Very rarely the Lab will take away HP. I wish it wasn't so rare, or I wouldn't have to level up to 125 to be able to train anything else.

Labbing for stats is good if you have another pet you're training but if you're relying on the lab for higher stats you will end up under-leveled with really high HP and STR, and almost no DEF.

Here are the full chances (click on the "click here" on that page for a small pop-up with the complete results).