Hereâ€™s where a basic epidemiology term comes in: specificity.
- Basically, the specificity of a test tells us how often we get false positive results (a positive result in an individual that does not have the disease). If the test is 95% specific, 1 in 20 positive tests will be a false positive (95% will be right, 5% will be wrong).
Another epidemiological term also comes into play: positive predictive value.
- This is the likelihood that a positive test means that the individual really has the disease.
We need to consider a few things when it comes to screening:
- How specific is the test?
- How low would the positive predictive value be in a healthy animal?
- What would the response be to a positive result?
So can testing this dog for blasto beÂ justified?
If the result is negativeâ€¦
- Thatâ€™s great. Thatâ€™s the expected and hoped-for result. If the dog is healthy and the urine test is negative, it is very unlikely this dog has blastomycosis.
If the result is positiveâ€¦
- Thatâ€™s not great. That doesnâ€™t mean the dogÂ has blasto, but it means that more work is needed. ToÂ me, that would involve a thorough physical examination and a chest radiograph to look for the characteristic lung lesions. If those were normal, odds are it is a false positive test but we still couldn’t rule out early disease. Iâ€™d probably recommend re-doing the urine test to see if the positive result was consistent and, perhaps more importantly, whether the value increased over time. If the latter, Iâ€™d be concerned that the dog was developing disease.
- I would not take a positive urine test and embark directly on a prolonged and expensive course of treatment in a clinically normal animal.