The rapid spread of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic worldwide requires the urgent adoption of effective preventive measures. Early diagnosis and rapid isolation of infected people are central to contain disease transmission. Although real-time reverse-transcription polymerase chain reaction (RT-PCR) is currently the reference assay for the diagnosis of SARS-CoV-2 infection, novel rapid antigen tests have emerged with several potential key advantages over molecular methods.
In contrast to the RT-PCR, the antigen test is relatively inexpensive, simple to perform, and easy to interpret, (2) does not require infrastructure, and enables obtaining point-of-care results within a few minutes. As a result, it allows immediate decisions about isolation and therapeutic interventions on infected individuals. Moreover, antigen tests are capable of identifying infected people early after infection, when viral loads are high and the likelihood of transmission is highest. Despite the lower sensitivity when compared with the molecular assays, the possibility of repetitive testing with a low-cost procedure and the real-time detection of the most infective patients make the antigen a potentially high valuable test in terms of surveillance, to track and prevent the spread of the infection.
Information on the performance of the point-of-care SARS-CoV-2 antigen tests is limited. The sensitivity of the first-generation antigens is overall low. In addition, most studies have been conducted in laboratory specimens, involved a relatively low number of samples, and the minority with available clinical data primarily included symptomatic patients. To assess the real performance of a point-of-care test, it should be used in real-life conditions, including consecutive patients, and obtain results on site.
The uncertainties about the antigen are the accuracy of the test in asymptomatic patients and how it performs in additional clinical settings, such as childhood, or old age, among others. Another relevant question is whether a more convenient sample would be a suitable alternative for diagnosis. Antigen tests are currently authorized to be performed on nasopharyngeal (NP) or nasal swabs, which need to be collected by healthcare professionals. Because saliva can be self-collected, antigen assessment in this sample would facilitate large-scale testing.
The Panbio COVID-19 antigen Rapid Test Device (RTD) (Abbott Rapid Diagnostic Jena GmbH, Jena, Germany) has been recently marketed for the qualitative detection of SARS-CoV-2 antigen in human NP swab specimens, with high sensitivity and specificity. We evaluated the performance of this point-of-care test in real-life conditions, in 3 primary care centers (PCCs) and an emergency department (ED). We assessed the accuracy of the test in symptomatic and asymptomatic patients, in different clinical scenarios, and in NP, nasal, and saliva samples.
Study Design, Setting, and Data Collection
A prospective study was conducted from September 15 to October 29, 2020 in 3 PCC and an ED. Consecutive patients, either with COVID-19 signs/symptoms or asymptomatic contacts attending the PCC, and a majority of symptomatic patients presenting to the ED were included in the study, and only patients who refused to participate were excluded. Demographic and clinical data from primary care patients were collected using a structured questionnaire. The questionnaire included information about 6 specific symptoms and their temporality and the number of days since the initiation of symptoms. Clinical data from patients who attended the ED were obtained from the electronic health records.
Patient Consent Statement
The patient’s written consent was obtained. The design of the work was approved by the COVID-19 Institutional Committee of Hospital General Universitario de Elche (Spain).
Know More- Panbio Nasopharyngeal Test
At the PCC, patients were asked to fill out the questionnaire about symptoms and to repeatedly spit up to a minimum of 1 mL of saliva into a 100-mL sterile empty container. Then, a nasal swab from 1 nostril and 2 consecutive NP swabs (1 swab for each nostril) were obtained by a qualified nurse according to the recommended standard procedure. At the ED, 2 consecutive NP swabs, also with a different swab for each nostril, were obtained by a clinician.
Nasal swabs, 1 of the 2 NP swabs, and the saliva samples obtained at the PCC were tested onsite within minutes after collection for antigen detection. One of the NP swabs obtained at the ED was also analyzed onsite for antigen detection immediately after collection. The antigenic assessment in all the samples was performed using the Panbio COVID-19 Ag RTD, an immunochromatographic test with a membrane strip precoated with antibodies to the SARS CoV-2 nucleocapsid. The kit was used according to the manufacturer’s instructions. In brief, nasal and NP swabs were immersed in 2 extraction tubes containing 300 µL of buffer from the kit. A third swab was soaked in the saliva sample and then immersed in a third 300-µL tube. The 3 tubes were ready to be applied to the corresponding antigen device.
Severe Acute Respiratory Syndrome Coronavirus 2 Ribonucleic Acid Detection
The second NP swab was preserved in a 3-mL transport tube containing guanidine salt solution (Mole Bioscience, SUNGO Europe B.V., Amsterdam, Netherlands). After collection of all samples, NP specimens were transported daily by the same healthcare workers who collected the samples at the PCC to the clinical microbiology laboratory for immediate molecular analysis by RT-PCR. Nasopharyngeal samples (NPS) from the ED were also sent to the same microbiology laboratory.
Nucleic acid extraction was performed using 300 µL NP specimen on Chemagic 360 Nucleic Acid Purification Instrument (PerkinElmer España SL, Madrid, Spain). Then, 10 µL eluate was used for real-time RT-PCR assay targeting the E-gene [LightMix Modular SARS-CoV (COVID19) E gene; TIB MOLBIOL, Berlin, Germany, distributed by Roche]. Testing was performed according to the manufacturer’s guidelines on Cobas z 480 Analyzer (Roche, Basilea, Suiza).
Continuous variables are expressed as median ± 25th and 75th percentiles (Q1, Q3), and categorical variables are expressed as percentages. Wilcoxon or Student’s t test was used to compare continuous variables, and the χ 2 or Fisher’s exact test was used for categorical variables comparison.
The percentage agreement (positive percent agreement [PPA], negative percent agreement [NPA], and overall predictive agreement) for Panbio antigen test in the NP, nasal, and saliva samples compared with the reference standard RT-RCR test in NP swab was calculated. Performance agreement was evaluated using Cohen’s kappa coefficient. Performance was also evaluated in NPS stratifying by age, sex, the number of cycles of amplification in RT-PCR (cycle threshold [Ct]), and duration of symptoms. Multivariate logistic regression was performed to assess predictors of the sensitivity of the antigen test in symptomatic patients. The estimated sample size for a sensitivity of at least 91.4% (according to the manufacturer), a precision of 2.5%, and a statistical power of 80% was 762 patients. For a specificity of at least 97%, sample size required was 377 patients.
During the study period, 913 patients were included; all of them had a NP swab for RT-PCR, 904 (99%) had a second NP swab for antigen test, 659 (72%) had a nasal swab, and 611 (67%) had a saliva sample collected. A total of 690 (75.6%) NP samples were collected from the PCC, and 223 (24.4%) were collected from the ED.
Clinical characteristics of the patients are Median (Q1–Q3) age was 40.6 (23.0–55.6) years, and 423 (46.3%) were men. The most common comorbidities were dyslipidemia in 80 (22.2%) patients, hypertension in 124 (17.0%), and diabetes in 60 (8.2%). There were 617 (67.6%) symptomatic patients and 296 (32.4%) were asymptomatic. Median (Q1–Q3) number of days from symptom onset was 3 (2–5) days, and the most frequent symptoms were cough (50.1%), followed by fever (46.8%), sore throat (31.9%), and nasal congestion (31.3%). Median (Q1–Q3) Ct was 24 (16–30); 22 (16–29) in symptomatic and 28 (21–32) in asymptomatic patients (P = .012); and 21 (15–27) in patients ≥50 years and 26 (18–31) in <50 years (P = .02).