Diagnostic tests in Covid19

The aim of this article is to summarize current knowledge of diagnostic test for SARS-CoV-2 during Covid-19 pandemic.

At the begining pandemic of COVID-19 Director-General of WHO urged everybody to “test, test, test”. In his opinion testing, isolation, and contact tracing should be our reponse to global pandemic [1].

As recommended by the World Health Organisation (WHO) and the Centers for Disease Control (CDC) there are common indications for diagnostic SARS-CoV-2 testing [2-3]:

1.???Anyone with Covid-19 symptoms, regardless of vaccination status

2.???Asymptomatic persons, regardless of vaccination status, who are close contacts of someone with known or probable SARS-CoV-2 infection should undergo diagnostic testing. Persons who are unvaccinated or who have not received a vaccine booster within the previous 6 months have a higher pretest probability of infection than those who are fully vaccinated.

3.???Testing should be considered in asymptomatic persons who have been in a setting where the risk of transmission is high (sporting event etc.)

In the European Union more than 140 different companies have had an antigen-based rapid tests for approved use [4]. These have various pathogen targets, detection methods, swabbing sites to obtain specimens and indications for use. According to the results of studies they have shown varying degrees of clinical accuracy (sensitivity, 36 to 82%; specificity, approximately 98 to 100%) [5-6]

In general there are three types of diagnostic tests:

1.???molecular or nucleic acid amplification tests (eg, PCR tests) that detect viral RNA;

2.???antigen tests that detect Surface viral proteins (eg, nucleocapsid, spike and receptor binding domains);

3.???serology tests that detect host antibodies in response to infection, or vaccination, or both

PCR tests detects RNA of the virus. Antigen tests detects its proteins (nucleocapsid, spike proteins etc.). Serology tests detects host antibodies in response to infection, vaccination or both. PCR tests and antygen tests can be used to diagnose acute infection. Contrary, serology tests provide only indirect evidence of infection but 1–2 weeks after the onset of symptoms. No test is perfect. The molecular or antigen tests have the highest sensitivity in early phase of infection when viral loads are high.

The current knowledge of the immune response to SARS-CoV-2 infection remains incomplete. The humoral and cellular immune responses occur within first two weeks after onset of symptoms. Humoral immune responses are mediated by antibodies directed to viral surface proteins [7-8]. In viral infections IgM antibodies typically appear several weeks earlier than IgG antibodies. However IgM and IgG antibodies against SARS-CoV-2 tend to appear almost simultaneously. Usually peaks about 11–14 days after onset of symptoms [9]

PCR tests are based on genome sequences of SARS-CoV-2. They are highly sensitive and specific. They are used as the reference standard for the diagnosis of active SARS-CoV-2 infection. [10-12]. Antigen tests are more available and easier to use than molecular tests. They can provide results within 15–20 min, but they has a lower limit of detection of genome copies per mL. According to the results of studies individuals with a viral load less then antigens test treshold are unlikely to transmit the virus. Antigen tests are useful as rapid triage tool to identify those most likely to transmit infection [13-15].

In the nasal passages of infected individuals high concentrations of virus can be detected even regardless of their clinical manifestations.[16]. SARS-Cov-2 virus is rarely found in blood or urine. [17] Infections of SARS-CoV-2 are classified as asymptomatic, presymptomatic, or symptomatic. It means that symptom-based testing alone is not adequate to control the spread of the virus.[18]. It is estimated that more than 20% of virus transmission could be attributed to individuals who were asymptomatic [19]. The available data from studies shows that SARS-CoV-2 viral RNA can be detected as early as 6 days before symptom onset. Its concentration peak around the time of symptom onset or a few days later. After two weeks after onset of symptoms RNA?usually becomes undetectable from upper respiratory tract. There is no substantial differences between adults and children. Although patients can remain RNA-positive for weeks after symptom onset live virus cannot be cultured from specimens collected later than 9 days after symptom onset. It may suggest that the mean period of infectiousness could be restricted to the period between 2 and 3 days before and 8 days after symptom onset [20]. After this period PCR could detect genomic fragments rather than an actively replicating virus [21].

In patients with symptoms of COVID-19 diagnostic tests are useful to confirm the clinical diagnosis, regardless of their vaccination status [22]. In general testing is not useful in the first 48 hours after exposure to SARS-CoV-2. It is estimated that the most appropriate window for testing is generally considered to be 5 to 7 days after exposure [23]. The symptoms of Covid-19 typically occur 2–14 days (average 5-6 days) after initial exposure. Given the high sensitivity and high specificity of molecular tests theirs false-positive or false-negative results are rare. In patient with high clinical suspicion of COVID-19 the test result may be negative. The specimen was inadequate or an error was made in the testing. In this case the test should be repeated [10-12]. If individuals with symptoms of COVID-19 who present 7–14 days after symptom onset combined the use of PCR and antigen tests should be considered. According to the results suggest that combining molecular and antibody testing in the second week after symptom onset can increase the rate of COVID-19 case detection by 40%.[24]

In the course of viral infection reinfection are common. It is mainly due to waning immunity. COVID-19 reinfection can be defined as the clinical recurrence of symptoms compatible with COVID-19 accompanied by positive PCR test more than 90 days after the onset of the primary infection [25]. According to data form large population-based protection against repeat infection over 7 months is 80,5 % - no difference by sex. However in patients aged 65 years or older is decreased to 47,1% [26].

In vitro IgG antibodies against the SARS-CoV-2 spike proteins are correlated with neutralising activities. However a high amounts of IgG antibodies have been shown in patients who have severe COVID-19 disease. It suggest that a robust IgG response might not be an indicator of protective immunity. [27]. A threshold for protective neutralising antibody responses has yet to be defined. It is affected by several factors: viral variants, viral loads etc. Furthermore it should be remembered that antibodies are only one component of an effective host response to infection. Cellular immune response also has a important. A positive antibody test result should not be used as proof of immunity. Moreover it is an absence of consensus on how to quantify protection from natural or vaccine-induced immunity. The protection against some new variants may be reduced [28].

An integral element of outbreak management is identification of people who are infected in the community. Case finding can be passive or active. Passive case finding relies on individuals with signs and symptoms recognising these as being associated with infection and self-reporting to health workers. Active case finding is a systematic search for individuals with infection and, of those found, all those who have signs and symptoms are proactively tested. Other forms of active case finding include mass population screening. Epidemiological modeling suggested that the effectiveness of outbreak control depends largely on the frequency of testing and the speed of reporting. Moreover it is only marginally improved by high test sensitivity. To ensure a safety in high-risk health environments the testing frequency should be twice a week?[29].

There is a wide selection of diagnostic tests. The choice of which test to use in what setting requires careful consideration of: the purpose of testing, tests accuracy and accessibility and the rapidity with which results are needed [30].?

References

1.???WHO. WHO Director-General’s opening remarks at the media briefing on COVID-19—16 March 2020. https://www.who.int/ director-general/speeches/detail/who-director-general-s-opening-remarks-at-the-media-briefing-on-covid-19---16-march-2020 (accessed Aug 27, 2021).

2.???World Health Organization. Antigendetection in the diagnosis of SARS-CoV-2 infection. Interim guidance. October 6, 2021 (https://www .who .int/ publications/ i/ item/ antigen -detection -in -the -diagnosis -of -sars -cov -2infection -using -rapid -immunoassays).

3.???Centers for Disease Control and Prevention. Testing for COVID-19. October 4, 2021 (https://www .cdc .gov/ coronavirus/ 2019 -ncov/ testing/ index .html).

4.???European Commission Directorate- General for Health and Food Safety. EU health preparedness: a common list of COVID-19 rapid antigen tests; a common standardised set of data to be included in COVID-19 test result certificates; and a common list of COVID-19 laboratory based antigenic assays. December 2021 (https:// ec .europa .eu/ health/ sites/ default/ files/ preparedness_ response/ docs/ covid -19_ rat _common -list_ en .pdf)

5.???Prince-Guerra JL, Almendares O, Nolen LD, et al. Evaluation of Abbott BinaxNOW rapid antigen test for SARS-CoV-2 infection at two community-based testing sites — Pima County, Arizona, November 3–17, 2020. MMWR Morb Mortal Wkly Rep 2021; 70: 100-5.

6.???Pray IW, Ford L, Cole D, et al. Performance of an antigen-based test for asymptomatic and symptomatic SARS-CoV-2 testing at two university campuses — Wisconsin, September–October 2020. MMWR Morb Mortal Wkly Rep 2021; 69: 1642-7

7.???Poland GA, Ovsyannikova IG, Kennedy RB. SARS-CoV-2 immunity: review and applications to phase 3 vaccine candidates. Lancet 2020; 396: 1595–606

8.???R?ltgen K, Boyd SD. Antibody and B cell responses to SARS-CoV-2 infection and vaccination. Cell Host Microbe 2021; 29: 1063–75.

9.???Zhao J, Yuan Q, Wang H, et al. Antibody responses to SARS-CoV-2 in patients with novel coronavirus disease 2019. Clin Infect Dis 2020 71: 2027–34.

10. WHO. Recommendations for national SARS-CoV-2 testing strategies and diagnostic capacities. Interim guidance. June 25, 2021. https:// apps.who.int/iris/bitstream/handle/10665/342002/WHO-2019-nCoV-lab-testing-2021.1-eng.pdf?sequence=1&isAllowed=yoV-lab-testing-2021.1 (accessed Aug 24, 2021).

11. Infectious Disease Society of America. IDSA guidelines on the diagnosis of COVID-19: molecular diagnostic testing. Dec 23, 2020. https://www.idsociety.org/practice-guideline/covid-19-guideline-diagnostics/ (accessed Sept 2, 2021).

12. European Centre for Disease Prevention and Control. Diagnostic testing and screening for SARS-CoV-2. May 21, 2021. https://www. ecdc.europa.eu/en/covid-19/latest-evidence/diagnostic-testing (accessed Sept 2, 2021).

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17. Bastos ML, Perlman-Arrow S, Menzies D, Campbell JR. The sensitivity and costs of testing for SARS-CoV-2 infection with saliva versus nasopharyngeal swabs: a systematic review and meta-analysis. Ann Intern Med 2021; 174: 501–10.

18. Furukawa NW, Brooks JT, Sobel J. Evidence supporting transmission of severe acute respiratory syndrome coronavirus 2 while presymptomatic or asymptomatic. Emerg Infect Dis 2020; 26: e201595.

19. Arons MM, Hatfield KM, Reddy SC, et al. Presymptomatic SARS-CoV-2 infections and transmission in a skilled nursing facility. N Engl J Med 2020; 382: 2081–90.

20. Health Information and Quality Authority. Evidence summary for the duration of infectiousness in those that test positive for SARS-CoV-2 RNA. Sept 15, 2020. https://www.hiqa.ie/sites/default/ files/2020-04/Evidence-Summary_COVID-19_duration-of-infectivity-viral-load.pdf (accessed Nov 30, 2021).

21. Alexandersen S, Chamings A, Bhatta TR. SARS-CoV-2 genomic and subgenomic RNAs in diagnostic samples are not an indicator of active replication. Nat Commun 2020; 11: 6059.

22. WHO. Recommendations for national SARS-CoV-2 testing strategies and diagnostic capacities. Interim guidance. June 25, 2021. https:// apps.who.int/iris/bitstream/handle/10665/342002/WHO-2019-nCoV-lab-testing-2021.1-eng.pdf?sequence=1&isAllowed=yoV-lab-testing-2021.1 (accessed Aug 24, 2021).

23. Centers for Disease Control and Prevention. Symptoms of COVID-19. February 2021 (https://www .cdc.gov/coronavirus/2019-ncov/symptoms-testing/symptoms.html).

24. Zhao J, Yuan Q, Wang H, et al. Antibody responses to SARS-CoV-2 in patients with novel coronavirus disease 2019. Clin Infect Dis 2020 71: 2027–34.

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