1. Introduction
Since December 2019, a novel coronavirus (nCoV) of animal origin started to infect humans and initiated a severe outbreak in China [
1]. As this virus was not sufficiently novel but is a sister virus to severe acute respiratory syndrome-related coronavirus (SARS-CoV), based on its taxonomy and phylogeny the official name of the nCoV has been changed to SARS-CoV-2 [
2,
3]. SARS-CoV-2 is highly contagious (10−20 times more than SARS-CoV) as it can be transmitted mainly via airborne droplets of asymptomatic individuals and have much higher viral loads in the upper respiratory tract compared to SARS-CoV, regardless of the similarity in their surface and aerosol stability [
4]. The transmissibility of SARS-CoV-2 has been reported to begin 2.5 days before and hit the peak 14 h prior to symptom onset [
5]. The clinical spectrum of SARS-CoV-2 associated disease (coronavirus disease 2019 (COVID-19) is broad, including asymptomatic, mild, and severe. In mild and severe cases, symptoms start with fever and cough, followed by dyspnea, and reaching a maximum approximately eight days after the first symptoms. Older adults, people with an immunosuppressive disease and pre-existing diagnosed chronic medical conditions are at a higher risk of getting a severe infection, resulting in an intensive care unit (ICU) admission at usually two weeks after symptoms onset [
6,
7]. Hence, the early detection of SARS-CoV-2 plays a vital role in controlling the spread of this highly contagious virus and decreases the fatality rate, which mostly affects high-risk people (
Figure 1A) [
5]. In the last few decades, many molecular and serological techniques have been developed and utilized for virus nucleic acid, antigens, and specific antiviral antibodies detection.
Serological methods have been recognized as simple, safe, and cost-effective virus detection approaches. However, until now, the World Health Organization (WHO) has recommended this method only for research purposes and not for patient care as it has low sensitivity and specificity. Moreover, after infection, the amount of antibodies usually takes one or two weeks to reach a detectable level, making this technique more suitable for population infection study [
8]. Since molecular techniques such as reverse transcription polymerase chain reaction (RT-PCR) can directly detect a specific sequence of virus genome with high sensitivity and specificity, they have become the standard virus detection techniques [
9,
10]. Nonetheless, standard RT-PCR approaches rely heavily on expensive equipment, well-trained staff, and equipped laboratories. Moreover, sample examination using this method usually takes between 4–6 h, excluding the ship** time to laboratories, which increases the total turn-around time with a higher risk of cross-contamination. Therefore, conventional RT-PCR approaches are limited in their ability to monitor SARS-CoV-2 outbreaks at a pandemic scale.
Since January 2020, when COVID-19 became a public health emergency of international concern, various researchers and companies have focused on develo** point-of-care (POC) testing devices so as to provide a rapid and reliable method for SARS-CoV-2 detection, enabling faster clinical decisions [
11,
12]. The implementation of POC testing devices allows an increased screening and detection capacity in a cost-effective manner, which can aid medical facilities in achieving a fast diagnosis, playing a crucial part in controlling the virus spread with less strict governmental actions such as closing schools and universities and locking down the entire country (
Figure 1B). After prolonged development period, POC testing is now gaining considerable traction due to the evolution of healthcare delivery methods, maturation of device fabrication technologies, and the expectations of the general public for rapid results. POC testing is therefore well-positioned to challenge the traditional centralized lab. This article summarizes key POC testing approaches developed for COVID-19 detection and provides insight into the potential future of these methods.
3. Future Direction and Outlook
Clearly, the covid-19 pandemic has triggered serious unprecedented impacts in almost all countries around the world while posing adverse and potentially long-lasting effects on those who are most vulnerable due to fragile healthcare systems. Alongside the overwhelming situation and deep stress felt by many populations, there are key lessons that can be learned. The COVID-19 pandemic showed us that a lack of international solidarity and commitment to share resources, knowledge, and experience makes controlling such a pandemic almost impossible. More specifically, global cooperation benefits vulnerable populations in avoiding the repeating of costly errors. Finally, the pandemic painted a clear example of the requirement for rapid, reliable, and sensitive diagnostic methods for widespread testing at a very early stage of disease in clinics, emergency departments (EDs), airports, and aged care facilities where ultrafast screening with high accuracy is necessary.
In this review, we have summarized the most promising SARS-CoV-2 POC detection methods, including immunoassay for antibody and antigen detection, RT-PCR as a gold standard approach, isothermal amplification as a fast method of nucleic acid amplification/detection, and CRISPR-Cas methods as a new emerging technique for nucleic acid detection (
Table 1). The processing time of all aforesaid methods has also been put into comparison and illustrated in
Figure 2D.
Table 1.
Comparison of selected assays for COVID-19 detection based on the execution time, sensitivity, specificity, LOD, and analyzed sample.
Table 1.
Comparison of selected assays for COVID-19 detection based on the execution time, sensitivity, specificity, LOD, and analyzed sample.
Immunoassays (Antibody) | Test/Author | Time (min) | Sensitivity | Specificity | LOD | Sample |
Li et al. [21] | 15 | 88.66% | 90.63% | - | Whole blood, serum, plasma |
Pan et al. [22] | 15 | 92.9% intermediate stage, 96.8% late stage | - | - | Whole blood, serum, plasma |
BioMedomics [23] | 10–15 | 100% | ~99% | - | Whole blood, serum, plasma |
Pharmact company [24] | 20 | 98.2% | 99.7% | - | Whole blood, serum |
Chembio diagnostics [25] | 15–20 | 96% | 98.7 | - | Whole blood, serum, plasma |
Immunoassays (Antigen) | CareStart [38] | 10 | 88.4% | 100% | 8 × 102–6.4 × 103 TCID50/mL | Nasopharyngeal |
Panbio [39] | 10 | 91% | 100% | 2.5 × 101.8 TCID50/mL | Nasopharyngeal |
Rapid Response [40] | 15 | 94% | 100% | - | Nasopharyngeal, oropharyngeal |
Sofia [43] | 15 | 96% | 100% | - | Nasopharyngeal |
Standard Q [41] | 20 | 96% | 99% | - | Nasopharyngeal |
Wantai kit [42] | | 94% | 98% | 20 pg/mL | Nasopharyngeal, oropharyngeal |
BD Veritor [44] | 15 | 84% | 100% | 1.4 × 102 TCID50/mL | Nasopharyngeal, oropharyngeal |
Rapid PCR | Xpert Xpress [61] | 25 | 99.4% | 96.8% | - | Nasopharyngeal swab, nasal swab, and nasal wash/aspirates |
QIAstat-Dx [63] | 60 | 95% | 100% | 500 copies/mL | nasopharyngeal swabs |
NxTAG COV [64,108] | 60 | 97.8% | 100% | - | Nasopharyngeal |
VereCoV OneMix [66] | 120 | - | - | 20 copies/mL | Nasopharyngeal |
VERI-Q Kit [67] | 55 | - | - | 8.9–9 copies/reaction | nasopharyngeal, oropharyngeal, sputum specimens |
Isothermal amplification | Yang et al. [83] | 30 | - | 99% | 1000 copies/mL | Nasopharyngeal |
El-Tholoth et al. [84] | 50 | 100% | - | 7 copies/reaction | Nasal swab |
ID NOW [90] | 13 | 95% | 97.9% | 125 copies/mL | Nasal, Throat, Nasopharyngeal |
Cue™ COVID-19 [93] | 25 | 99% | 98% | 20 copies/sample | nasal swab |
CRISPR-Cas | DETECTR [104] | 40 | - | - | 10 copies/µL | Nasopharyngeal, oropharyngeal, mid-turbinate nasal swabs, anterior nasal swabs, nasopharyngeal wash/aspirate and nasal aspirate |
Sherlock [101,109] | 60 | 100% | 100% | 6.75 copies/µL | nasopharyngeal, oropharyngeal, bronchoalveolar lavage |
iSCAN [106] | 60 | - | - | 10 copies/reaction | Oropharyngeal, nasopharyngeal |
Due to the surging number of confirmed COVID-19 cases throughout the world, fast and reliable POC tests for early detection are greatly needed. A reliable POC diagnostic device could reduce transportation needs, risk of spreading infection, strain on the healthcare system, and cost of care for both individuals and the government (
Figure 3A). In spite of the outbreaks caused by viral infectious diseases such as MERS, SARS and Ebola, existing programmable POC diagnostic platforms were not mature enough to promptly address the COVID-19 viral threat. However, during 2020 substantial efforts have been made to enhance COVID-19 detection using POC testing devices. This resulted in a variety of new and improved POC approaches (
Figure 3B) stimulating a fresh revolution in this field. They each have their own advantages for various purposes in different stages of infection between exposure to the virus and the onset of symptoms and recovery.
Antibody detection methods are not suitable for early detection of COVID-19 due to the late presentation of the antibody response; however, they have an important role in seroprevalence analysis, which helps countries to estimate the rate of exposure and take precautionary measures to handle waves of the pandemic [
110]. Moreover, immunoassay tests are essential to identify the level of antibodies before and after vaccination as it can show who has already been exposed to the virus and who has achieved immunity after immunization by a vaccine. On the other hand, virus antigen and nucleic acid detection approaches are mainly employed for early diagnosis as SARS-CoV-2 can be detected as early as the first week after exposure, before symptoms appear [
111]. Rather than being a simple, fast and cost-effective solution for early detection, current rapid antigen diagnostic tests show a highly variable range of sensitivity and specificity from 0–94% due to low viral load, quality of sampling, and intrinsic limitations in the detection technology [
45,
112,
113]. Therefore, major attention is currently focused on develo** nucleic acid-based POC testing as this exhibits a robust combination of accuracy and reliability.
According to the WHO, most cases of COVID-19 in countries beyond China originated from internationally imported patients [
114]. At the time of writing this review, imported cases of SARS-CoV-2 infection have been reported in 197 countries and territories. Therefore, there is no doubt that using a portable POC testing device at the border of each country, including border crossings, airports, and train stations would drastically reduce the risk of imported cases of COVID-19. While the symptoms of SARS-CoV-2 are similar to cold and influenza, a multiplexed POC platform with a high degree of accuracy that avoids cross-contamination could provide an opportunity to distinguish these diseases from each other. Hence, the psychological burden of COVID-19 would be reduced considerably, resulting in a safer global community during pandemic scenarios. Since COVID-19 is known as a highly contagious respiratory tract infection that is mainly transmitted via airborne droplets, it lends evidence to the idea that saliva can be a promising source of SARS-CoV-2 sample for detection. Recent research has proven saliva to be a minimally invasive and self-administrated sampling method featuring higher sensitivity and consistency compared to the standard sampling approach (nasopharyngeal swabs), which is more invasive and requires healthcare workers [
115]. Thus, a POC device compatible with simple saliva sampling could be a favorable platform for either SARS-COV-2 or other respiratory disease detection.
Microfluidics and microfabrication technologies offer significant advantages over conventional methods [
116,
117]. Since microfluidic devices can integrate different modules of pipetting, filtering, mixing, separating, and concentrating in a single miniaturized chip, they hold great promise as the future of low-cost POC testing devices featuring a rapid turnaround time (min) from sample-to-result [
118]. The future development of portable microfluidic-based cartridges will enable POC testing outside of the clinical diagnostic laboratory and enable decentralization. Moreover, the integration of smartphones and artificial intelligence (AI) with detection systems proffers effective communication and surveillance ability. It is anticipated that the integration of technological gadgets such as smartwatches and fitness trackers have the potential to integrate with the next generation of “smart” POC devices.
At this point, regardless of all the developments in POC devices, currently available approaches would be difficult to apply routinely in the clinical setting. The main concern is cost. The average cost for develo** a POC diagnosis device from conceptualization into the market is remarkably high, as building a miniaturized integrated device that can provide reliable results requires sophisticated technologies. There is no exemption for COVID-19, as the cost for detection of SARS-CoV-2 infection is currently estimated ranging from US
$15 (serological test) to
$45 (molecular tests) [
119]. In conclusion, we argue that it is no longer a question of if POC testing will be implemented clinically, but when, in which patient cohorts, and at what cost.