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Let's talk about *window period* and how it makes everything in diagnostics complicated and difficult.

Window period is the time between infection and when a diagnostic test becomes useful.

This is not Star Trek: our tests aren't perfect or comprehensive.
Typical viral infection:
1. Virus enters your tissues.
2. Virus starts spreading beyond local point.
3. You develop a local immune reaction (swelling, redness)
4. Local immune reaction prompts systemic reaction (fever, aches)
5. Your immune reaction overwhelms virus.
1 is too early to detect anything unless we biopsy the target tissue and have a perfect test.
Your symptoms don't show until 4 (maybe 3).

Now we have to consider what we're testing for.
Here are the diagnostic windows for HIV. Imagine you're testing for p24 antigen by immunoassay.

You can detect it between day 22 and day 45. Not before, not after.

You can be viral RNA (+), antibody (-), and antigen (-) day 11-17.
This is assuming perfect test performance. No false negatives, no false positives.

Enter SARS-CoV-2. We have an RNA test (qRT-PCR) that detects early but may not detect shedding in recovered or convalescent patients. There's minimal window period in *hospital presentation*.
Now there's an antibody test, which is being pushed into front-line use because it's *FASTER* than the qRT-PCR test for viral RNA, but it has a much longer window period, regardless of sensitivity.

Recently infected people will still be seronegatives.
All of this makes studies of test concordance (agreement) more difficult. Patients would be expected to differ between serological and nucleic acid testing. It's inappropriate to expect them to agree, but there's pressure to establish universal predictive value.
There's also some misuse of the tests for the detection window being targeted: antibody tests being used for early detection is entirely inappropriate. They're great at establishing exposure after the fact, but terrible at screening populations.
The fundamental principle some ordering physicians miss is that we can only detect what exists in the fluid or tissue they're testing.

This isn't Star Trek, the tests don't go through the whole body, and they can't test every type of evidence of infection all at once.
Things are going to get more complicated the closer we get to primary care physicians, walk-in clinics and home testing.

Right now, we stratify patients by risk. Hospital presentations are already patients with symptoms. When we start screening asymptomatic patients...
... based on exposure, the positive samples will likely be much earlier, certainly in the window period for antibodies or antigens, in rare cases even undetectable for qRT-PCR for viral RNA.

That's going to be a bit of a mess of false negatives, and will damage trust, I think.
In short, testing by different methods will give different results, and we're going to be dependent on a partnership of laboratorians and clinicians, both informed and aware of test performance, in order to get accurate results reported to patients.

==Fin.==
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