If you were convicted of drink driving A blood sample will be taken, but that is only tested for CDT which is an indicator of alcohol dependence, the only time a drugs urine sample would be required is if your conviction was for drug driving, or there was something in your answers that indicated that a drugs test was necessary.
Whatever sample is taken, it is not tested on the spot, the doctor sends it to an independent laboratory for analysis, with the results then going to DVLA.