COVID-19 Diagnostic Test: Xpert Xpress SARS-CoV-2

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Last edited and updated on: by Sagar Aryal

Medical Disclaimer: The information presented on the website is only for academic and study purposes and must not be used for the purpose of diagnosis. If you are not feeling well, please consult with your physician or doctor.

Introduction

COVID-19 disease caught the world unaware, unprepared for a pandemic as bad at what we have faced the past 3 months. Countries which the most sophisticated technologies and strong health systems have scrambled for diagnostic testing kits that could salvage the increased cases of the infection that has caused deaths of over 30,000 as of March 29th, 2020.

This viral infection that was initially found in Wuhan City, Huabei, China, has shaken the globe in many aspects. The health sector has been tested on how ready it is to handle emergencies such as this. The WHO, the CDC and the FDA along with government authorities are working around the clock to come up with measures, protocols, policies, and giving updates on new developments in regard to the infection. One of the aspects that were most tested is the ability to diagnose the disease. There has been a lack of clear diagnostic protocols specific to the virus.

Initially, the first medical practitioners that identified the virus, used the Real-Time RT-PCR, as recorded by The Lancet, after the Viral genome was sequenced and release to the public for purposes of designing vaccines and drugs for the treatment of COVID-19.

After observing the curve under which the virus was spreading and a lack of a specific diagnostic test, it was clearly a matter of urgency on how important it was to have a definitive test for the novel Coronavirus. And therefore, the FDA under the Emergency Use Authorisation (EUA) has authorized the use of some diagnostic tests designed by certified companies in order to increase the efficiency of detecting the virus before it spreads to a larger population.

Cepheid Receives EUA from FDA for Rapid SARS-CoV-2 Test (COVID-19) — Discussion w/CMO David Persing

Here we will discuss the most recent technique that was designed by Cepheid Xpert Technologies.

Xpert Xpress SARS-CoV-2

  • This a rapid molecular in vitro diagnostic test designed by Cepheid, the company that has and uses the GeneExpert technology system to detect for Nucleic acids in samples.
  • The Xpert Xpress SARS-CoV-2 test was devised to detect and diagnose SARS-CoV-2, only.
  • Its working principle is based on the amplification of the nucleic acids for the qualitative detection of the SARS-CoV-2 viral RNA.
  • Considering the manifestations of the viral infection, symptoms which are associated with coughing due to the infection of the upper respiratory tract, this technique uses samples collected from the nasopharynx and/or the nasals.
  • Being a rapid diagnostic technique, it can be able to detect for RNA nucleic acids of SARS-CoV-2 within 45 minutes which makes it a very effective technique this far to enable taking appropriate measures when it comes to Respiratory isolation, quarantining and basically, treatment.

Xpert Xpress SARS-CoV-2

Image Source: Cepheid

Objectives of Xpert Xpress SARS-CoV-2

  • To detect RNA nucleic acid from a nasal wash or nasal swab sample of a suspected COVID-19 infected patient.

Principle of the Xpert Xpress SARS-CoV-2

  • The Xpert Xpress SARS-CoV-2 was developed by the Cepheid Xpert System for use in detecting the nucleic Acid within a single apparatus (cartridge). Designed with all its components under a single tube, the cartridge is incorporated with liquid reagents and solid dry reagents which allows the interaction with the sample, extracts the nucleic acid, purifies it, amplifies it and detects the viral RNA by Real-Time RT-PCR.
  • The cartridge is designed with an automated Line known as  ROBAL (Reagents on Board  Automated Line), whose function is to fill in liquid reagents and dry reagents in the form of lyophilized beads that initiates the amplification reaction, after adding the liquid sample.
  • The Xpert Xpress SARS-CoV-2 test contains primers and probes and an internal control used in the RT-PCR for in-vitro detection of the virus form specific samples. According to the manufacturing company, Cepheid and the FDA, this test should only be used by qualified laboratories. This means that the persons responsible for performing the test must be proficient in using Real-Time RT-PCR Assays.

How Cepheid’s SARS-CoV-2 Test Cartridge (COVID-19) is Made & How it Works with CMO Dr. David Persing

Instrumentation of the Xpert Xpress  SARS-CoV-2

The Technique uses a specially designed GeneExpert cartridge tube, whose parts are manufactured by companies within the United States, and those parts are combined together to form a cartridge tube where the test is done. The components of the cartridge tube include:

  • White cartridge body – its the main component of the tube incorporated with several mini tubes that hold the liquid and dry reagents, and the sample. Inside the cartridge, the tube is where the reaction takes place.
  • Reagent on Board Automated Line- this is a blue line that is used to fill reagents (liquid and solid) into the cartridge body
  • A slender tube – this is a thin tube placed at the end of the cartridge tube, where the detection of the virus takes place, the heating and cooling mechanisms of Polymerase Chain Reaction (PCR)
  • Lid – it closes the top part of the cartridge tube especially after filling in the reagents and the sample
  • Foot – this represents the stand of the cartridge tube

The procedure of Xpert Xpress SARS-CoV-2

The diagnostic kit is a ready-to-use test, that’s doesn’t use a lot of external procedures. Some of the procedures involved include:

  1. Collect the patient sample Nasal swab/wash or/and a nasopharyngeal swab and put it in a sterile tube.
  2. Using a pipette that has been included in the kit, transfer the sample into the Cepheid cartridge tube which already contains the reacting reagents.
  3. Close the cartridge with the lid and put in a gene expert system, for 45 minutes.
  4. Read and interpret the result.

Cepheid’s Xpert® Xpress SARS-CoV-2 Test (for the Virus that causes COVID-19 & the GeneXpert® System

Results and Interpretation of Xpert Express SARS-CoV-2

The results based on the reactions that occur within the cartridge body are meant to detect the RNA nucleic acids of SARS-CoV-2. By using the nasopharyngeal swab samples and/or nasal wash or aspirates collected in the early stages of infection, are used for the test. Positive results indicate an active infection of SARS-CoV-2, which needs to be correlated with clinical diagnosis (observed signs and symptoms) and patient history.

A negative result does not exclude the possibility of SARS-CoV-2 infection, therefore it shouldn’t be used as the sole basis for treatment and patient management decisions. It is important to combine the laboratory diagnostic test with clinical diagnosis, patient history, and demographic evidence.

Advantages of Xpert Xpress SARS-CoV-2

  1. It is rapid and can detect the presence or absence of SARS-CoV-2 RNA within 45 minutes.
  2. It is accurate and the possibility of contamination is low.

Disadvantages of Xpert Xpress SARS-CoV-2

  1.  It can present a false-positive result, which might indicate the presence of other viral and bacterial infections.
  2. A negative result does not preclude SARS-CoV-2 and therefore, clinical diagnosis and epidemiology of the patient should be investigated to rule out possibilities of a false negative.

Application of Xpert Xpress SARS-CoV-2

  • This diagnostic test was specifically designed for the detection of SARS-CoV-2 RNA Nucleic acid in suspected cases of COVID-19 infection.
  • It must be used in a healthcare system that has a GeneXpert System only.
  • It must be used by a well trained and qualified person who can perform the Real-Time RT-PCR, only.

References and sources

  1. Faith Mokobi Zablon and Sagar Aryal. 2020. The Novel Coronavirus, COVID-19: An Overview. The Biology Notes.
  2. https://www.fda.gov/emergency-preparedness-and-response/mcm-legal-regulatory-and-policy-framework/emergency-use-authorization#2019-ncov
  3. https://www.fda.gov/emergency-preparedness-and-response/counterterrorism-and-emerging-threats/coronavirus-disease-2019-covid-19
  4. https://www.cepheid.com/coronavirus
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