Let it play out and see how it goes. Insurance might pick it up under appeal. They can file up to 3 appeals to your insurance company.
In the worstcase (insurance rejects it) then at least your cost isn't huge. Still stinks though! Prometheus should bill you only $250 (price I know for infliximab concentration test, which you didn't get) if the test isn't covered. I'm not sure what the charge is for the Thiopurine metabalites test, might be similar. They also have a patient assistance program that drops the test down to $75 depending on proof of income.
http://www.prometheuslabs.com/Resources/Financial_Asssistance_Application_Form.pdfJust an FYI, if you need another thiopurine metabolites test, it can be done through most labs now. Prometheus created the test, but others have copied it. So, you can get a lab order to go to quest diagnostics, labcorp, or any of dozens of other labs. Sometimes health insurance tells you test are free if you use Labcorp, or somebody else whom they have a special agreement with on cost.
Oh the joys of chronic illness, dealing with health insurance companies, labs, doctors, and all of this other BS. *grumble*, it shouldn't be hard at all, but unfortunately it is!