What does microbiology has to do with Covid testing? On paper, not much since the whole ordeal is more aligned with molecular pathology. But, my department still helps out with some aspects of testing to ease the burden.
We use the Sofia SARS Antigen FIA from Quidel corporation. As the picture from the company website states, it's a fluorescent immunoassay. In simple terms, it means it uses antigen-antibody reactions. The result is the release of fluorescent molecules which a detector picks up. You can read in greater details follow the link and read the various inserts.
For the curious, the antigen of the test is the nucleocapsid protein (shell) of the virus. The test calls for nasopharyngeal swabs (the deep kind) from patients. Our medical director sought to validate this test to work on saliva due to the unpleasant deep swabs. Alas, the effort did not work out, so everyone is still stuck with the uncomfortable poke in the back of your nose.
When do we perform this test?
We only do this for patients in the ER, who the clinician are considering admitting. A viral transport media (VTM) accompanies each patient swab. All Covid antigen tests are stat tests.
The test kit has several components:
- Reagent tube with buffer
- Ampoule with salt solution
- 120µl pipette
- Testing cassette
The fist part of running the test has to be inside a biosafety hood. To start, you add the solution into the tube and inoculate the patient sample. Incubate it for 1 minute. Then, use the pipette to transfer the solution onto the cassette.
The patient sample goes into the well. The arrows indicate the direction you put into the Sofia 2 Analyzer. I edited out the QR code even though it's the cassette's own serial number. That part needs to be clear in order for the analyzer to process it. So, don't obstruct it in any way.
Here, we have two choices depending on the work load.
The cassette needs to incubate at room temperature for about 15 minutes. On the analyzer, you can see who options:
walk away and
Walk away means you let the incubation happen inside the machine and it will automatic time and read it.
Read now will tell the machine to read it immediately. The Sofia 2 will display a huge positive or negative sign when it finishes the test. So far, I have never gotten an invalid result.
Depending on the result, there are different actions.
A positive result requires a stat callback to the ER. We cache the VTM. No further action.
A negative result requires we order a PCR test. We send the VTM to molecular department.
A negative result doesn't mean you do not have the virus. It could mean your viral load is below detection limit. Quidel claims their test kit has PPA of 96.7% and NPA of 100%. This is relative to the PCR tests. The PPA means it's in agreement 96.7% of the times.
So, how often do we perform the antigen test?
The answer from personal experience is around 10-12 a day. That means there are 10-12 patients in the ER each day with suspected Covid infection. It's not uncommon to see 2-4 positive results a day either. This is to give you a perspective of what is happening. And this only applies to what I am seeing at my workplace.
Posted with STEMGeeks