||Lithium levels are checked to make sure the patient is taking the right levels of lithium. Lithium is a great drug for bipolar disorders (relatively inexpensive), but has a very narrow window of therapy, meaning that the dosage range is very narrow and too much or too little a dose can cause toxicity or no effect on the patient.|
The testing methodology used to be 'flame photometry' but this has largely been replaced with 'ion selective electrode' technology. They are working on a rapid microchip assay, which to our knowledge has not yet been approved by the FDA. The fact that lithium enters the body as lithium ion (carrying a positive electrical charge) allows it to be measured electrochemically (ion selective electrode) or by a spectral change in a calibrated flame (flame photometry).
These assays are different than most drugs-of-abuse tests, which are generally enzyme linked immunoabsorbent tests (ELISA) or gas chromatography/mass spectrometry assays (GC/MS).
Lithium level testing is done to assure adequate levels only, and is not performed as part of the "routine" blood work for a physical, unless specifically ordered. Since anything that can cause dehydration can cause a potential fatal elevation in lithium level, people taking duiretics or having kidney problems should avoid this drug.
So, the testing that is done for a lithium level test is unrelated to the testing done to detect the presence of most recreationally-used drugs. However, it is important to remember that once a blood sample has been submitted, it is possible to do other tests on that blood. If the hospital or testing facility is instructed to conduct other tests (whether by law enforcement, parents, insurance company, etc.), it is possible that they could do so.