We already know PCR is used to diagnosis COVID-19 and can also...

We already know PCR is used to diagnosis COVID-19 and can also be used to detect pneumonia, however, they take a long time to process in a non COVID-19 environment as they aren't process in batches, hence the cost of process in the laboratory is more expensive and has clinically proven to have very low specificity.

In regards to this study where they are trying to differentiate if the pneumonia is IAP or NCP, the sample size is very small which they've acknowledged and they also used the current CT scan to detect for pneumonia which has clinically proven not to be very accurate and expensive as the test require additional hardware.

"Although real‐time RT‐PCR testing lacks the ability to diagnose pneumonia by design, it possesses good sensitivity and remains currently the gold standard for confirming SARS‐CoV‐2 infection"

"There are several limitations to our results. The scarcity of available NCP (n = 41) and IAP (n = 37) patients for analysis and the lack of external validation may limit the generalizability of our diagnostic model; future validation studies are certainly needed"