Many but not all applications of saliva on the LFA do require sample dilutions.

One of the major factors leading to the common dilution of saliva is its viscosity, which is typically high and variable depending on the individual and level of salivation at the moment of testing. This viscosity is a result of mucin proteins which are one of the primary components of saliva. Food particles contribute to this issue of viscosity as well and pose a problem when trying to flow a sample through the test strip. This ultimately will result in highly inconsistent test results. Besides viscosity, the mucin proteins also to be very "sticky" and often interfere with antigen-antibody binding. Overall this makes saliva a very challenging matrix to work with, especially considering that many salivary biomarkers are present in extremely low concentrations. Hope that helps!

For more information, below is a review article discussing the use and treatment of saliva on lateral flow assays.

Article Quantitative Lateral Flow Assays for Salivary Biomarker Asse...