You will be faced by many questions concerning diagnosis, treatment and control measures when you enter practice, and you should be confident in your answers. This is quite achievable, so look at some of the diagnostic situations and prepare your answers for the second review session on April 27th. That would correspond to exactly the same number of days that it would take for worms to develop into antigen-producing females. If the puppy were bitten on the day it was born, at 120 days you could expect the dog to become antigen positive. In other words, a positive test on the antigen test should be rechecked before launching into adulticide therapy.
Technical errors are much more likely than the true false-positive.
Not enough worms to produce sufficient antigen for the test to register. Each manufacturer can provide you with the specifics for their tests. Few worms and all of them males, because the antigen produced is from female heartworms. Antibody complexing enough of the circulating antigen to reduce the concentration below the threshold of the test. This would be most likely in a light infection where you could expect the antigen concentration to be low in the first place, and lowered still by the antibody binding.