COVID-19 Clinical and Laboratory Diagnosis Overview

COVID-19 was identified in Wuhan, China in in December 2019, and rapidly spread worldwide, being declared global pandemic one month later on 30 January 2020. Since its emergence, COVID-19 has raised global concerns associated with drastic measures that were never adopted in any previous outbreak, to contain the situation as early as possible. The 2019 novel corona virus (2019-nCoV) or SARS-CoV-2 is the causative agent of COVID-19. 2019-nCoV genetic sequence was rapidly identified within few days since the first reported cases and RT-PCR kits became available for COVID-19 diagnosis. However, RT-PCR diagnosis carries a risk of false-negative results, therefore additional serologic test are needed. The most important approach in the battle against COVID-19 is rapid diagnosis of suspicious cases, timely therapeutic intervention and isolation to avoid community spread. In this review, we summarize the clinical scenario that raises suspicion of COVID-19 and available laboratory diagnostics.


Introduction
The 2019 novel corona virus (2019-nCoV)/SARS-CoV-2 sequence was first identified in January 2020, from bronchoalveolar lavage (BAL) samples of five patients in Wuhan, China, presenting with unusual respiratory symptoms; fever, cough, and dyspnea accompanied by complications of acute respiratory distress syndrome with diffuse lung opacities and consolidation detected in chest radiography. Next generation sequencing results revealed an unknown β-CoV strain with 79.0% nucleotide identity with the sequence of SARS-CoV, 51.8% identity with the sequence of MERS-CoV and 87.7% nucleotide identity with bat SARS-like CoV ZC45 [1]. Therefore, it was announced that the 2019-nCoV is of bat origin. In fact, bats are the key reservoir of CoVs, and many human CoVs most probably have originated from bats [2,3].
The disease caused by 2019-nCoV/SARS-CoV-2 was named as corona virus disease 2019  by the World Health Organization (WHO) [4].
On 30 January 2020, COVID-19 was declared by the WHO as a public health emergency of international concern (PHEIC) [5]. In 2005, the WHO gained the power to declare an international emergency [6], since then, international emergency was declared five times; H1N1 swine flu in 2009, the Ebola outbreak in West Africa in 2013, the polio outbreak in 2014, the Zika outbreak in 2016, and Ebola outbreak in the Democratic Republic of Congo in 2019 [6].
However, none of these previous emergencies led a worldwide pandemic [7]. Because of the rapid increase in number of COVID-19 cases and uncontrolled worldwide spread, it was declared by the WHO a pandemic [8]. As of May 1, 2020 the virus has infected 3 175 207 total confirmed cases with 224 172 deaths [9]. COVID-19 pandemic was associated with strict measures to contain the situation where many countries closed its borders associated with partial lockdown of most daily activities and social distancing. COVID-19 properties alert global concerns. The incubation period of COVID-19 is believed to be as long as 14 days, with potential asymptomatic transmission [10,11] in addition to being highly contagious and having higher transmissibility (R0: 1.4-5.5) than both SARS-CoV (R0: 2-5) and MERS-CoV (R0: <1) [12], although the mortality rate is lower 3.4% for COVID-19, compared to 10% for SARS-CoV and 34% for MERS-CoV (13-15).

Clinical presentation
The China National Health Commission proposed guidelines for initial diagnosis and disease severity triage into mild, severe, and critical categories. Around 70% to 80% of patients are mild, and 20% to 30% are severe or critical [16]. Table 1 Clinical diagnosis requires epidemic exposure, in addition to two clinical findings of the following: fever, radiographic features, normal or lowered white blood cells, or reduced lymphocyte count [16].

Real Time-PCR (PT-PCR)
The current diagnostic test for COVID-19 is RT-PCR assay [17]. It would not be possible to do PCR test to all suspected individuals, so the Centers for Disease Control and Prevention (CDC) released guidance for COVID-19 PCR testing were cases are prioritized [18]. Table 2 The recommended specimen for testing is lower respiratory tract specimen; sputum and/or endotracheal aspirate or bronchoalveolar. If not possible or in asymptomatic contacts, upper respiratory tract specimen; nasopharyngeal and oropharyngeal swab or wash in ambulatory patients is collected, with preference of combined nasopharyngeal swab and oropharyngeal swab collection (19).
High viral loads in both upper and lower respiratory tract are detected 5 -6 days of the onset of symptoms [20][21][22][23]. Lower respiratory tract specimens yielded highest viral loads for the diagnosis of COVID-19 [24]. As for upper respiratory tract specimens higher sensitivity of nasopharyngeal swabs (63%) was detected compared to oropharyngeal swabs (32%) [25].  [28] where there is a lag period of 14-28 days after the onset of illness till the antibodies appear in patients' serum [29]. In some people with COVID-19, disease 6 confirmed by RT-PCR, weak, late or absent antibody responses have been reported [30][31]. The strength of antibody response is dependent on multiple factors as age, nutritional status, disease severity, and certain medications or infections that may suppress the immune system [30,31,32].
In March 2020, the FDA issued a policy that allows developers of certain serological tests to begin to market or use their tests once appropriate evaluation to ensure test validation are performed. The FDA issued this policy to allow early patient access to certain serological tests. It is recommended to use combined IgG&IgM antibody testing for more accurate results [28].
The average time for seroconversion in reported studies is 12 days, while positive RT-PCR is detected 5-6 days from the onset of symptoms, making antibody testing still inferior to RT-PCR in COVID-19 diagnosis but more likely used when RT-PCR is not available or accessible [35].
A sample of reported studies using different serological tests to detect antibodies against SARS-  To monitor disease progression, it recommended to combine both serial viral load monitoring and antibody response as viral load found to be inversely related to serum antibody response [22].
Detection of antibody responses to COVID-19 in the population is important to aid vaccine development, and to add to our understanding of the extent of infection among individuals who passed asymptomatic and/or not identified during surveillance efforts. On the other hand, positive antibody testing doesn't guarantee safety from reinfection by COVID-19 or acquisition of herd immunity, and therefore shouldn't be considered as an excuse to ignore public health advice which may lead to increasing the risk of continued transmission.