Serological analysis for COVID-19 diagnosis

The SARS-CoV-2 virus is a single-stranded positive-sense RNA virus belonging to the Coronavirus family. This virus is responsible for the COVID-19 disease, declared a pandemic by the World Health Organization in March 2020. COVID-19 is a disease that primarily affects the respiratory tract, leading to symptoms such as dry cough or fever, and in severe cases, bilateral pneumonia that sometimes progresses to the patient’s death.

Diagnosis through RT-PCR

The most common method for diagnosing COVID-19 is RT-PCR (Reverse Transcription Polymerase Chain Reaction in Real Time), which aims to detect active SARS-CoV-2 infection. 

However, due to the sensitivity of RT-PCR, there may be cases of patients who are in the early stage of infection, and consequently, their viral load is so low that RT-PCR detects them as negative. This is known as a “false negative,” and to deal with it, it is recommended to complement the diagnosis through RT-PCR with serological testing, especially in cases where RT-PCR is negative.

ELISA Method

Serological tests for the diagnosis of COVID-19 detect specific antibodies against SARS-CoV-2 antigens in patient serum samples, with the most commonly used method being ELISA, which allows for the detection and quantification of both early IgM and late IgG antibodies specific to SARS-CoV-2.

As a preliminary step to performing this technique, serum from patients must be obtained. The Digicen 22 is a universal centrifuge capable of centrifuging up to 32 tubes of 5 ml of blood samples thanks to a rotor (easily installed using the REI System), dedicated especially to this type of samples.

To do this, a blood sample is taken in a 5 ml blood collection tube without anticoagulant and centrifuged at 1,500 G for 15 minutes. After centrifugation, the rotor with the sample is removed, and the sample is divided into two fractions. The upper fraction, clear and transparent in appearance, corresponds to the blood serum, which is transferred to another tube to begin the immunoassay.

Overall, complementary diagnosis through RT-PCR and ELISA would allow determining if patients with negative RT-PCR are:

• Actually negative for the infection, as they have no antibodies of any kind against SARS-CoV-2; or they present IgG antibodies, indicating that they have already had the infection.

• “False negatives”; as they present IgM antibodies, indicating that they are in the early stage of infection. In this case, appropriate isolation measures should be taken.

The detection of antibodies against SARS-CoV-2 through ELISA contributes to a more accurate diagnosis of COVID-19, and for the proper functioning of this technique, it is necessary to isolate the serum from the total blood through a simple centrifugation step beforehand

Read more about the performance of PCR tests.