many patients with COVID-19 appear to have negative PCR, while other had positive test even without any disease or infection.
This depends on the symptoms. They might have the virus (the PCR detects it) but they don't know until it is confirmed. Not all of us react to the virus in the same way. We show different symptoms. The positive means we have the virus, the negative means we don't have it. PCR tests may make mistakes and is why we have to redo them. It is nlt frequent, and is a very reliable assay. (PCR tests may make mistakes in the opposite way, however we prefer them with this kind of mistakes, covid and barely no symptoms). It is difficult when we do not show the same symptoms.
The specifity might be a parameter mostly of qtPCR. It might also be applied generally to methods. How well is a method is based in... Those parameters.
This is not an easy answer. Note that each assay (different suppliers and different amplification platforms) has a LDL (lowest detection limit). If the result is negative, all it means is that the assay could not detect COVID-19 RNA from the sample. If the RNA concentration in the sample is lower than the LDL, no amplification will take place. Resulting in a negative result. The correct term to use is rather "Not Detected" instead of "Negative".
Another important aspect; if the sample was taken incorrectly, this will lead to low or no viral RNA, potentially resulting in a false negative result.
Most tests are multiplex, detecting more than one gene area, improving specificity. However, with so many symptoms overlapping with those of other flu symptoms, the question could also be how specific are the symptoms?
To calculate specificity and sensitivity, you need to know the number of false positives and false negatives
Comparing the electrophoretic and hybridization-enzymatic methods for amplicon detection, the sensitivity of electrophoresis is found to be 89.9%, specificity 91%; for hybridization-enzymatic detection, these values are 96.1 and 100%, respectively. Samples that yield false-negative results are repeatedly tested by two-staged amplification (nested PCR) with external and internal primers. The main cause of false-negative results is a low concentration of proviral HIV-1 DNA in some blood samples.
you can also refer to this article- K A Sarkisian, G A Shipulin, M S Vorob'eva, O Iu Shipulina, V I Shakhgil'dian, A V Kravchenko, E V Buravtseva, L V Serebrovskaia, V V Pokrovskiĭ, E V Korystova
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