Why isn't saliva sampling used when sampling for the presence of respiratory diseases?

Question

Previously I've asked a question focused on COVID-19: Why did countries initially presume that detecting COVID-19 requires a deep nasal swab?

@Fizz provided the following response:

So clearly it was preferred for Covid-19 because of the "received wisdom" from other respiratory virus illnesses. (To pick a random example, the 2005 WHO guidlines for avian influenza don't mention saliva samples, but various kinds of swabs and washes (for the upper respiratory tract): nasal swab, nasopharyngeal swab, nasopharyngeal aspirate, nasal wash, throat swab.)

If you want to ask why saliva sampling is not a method more widely used (instead of, or, in addition to nasopharyngeal swabs) in other respiratory virus diagnostics, modify your question according (incl. tag).

So my follow up question is: how did we arrive to the "common wisdom" of using nasal swabs in respiratory virus diagnostics? Was it just presumed that it's more efficient than saliva testing or was there any research on this matter?

Answer 1

It's because influenza attacks ciliated cells, and you get the best specimens where you include cellular material

Acceptable respiratory specimens Most tests can be used on a variety of respiratory specimen types, however not all specimen types yield equivalent results, and other factors can influence specimen quality. Nasal aspirates, nasal washes, sputa and nasopharyngeal swabs, especially those specimens containing cellular material, are preferable to nasal swabs and throat swabs. They should be collected as close to the onset of symptoms as possible and not after 4–5 days in adults as virus shedding typically diminishes. In young children, viral shedding may occur for longer periods, and the collection of specimens for testing after 5 days of illness may still be useful. There are very limited data on the shedding of avian influenza viruses in human infections. For guidelines on human specimen collection and handling see WHO guidelines for the collection of human specimens for laboratory diagnosis of avian influenza infection (http://www.who.int/csr/disease/avian_influenza/guidelines/humanspecimens/en/).

Saliva is a biofluid made by salivary glands and doesn't include much cellular material. You can get virus from saliva but it's more sensitive from a swab, and more sensitive again when you do both.

https://www.cdc.gov/flu/professionals/diagnosis/clinician_guidance_ridt.htm

https://pubmed.ncbi.nlm.nih.gov/28111058/