ELISA (serologic(blood) ) test for Johne's is very reliable for animals with clinical signs suggestive of Johne's Disease; for animals that are clinically normal... not so much - may miss as many as 35% of those in the incubation phase. Most infected cows seroconvert around the time they break with clinical disease(diarrhea, wasting).
Fecal PCR and culture are very reliable... culture takes anywhere from 6-16 weeks for results, depending upon method.
Neither of these are tests that many veterinarians can run 'in-house'... they have to go to a diagnostic lab(private or state/university-affiliated) .
BVD-PI status would also be a consideration, though if this cow is in the 'mucosal disease' presentation of that deal, I'd expect her stools to be bloody, and she'd be dead in short order.