"I may need to pass a drug test for cannabis, despite having a California medical card, and managed ulcerative colitis. I have tried test strips available from online vendors (e.g. Amazon), but I am not confident in the results. Can I contract for a drug test with a lab in the Bay Area?"
You can buy a drug test kit at any Walgreens drug store but the sensitivity and specificity of the test cannot be guaranteed. It you are involved in an incident at work involving public safety or property damage and testing is mandated you will undergo industrial drug testing that is highly sensitive and specific given the civil litigation impacts of false test results in such a setting. Hair samples can be tested that will yield positive results when blood and urine tests are questionable. If you are employed in a setting that mandates drug testing, I encourage you to consider use of transdermal topicals for pain as there are no documented case reports of positive blood tests in people who use cannabis infused transdermal body balms, lotions and massage oils in the standard concentrations present in dispensary grade products.