Unfortunately, the cannabinoids CBD and THC present in the most popular transdermal patch do enter the blood stream and are associated with psychoactive effects. I recently researched the literature to find out if positive blood tests for cannabinoids have been documented in patients using transdermal body balms in the concentrations available in standard dispensary preparations. To date, there have been no reports of positive blood tests using transdermal body balms, lotions and massage oils. This may be a safer option for you.
If you hold a Class A license I caution you to be cautious with your use of medical cannabis. If you are in an accident in which injury, property damage, civil litigation or criminal action occurs you will be drug tested using industrial testing methods that are highly sensitive and specific. Additionally, you will place the physician who authorized you at professional risk.
I agree with Dr. Sumchai, that topicals like salves and balms are not likely to cause a positive drug test. While CBD is not psychoactive, most transdermal CBD patches will have tiny amounts of THC that are not enough to cause mental impairment (particularly in the company of CBD which mitigates the psychoactivity of THC), but are enough that you could test positive for THC. Cannabis salves and creams generally have THC which does not pass through moist, intact, healthy skin, but might if the skin is very dry with micro-abrasions. If the latter occurs, then theoretically, it might be possible to test positive. Using a topical on healthy skin would be the safer route.