That’s not a test tube, that’s a bottle.

and zecora’s the father

You know, I did some research on the topic of swamp fever (or Equine Infectios Anemia) and I found out there are two tests used to diagnose this retroviral disease
 
The Coggins Test and the ELISA test  
(This video explains that this test is used on avian diseases, but the same/similar procedures and techniques still applies)
 
If the videos are TL;DW for you, basically those tests use a solution of antibodies (or secondary antibodies) against the viral proteins (or a partition/epitome of the viral protein) to test for agar growth lines in the Coggins test or change in color in the enzyme-coated antibody solution in the ELISA test
 
Thus, though this test was simplified for this kids’ show, the idea of using versatile shapeshifting magic antibodies as a test fluid may be a plausible theory on how the GP doctor diagnosed Zekky with swamp fever…

doctor muffin top: your pregnant.