If an animal tests negative for a pathogen 3 weeks after arrival at the facility, which conclusion is appropriate?

• A. The animal was free of this pathogen 2 weeks after arrival.
• B. The animal was free of the pathogen when shipped.
• C. The animal is free of the pathogen at the present time.
• D. All of the above

Answer: (B)

Testing an animal three weeks after arrival is used to look for infections that could have been acquired during transit or shortly after shipment. A negative result at that time provides the strongest, most defensible conclusion that the animal was not carrying the pathogen at the moment it left the supplier. If the animal had been infected before shipment, the test conducted within this window would more likely have detected it by three weeks after arrival. Therefore, the appropriate conclusion is that the animal was free of the pathogen when shipped.

The other implications aren’t as well supported. You can’t reliably claim something about an earlier time (two weeks after arrival) from this single late test, since the infection status could differ between those earlier days. And while the animal is negative at the time of testing, you can’t guarantee it will remain negative in the future if exposure occurs after the test.