-CoV-2 antigens (typically N protein); the rapid molecular tests detect viral RNA.
Rapid molecular and antigen tests differ from standard RT-PCRs in several ways. They offer the potential to improve access to testing through point-of-care assays and to improve efficiency by reducing process steps between sample collection and results. Some rapid tests also negate the need for a central laboratory and specialized equipment. The sample type for rapid tests is a mid-turbinate or nasal swab, rather than a nasopharyngeal swab; this may be a more acceptable to patients. Rapid tests may ultimately contribute to better SARS-CoV-2 containment via more efficient detection and subsequent isolation. As a result, some hospitals are using or considering the use of rapid tests as part of their SARS-CoV-2 testing algorithms.
As noted above, rapid tests may detect either nucleic acid or viral antigens. Early evaluations of COVID-19 antigen tests show similar specificity to standard RT-PCRs, but inferior sensitivity evaluations of rapid molecular tests show similar specificity to standard RT-PCR tests, and similar sensitivity (the exception being the Abbot IDNOW; (Dinnes, August 2020). These studies are primarily early-stage evaluations of variable quality that utilize laboratory remnant samples; performance may also be impacted by viral load and timing of testing with relation to acquisition of infection, and sampling variability. These limitations are similar to those of standard RT-PCR, but an important difference in rapid antigen testing in particular is that some studies have shown that persons who have symptoms for more than 5 days may no longer have antigen at the level of detection (CDC, August 2020). Uncertainty remains about how the rapid tests will perform in real-life clinical space, particularly among asymptomatic patients, about whom data is limited. The FDA has issued individual Emergency Use Authorizations for several antigen and rapid molecular tests.
In November 2020, FDA issued an Emergency Use Authorization for the Lucira COVID-19 All-In-One Test Kit, a rapid molecular PCR test that provides results within 30 minutes, allowing self-testing in home and community settings. According to an unpublished study by Lucira [PDF], symptomatic individuals suspected to have COVID-19 self-collected nasal swabs for testing. The self-administered results were compared to a “high sensitivity molecular FDA authorized SARS-CoV-2 assay” that is not specifically named. In 101 participants, the total positive percent agreement across all samples was 94.1% (48/51) or 96.0% (48/50). When samples with high Ct values (>37.5) were excluded, the test achieved 100% (45/45) positive percent agreement.
Rapid antigen and molecular tests may differ in their performance characteristics, and so it is not possible to extrapolate the evaluation of a specific rapid test to others. Until more reliable and positive data is available on the use of these tests in clinical settings, standard RT-PCR will likely remain the gold standard used to diagnose acute SARS-CoV-2 infection. However, in specific scenarios a rapid test may be a reasonable initial screening test; these include situations where rapid isolation is important, such as in congregate settings, or instances where a symptomatic person has had a known contact with someone who known to have COVID-19 (CDC, August 2020).
Several randomized control trials underway aim to assess rapid COVID-19 tests, including two from the United States – Rapid, Onsite COVID-19 Detection at the University of Wisconsin, Madison, and Comparison of the Efficacy of Rapid Tests to Identify COVID-19 Infection (CATCh COVID-19) at the Texas Cardiac Arrhythmia Research Foundation.
IDSA guidelines make no recommendations for or against using rapid tests (i.e., result time ≤ 1 hour) versus standard RNA testing in symptomatic individuals suspected of having COVID-19, citing knowledge gaps.
Rapid, point-of-care antigen and molecular-based tests for diagnosis of SARS-CoV-2 infection (Dinnes, August 2020)
Overall, in this Cochrane review of rapid tests for SARS-CoV-2, the snesitiity of antigen tests across was variable, and the average sensitivtity was low at 56.2%. For rapid molecular tests the sensitivity across studies was more consistent, and the aevrage sensitivity was 95.2%. it is unclear how the tests evaluated in this review will perform in clinical practice.
- 18 study cohorts of people with suspected current SARS‐CoV‐2 infection, known to have, or not to have SARS‐CoV‐2 infection, or where tests were used to screen for infection.
- There were 3198 unique samples, of which 1775 had confirmed SARS‐CoV‐2 infection.
- Ten studies took place in North America, two in South America, four in Europe, one in China, and one was conducted internationally.
- To assess the diagnostic accuracy of point‐of‐care antigen and molecular‐based tests to determine if a person presenting in the community or in primary or secondary care has current SARS‐CoV‐2 infection.
- For antigen tests, sensitivity varied from 0% to 94%; the average sensitivity was 56.2% (95% CI 29.5 to 79.8%) and average specificity was 99.5% (95% CI 98.1% to 99.9%.
- Data for individual antigen tests were limited with no more than two studies for any test.
- For rapid molecular tests, sensitivity showed was more consistent than in antigen tests (from 68% to 100%).
- Average sensitivity was 95.2% (95% CI 86.7% to 98.3%) and specificity 98.9% (95% CI 97.3% to 99.5%).
- The authors did not find any studies at low risk of bias for all quality domains and had concerns about applicability of results across all studies.
- The authors note their confidence in the literature evaluated was limited. because many studies did not follow the test manufacturers’ instructions
- The studies frequently did not use the most reliable methods or did not report enough information for us to judge their methods.
- The evaluations were largely based on remnant laboratory samples.
Comparison of Abbott ID Now (IDNCOV) and Abbott m2000 Methods for the Detection of SARS-CoV-2 from Nasopharyngeal and Nasal Swabs from Symptomatic Patients (Harrington, August 2020).
- 524 paired nasal swabs (NS) tested on Abbot ID NOW (IDNCOV) compared with nasopharyngeal swabs from symptomatic patients meeting criteria for a diagnosis of COVID-19.
- To compare Abbot ID Now (IDNCOV), with the Abbott RealTime SARS-CoV-2 (ACOV) assay performed on the Abbott m2000 system.
- The overall positivity rate in this sample collection was 35%.
- The overall positive agreement was 75% (95% CI, 67.74% - 80.67%) and 99% negative agreement (95% CI, 97.64% - 99.89%) between IDNCOV and ACOV for all specimens tested.
- No information is given about length of symptoms before testing or Ct values, both of which may impact the sensitivity of rapid tests and the test performance seen in this study.
Rapid detection of COVID-19 Coronavirus using a reverse transcriptional loop-mediated isothermal amplification (RT-LAMP) diagnostic platform (Yu, July 2020).
- 248 samples from COVID-19 patients diagnosed in Shenyang province, China.
- To evaluate a LAMP-based assay for the detection of SARS-CoV-2.
- This LAMP-based assay had a sensitivity of 89.9%, with 25 false negatives due to relatively low viral loads.
Rapid and visual detection of 2019 novel coronavirus (SARS-CoV-2) by a reverse transcription loop-mediated isothermal amplification assay (Yan, June 2020).
- 130 specimens (a mixture of upper and lower tract) from patients with clinically suspected SARS-CoV-2 infection.
- To evaluate a reverse transcription loop-mediated isothermal amplification (RT-LAMP) assay for detection of SARS-CoV-2, and compare it with RT-PCR.
- Based on 58 positive and 72 negative samples compared to RT-PCR.
- The sensitivity of the RT-LAMP assay was 100% (95% CI 92.3%–100%).
- Specificity was 100% (95% CI 93.7%–100%).
- The RT-LAMP assay showed no cross-reactivity with other respiratory pathogens.
- 103 complete genomes of SARS-CoV-2 were obtained from four databases when designing the primers. These genomes may not have shown mutations occurring at the primer sequence region of the target gene.
- Some specimens were from bronchiolar lavage, but the authors do not say how many. Given the higher sensitivity with RT-PCR in this sample type, these findings may limit generalizability to settings where only nasopharyngeal swabs are used.
Evaluation of a novel antigen-based rapid detection test for the diagnosis of SARS-CoV-2 in respiratory samples (Porte, June 2020)
Overall, in this retrospective study, a novel rapid antigen detection test had high sensitivity and specificity in patients who primary were tested within 2 days of symptoms. The test performed better in samples with higher viral loads.
- 127 nasopharyngeal and oropharyngeal samples derived from patients with respiratory symptoms and/or fever and an epidemiological risk factor for SARS-CoV-2 infection at a private medical center in Chile
- 82 of the samples were positive for SARS-CoV-2 RNA by RT-PCR
- Median patient age was 38 years, and 53.5% were male
- 93.7% samples were from the first week after symptom onset.
- To evaluate a novel antigen-based RDT for the detection of SARS-CoV-2 in respiratory specimens from suspected COVID-19 cases.
- The median duration of symptoms was 2 days (IQR 1–4 days)
- The median Ct value of RT-PCR-positive samples was 17.7 (IQR 14.2–25.1)
- Overall sensitivity and specificity were 93.9% (95% confidence interval 86.5–97.4%) and 100% (95% confidence interval 92.1–100%), respectively, with a diagnostic accuracy of 96.1% and Kappa coefficient of 0.9. Sensitivity was significantly higher in samples with high viral loads.
- The sample type used in the study was not a type recommended by the test manufacturer
- The study was conducted during the summer, when other respiratory viruses were in low circulation. In the setting of other respiratory viruses, the test may have a different sensitivity/specificity
Low performance of rapid antigen detection test as frontline testing for COVID-19 diagnosis (Scohy, August 2020).
- 148 nasopharyngeal swabs tested by RT-PCR and rapid antigen testing.
- To determine the sensitivity and specificity of the Coris COVID-19 Ag Respi-Strip test, a rapid immunochromatographic test for the detection of SARS-CoV-2 antigen, in comparison to RT-qPCR.
- According to RT-qPCR results, 42 samples were negative and 106 were positive, with a Ct value of 33 (mean: 31.4; range: 16–38).
- The median time of symptom duration before the sampling date was 4 days (mean: 6.6; range: 0–34).
- 106 of samples were positive by RT-PCR; 32 were detected with the antigen test (30.2% sensitivity).
- For samples with Ct < 25 (n = 10), <30 (n = 34) and <35 (n = 64), 1.8 × 105, 9.4 × 103 and 494.8 copies/mL respectively, COVID-19 Ag Respi-Strip had a sensitivity of 100%, 70.6% and 46.9%.
- The median time of symptom duration was 4 days, which may have contributed to the poorer sensitivity of the antigen test. If utilized earlier, sensitivity may have been higher.
- The median cycle threshold was fairly high; it is possible the antigen test would have performed better at lower thresholds (higher viral loads).
Performance of Abbott ID Now COVID-19 Rapid Nucleic Acid Amplification Test Using Nasopharyngeal Swabs Transported in Viral Transport Media and Dry Nasal Swabs in a New York City Academic Institution (Basu, July 2020). In this comparison of the test performance of the Cepheid-Xpert-Xpress SARS-CoV-2 and the Abbot ID NOW rapid tests, the sensitivity of Abbott ID NOW COVID-19 for dry swabs of nares and nasopharynx was 51.6% and 66.7%, compared to Xpert Xpress platform.
Clinical Evaluation of Self-Collected Saliva by Quantitative Reverse Transcription-PCR (RT-qPCR), Direct RT-qPCR, Reverse Transcription–Loop-Mediated Isothermal Amplification, and a Rapid Antigen Test To Diagnose COVID-19 (Nagura-Ikeda, September 2020). In this study of saliva samples from 103 known-positive COVID-19 cases, SARS-CoV-2 was detected in 50.5-81.6% of cases with molecular assays and 11.7% of cases with rapid antigen testing.