What medical tests are essential before breeding my Doberman?

I'm not sure I completely understand your question but I will do my best to answer it. In any breeding situation, the best thing one can do healthwise is verify as much as reasonably possible. Ways to do this are to obtain x-rays to verify the health of the hips and elbows. In the case of a Doberman in particular, an echocardiogram to check for any signs of heart disease or defects is paramount. Another Doberman specific test would be to check for von Willebrand's disease, which is a disorder that reduces clotting abilities. Having Gowri's eyes examined by an ophthalmologist for CERF certification is also valuable in breeds that are known for genetic predisposition for ophthalmic disease. If you were asking about what blood tests to get to verify status in the heat cycle, obtaining progesterone levels can be valuable. Obtaining vaginal cytology (vet looks under a microscope at the type of cells) is often the best method to determine readiness for a successful breeding. The discharge you noted may or may not be normal, but in the absence of any other clinical signs of illness I would not be terribly concerned. Hope this answers your question.