Adequate diagnostic testing for SARS-CoV-2 is a critical component to the overall prevention and control strategy for COVID-19. Rapid Antigen Diagnostic Tests (RDT) can be an appropriate alternative to RT-PCR for expanding testing capacity and reduce burden of laboratories of many diagnostic settings. The ability to perform this test in patient care centers would simplify the process of testing, and provide rapid results to the doctor, thus it can improve the decision-making process and reducing pressure on the healthcare providers [3, 4].
In this study the Panbio™ tests has an overall sensitivity of 77.5% (61.6% − 89.2%). This similar to study reported from Valencia, Spain (79.6%) (13), Madrid, Spain (73.3%) [14], Switzerland (81%) [15], Brazil (84%) [16]. Geneva University Hospital (85.5%) [17], Marseille, France (75.5%) [18] and Sint Maarten, Dutch Caribbean. (84%) [19] .This finings is slightly lower sensitivity of 90.5% report from Spain among COVID-19 patients with ≤ 7 days onset of symptoms or from exposure to a confirmed case[20]. However, this test is lower than 91.4% sensitivity of test report by the manufactures [10].
The WHO guidelines require that SARS-CoV-2 RADTs demonstrate ≥ 80% sensitivity and ≥ 97% specificity compared to the RT-PCR reference assay. Ag-RDTs should be prioritized for use in symptomatic individuals meeting the case definition for COVID-19 [4]. In this study, the sensitivity of Panbio™ test was found high in samples collected from individuals with 1–5 days duration COVID − 19 symptoms (94.4%) (95% CI: 90.3%-98.5%). This is similar to 91.8% and 94.8% sensitivity report from Spain (20) and Geneva hospital [17], respectively. However it is higher than two reports from Spain (80.4% and 85.3% ) [13, 14]. In the current study the sensitivity of Panbio declined in sample collected after 1-7days of onset of sign/symptoms (85.7% ) (95% CI: 80.3% -91.1%) and greater than equals to 7 days (50% ) (95% CI: 34.3%- 65.7%). This is similar to report of sensitivity report of 86.5% and 53.8% report on sample collected from symptomatic patients with < 7 days and ≥ 7days ,respectively from Madrid, Spain [14]. A similar decline in sensitivity of Panbio test with increased in number of days of sign/symptoms COVID-19 were found in study report from Geneva hospital [17]
It is known that following SARS CoV-2 infection, the virus undergoes a period of incubation during which viral titers are usually too low to detect, after which the virus undergoes exponential growth, leading to a peak in viral load and infectivity, and ending with declining viral levels and clearance when infectivity is low [16].
In this study, the sensitivity of Panbio™ COVID-19 Ag Rapid Test was higher among pediatrics (100%) than adult (75%). This is difference from study conducted in Spain, which found higher sensitivity test among adults (82.6%) than pediatric (62.5%) (130. In other previous study found that the viral load in respiratory tract was not significantly different by age [21].
The sensitivity is higher among contacts with household’s contact (90.0%) than non- household (50.0%). The current finding is different to report of 50.8% and 35.7% from Valnacia, Spain among asymptomatic households and non-households, respectively [22]. Household contact might have serious impact on risk groups like old age and those with chronic problem in the households. As most of those contacts with sign/symptoms COVID − 19 in the current study were from confirmed cases in house hold. In general, it is very important to identify infected individuals who present with COVID-19 symptoms and contact with infected individuals who might be responsible for transmission SARS-COV-2 in the community.
In this study, the highest sensitivity (100%) of the Panbio™ COVID-19 Ag Rapid Test was obtained in those samples with low Ct-values of < 20. This similar to report from Geneva hospital (98.4%)) [17]. However, the sensitivity decreases fast for those threshold cycle (CT) < 25 (80%). This was different from study reported form Spain (99.5%) [20] and Geneva (95.5%) [17]. The lowest sensitivity value in the current was found for specimen having 30 < Ct ≤ 37 (70.6%). This was higher than 40.9% report from Geneva hospital [17]. This difference might be due to variation in epidemiology of SARS-COV-2 by geographical locality, sample size and viral copies SARS-COV-2 and other.
According to WHO recommendation, Ag-RDTs perform best in individuals with high viral load, early in the course of infection and in settings were SARS-CoV-2 prevalence is ≥ 5%. When there is no transmission or low transmission, the positive predictive value of Ag-RDTs will be low, and in such settings, RTPCR are preferable for first-line testing or for confirmation of Ag-RDT positive results [4]. During the month of September, the prevalence of SARS-COV-2 in Ethiopia was 7.9% [2] and 15.5% in the Harari region state [8].
The Panbio tests current study had an overall specificity of 98.5% (95.6%- 99.7%). This is similar to report from Switzerland (99.1%) [15], Brazil (98%) [16] and Sint Maarten, Dutch Caribbean (99.9%) [19]. This finding is slightly lower than 100% report from Valencia, Spain [13] and Geneva [17] and higher than report from Marseille, France (94.9%) [18].
In this study the Panbio rapid tests had Positive Predictive Value 91.2% (95% CI: 76.9% − 97%) and negative Predictive Value 95.5% (92.3%, 97.4%). This is agreement with studies conducted in Marseille, France and Sint Maarten, Dutch Caribbean found positive predictive values of 95.6% and 91.7% ,respectively. However higher than NPV report 72.2% Marseille, France and slightly lower than NPV report found Sint Maarten, Dutch Caribbean ( 98%) [18, 19].
The above difference in Panbio performance might be due to difference viral load, prevalence /incidence of SARS-COV-2, patient clinical characteristics and age, type of specimen processed, and duration symptoms others [6].
In the current study, the agreement between the two methods were 94.5% (Kappa = 0.8, 95% CI: 0.7–0.9). This similar to two studies report from Spain (k = 0.87 and 0.90) [13, 20].
The study used reasonable sample size, followed strict standard operating procedure for sample collection and examination with Panbio and RT PCR by including both symptomatic and asymptomatic individuals. The limitations of this study were symptoms and its duration were self-reported which might under or overestimate the sensitivity of this study. Viral culture had not done to assess the viability/ infectivity of SARS-COV-2 among those with SARS-COV-2 positive individuals.