A saliva test may be able to predict future health risks. White blood cells secrete immunoglobulins (IG) which are necessary to fight off illness. Secretory IgA (sIgA) can be measured through saliva and it is the body’s first line of defense against infection.
DNA swab of saliva taken from senior man
The study looked at 639 participants saliva samples from 1995. IgA secretion rate was measured along with mortality rate which was tracked for over 19 years. 
The findings uncovered that IgA secretion rate was negatively associated with all-cause mortality.
Saliva sample may be an effective health tester because it is less invasive than blood tests. There are a number of factors that can affect how well we produce antibodies and maintain their levels. 
There are some that we have no control over, such as age, heritability or illness, but our general state of health can also affect their levels; stress, diet, exercise, alcohol and smoking can all influence those levels.
However, the research team is still unsure as to whether saliva tests could be used at regular check-ups but does state that it could be a good and early indicator of health.
The team hopes to conduct a larger study to further confirm findings.