Video: What is a PCR?

 

Maria Paz Sánchez-Seco, Head of the Arbovirus Laboratory at the National Microbiology Centre (NMC), explains what a PCR is: the technique used to determine whether or not a patient is infected with the Ebola virus, how this test is performed and the conditions in which samples are transported, the equipment that must be worn by staff and how the virus is removed from the sample.

WHAT IS A PCR?

It is a technique through which a target signal is amplified by basically raising and lowering temperatures; if the target signal you are looking for is present in the sample, you will obtain a signal you can see and that gives you a positive result.

HOW IS A PCR PERFORMED?

The sample arrives in the appropriate triple packaging biosafety conditions from a company that guarantees its safekeeping from the moment the sample is taken until its reception at the NMC. We open the outermost tertiary packaging and the secondary packaging is taken into the P3. We pass the samples from the P3 and start working in the P3 under high biosafety conditions.

We then proceed with deactivation. Deactivation is performed in class 2 biosafety cabinets and the staff responsible for this step must wear full personal protection equipment consisting of sealed disposable coveralls, a type CP3 mask, goggles and a protection face mask. All work is carried out inside a type 2 biosafety cabinet with a laminar air flow that prevents anything from inside the cabinet escaping and represents yet another layer of protection for staff. The virus is deactivated inside the biosafety cabinet: the virus is killed, once, after shaking it and approximately 10 minutes have passed there is no viable virus, there is no virus in that sample.

We remove the samples and they are taken back to the SAS at the barrier that separates the P3 biosafety level from the P2 biosafety level. The P2 laboratory is a laboratory like this one where work is also done inside biosafety cabinets for extraction of the nucleic acid. The nucleic acid is extracted and amplified in a thermo-cycler for the PCR. We get our results from that PCR: our result is positive if we see amplification, although it needs to be confirmed with a second PCR in another region of the genome in order to avoid false positives. If the result is negative, that PCR includes an internal control that enables us to say the result is negative and not a false negative. The samples from which we have obtained a positive result are sent to a confirmation laboratory: i.e. a class 4 laboratory serving as a reference laboratory for the WHO. In our case, we send them to the Bernhard Nocht Institute for Tropical Medicine in Hamburg.