Nicotine saliva tests are not as dependable as urine tests are, which is a shame because many health and life insurance companies use the test to determine qualification and rates for health insurance.
My job's health insurance required me to get a nicotine and drug test and used the saliva test for it. Even though I do not smoke and I am never around a smoker, I tested positively for nicotine and my insurance rates went up. I then appealed the results saying that I'm a non-smoker and requested a urine test. The urine test gave a negative result for nicotine.
I'm a vegetarian and it turns out that some of the foods vegetarians eat often can trick the saliva test into thinking that it's nicotine. I read about this during my appeal and stopped eating cabbage, garlic, almonds and broccoli which I used to consume a lot of. So there are some factors that can cause a false result with the saliva test and I prefer to get a urine test for that reason.