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Our Knowledge in Diagnosing COVID-19

There are different diagnostic techniques available for dealing with the current COVID-19 pandemic. These include serological (ELISA, rapid test, CLIA) and molecular (mainly PCR) tests. At HIPRA we have more than 20 years of diagnostic experience in PCR and ELISA.

Diagnostic Techniques

The Coronavirus 2 (CoV-2) responsible for the Severe Acute Respiratory Syndrome (SARS-CoV-2) is an enveloped non-segmented RNA virus with positive polarity. It is the cause of the so-called Coronavirus Disease of 2019 (COVID-19).

There are two main types of tests for laboratory diagnosis of COVID-19: SARS-CoV-2 viral RNA screening tests (primarily using qPCR) that identify infected individuals with the virus during the acute phase of infection, and serological tests that identify individuals who have been in contact with the virus and have developed a specific humoral response against the virus.

PCR

Molecular technique to detect CoV-2. This type of technique is performed in the laboratory, mainly from nasopharyngeal samples. A positive PCR result indicates that the patient has COVID-19 and is therefore infected. The sensitivity and specificity of this type of techniques are excellent. However, there is a risk of false negative results1, due to:

  • Sample type: nasopharyngeal samples are usually taken. In some cases, SARS-CoV-2 may not be present in the upper respiratory tract. 1
  • Sample intake: nasopharyngeal samples are not a simple easy to obtain. The personnel who do this must be trained to do so properly. 1
  • Quality and consistency of the PCR. 1

Nasopharyngeal sample

SARS-CoV-2 Detection

Indicates infection

ELISA

Serological technique based on an enzyme immunoassay. Detects and quantifies the amount of antibodies to SARS-CoV-2 from serum or plasma samples. The results it provides are quantitative, so the amount of antibodies present in the sample can be determined. A positive result indicates that the patient has had contact with the virus. Sensitivity and specificity values are better than the rapid test, avoiding erroneous results.

There are two main types of ELISA kits depending on the type of antibody that they detects: IgM or IgG. IgM antibodies are produced more quickly as they are less potent. IgG antibodies are more potent and specific to infection.

Further studies are still necessary to determine the day post-infection (d.p.i.) on which each of these is produced and how long they both last. Some studies show:

  • IgM: start of IgM production at 7-12 d.p.i. 1,3
  • IgG: start of IgG production at 10-14 d.p.i. 1.3

Serum or plasma sample

Antibody Detection

Indicates previous contact with the virus

In view of the difficulty in obtaining a correct sample for PCR analysis and the delay in antibody production against SARS-CoV-2 (minimum 5-7 d.p.i.), several authors recommend combining PCR and ELISA for a correct approach to patient diagnosis. 1

BIBLIOGRAPHY

1 Antibody Responses to SARS-CoV-2 in Patients of Novel Coronavirus Disease 2019. Zhao J. et al. Clin Infect Dis. 2020:ciaa344. doi: 10.1093/cid/ciaa344. https://academic.oup.com/cid/advance-article/doi/10.1093/cid/ciaa344/5812996
2 Antibody Detection and Dynamic Characteristics in Patients With COVID-19. Xiang F et al. Clin Infect Dis. 2020;ciaa461. https://academic.oup.com/cid/advance-article/doi/10.1093 /cid/ciaa461/5822173

3 Tan, W. et al. Viral Kinetics and Antibody Responses in Patients with COVID-19. medRxiv
(2020).

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