Many substances that can be found in blood are distributed into saliva (but in different concentrations) so saliva as a biological material can be used for a wide range of tests. All of these have their advantages and disadvantages, and many are still not routinely used. The important thing to have in mind is that, in blood, substances are bound to plasma proteins to some extent, and only the free (unbound) fraction can pass into saliva, so the concentration in saliva is proportional to the concentration of free substance in blood.
Let's have a look at some possible applications:
Monitoring the concentrations of certain medicines can be useful, and saliva is a good material for monitoring neutral substances, weak acids or weak bases. Saliva is often used in therapeutic drug monitoring, metabolic phenotyping (determination of activity of metabolic enzymes in a patient), compliance monitoring and determining the free fraction of the medicine. These are all used in highly individalised approaches to treatment, and are not routinely used.
These can be found if you visit the websites of some laboratories that do these tests, and see what they offer. Some of those that I have found include:
- steroid hormone testing (this is by far the most common one I found)
- peptide hormone testing
- antibody testing (for some infectious diseases, HIV e.g, and possibly allergies)
- there is a commercially available nitric saliva test
(by testing I mean - detecting presence and determining the concentration).
This procedure is definitely non-invasive, and as such it can be very convenient when multiple samples have to be drawn at certain time periods. This is often the case with steroid hormones, and this is why saliva can be preferred to blood - sampling saliva is less invasive than venipuncture.
On the other hand there are some analytical limitations: saliva is a complex matrix and some substances in it interfere with analysis; the concentration of tested substances is usually low in saliva, so method sensitivity is an issue. These sensitive methods are often advanced and sophisticated, and may be quite costly.
There are other options which are being used or studied (some are successful and entering diagnostic use, some require further research):
- Markers for systemic malignancies
- Biomarkers for oral diseases
- Biomarkers for some autoimmune diseases
- Forensic analysis samples
- Illicit drugs and their metabolites
I take it that your question was out of curiosity. Clinical applications of these tests, whether they are necessary and cost-effective, and other clinical justification issues are up to a physician to determine in each individual case.
An interesting aside:
There are several collection methods. In some you don't actually have to spit into a tube - you chew a special swab and place it into a tube, and the saliva is obtained by centrifugation. Pretty neat, huh?
Image source: Salivette® instruction manual
- Rocky Mountain Analytical - Our Tests - commercial website, used as an example only
- Diagnos-Techs, Inc.™ - Why Saliva - commercial website, used as an example only
- Steroid Analysis in Saliva: An overview
- Saliva as a Diagnostic Fluid