I’m sorry to say that positive drug tests do not happen as they do on CSI. A lab technician does not put a few drops of clear liquid into a test tube which promptly turns blue indicating guilt. It is a very complex process prone to biases, interpretations and errors.
The below link takes you to Trust But Verify. Under research, a link looks at a study commissioned by the WADA looking at dietary factors that may affect drug test results. Very technical but you should be able to get the gist through the commentary.