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 PCR) that identify if certain viral RNA sequences are present in the sample, and serological tests that identify individuals who have been in contact with the virus and have developed a immune response against it.

The rapid antigen tests can be performed outside the lab but professional sanitary staff is needed to take the sample. The results are obtained in 15-20 minutes and the aim of the test is to perform big scale testing in emergency cases.

Rapid Antigen Test

The rapid test detects the presence of a protein from the SARS-CoV-2 virus in the nasopharyngeal sample. The aim of its use is to prevent the spread of the virus by reducing the time period needed for PCR results.

Interpretation:

According to manufacturer indications: A negative result does not exclude the possibility of patient infection with COVID-19 and it should not be used as the only base to determine a treatment or making therapeutic decisions. A negative result should be combined with clinical observation, patient history and epidemiologic information.



Check the European Comission recommendation regarding the use of rapid antigen tests for the diagnosis of SARS-CoV-2 infection.

PCR

Molecular technique to detect CoV-2 RNA sequences which needs to be performed in the laboratory. The presence of RNA sequences of CoV-2 can be assessed in different kinds of samples such as individual or pooled* nasopharyngeal samples, waste waters or surface samples.



*The pooled samples are obtained by mixing a maximum of 5 individual samples and carrying out only one PCR.



Interpretation:



A positive PCR result indicates the presence of the virus, therefore infection, in the case of individual samples, or presence of the virus in the building in the case of waste waters samples. In the case of a positive result from a pooled sample, it means that one or more of the 5 patients is infected, and in order to know which one/s, the individual PCR’s need to be carried out.



The sensitivity and specificity of this type of technique is excellent, so the quantity of 5 assures the detection of one positive among them. However, there is a risk of false negative results, 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 must be properly trained to do it. ¹Quality and consistency of the PCR. ¹
• Denaturation of the sample during transport due to an excessively long transit period or unsuitable conditions.
• Presence of PCR inhibitors in the PCR originating from the sample itself or from the transport.

ELISA

Serological technique based on an enzyme immunoassay. Detects and quantifies antibodies against SARS-CoV-2 from serum or plasma samples. The results are quantitative, so the amount of antibodies present in the sample can be determined.

There are two main types of ELISA kits depending on the type of antibody detected: IgM or IgG:

• IgM antibodies are produced more quickly as they are less specific.
• 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

Interpretation:

A positive result indicates that the patient has had contact with the virus. The amount of antibodies present in the sample can’t be interpreted by itself nor indicate the level of immunity of the patient. Its correct interpretation requires a context of complementary information (symptoms, other tests) and a professional doctor’s evaluation.

The interpretation of negative ELISA results depends on the type of antibodies (IgM, IgG), the stage of infection (initial or late phase) and the sensitivity of the technique. Each one of these facts should be assessed by a doctor in charge.

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 

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 

Combined interpretation:

 It must be taken into account that the diagnostic results are highly variable between individuals, the combined interpretation of the two laboratory techniques results together with the symptoms increases the available information and can give a more accurate context to assess the patient status.

A simplified interpretation key would be:

The European Centre for Disease Prevention and Control (ECDC) has published testing strategies and objectives for COVID-19.

Check the latest updates issued by the Spanish Government’s Ministry of Health with regard to the different measures and recommendations for dealing with the COVID-19 pandemic.