How Diagnostic Tests Work…

I have just had a good friend have all sorts of trouble with her lovely old Labby- He suddenly got sick, really unwell, and in a matter of weeks they had to help him along to spirit… He suddenly developed Diabetes, and then they thought he had developed Cushings disease as well. However- the tests they did came back as negative, even though he had the symptoms!

This was confusing and painful…

The thing is- Diagnostic, or Laboratory tests are not perfect! I always tell my clients this- even the very best tests have some False results- both False Negatives (so they tell you your pet doesn’t have some problem, when really he does)- and False Positives (so they tell you your pet does have some problem, when really he doesn’t!)

Confused yet?

Stay with me- and I will show you what I mean.

There are two important measurements of diagnostic tests- SENSITIVITY, and SPECIFICITY..

Sensitivity tells you how good a test is at detecting when your pet has a given dis-ease. So- If a test has a Sensitivity of  %95, then it will detect 95 out of every 100 patients with the given dis-ease.

Specificity Tells you how accurately a Positive Test identifies patients who actually have the given dis-ease… So if a test has %90 Specificity, then 10 times out of 100, if your pet tests positive to the dis-ease  with that specific test, he doesn’t actually have the dis-ease.

Or Don’t Work?

So- When you are testing for diseases that are harder to diagnose with Laboratory or blood test (and Cushings is one of these)- it gets really tricky.

Let’s take a quick look under the hood- for Cushings, there are several tests you can use-

1. The Low Dose Dexamethasone Response Test is about %95 Sensitive, but only around %70 Specific (so it will detect 95 out of 100 pets with the disease, BUT  out of 100 positive tests, 25 patients will not have the disease)
2. The ACTH Response Test is about %80 Sensitive, But it’s Specificity can vary from %35-%80
3. The Urine Cortisol:creatanine ratio Test is about %90 Sensitive, but it’s Specificity is only  around %25 (so only about a 1/4 of dogs with abnormal levels in this test actually have Cushings disease, BUT it will pick up %90 of dogs tested that do have the dis-ease.)

I hope that makes some sort of good sense

So What do You Do then?

Please remember that this is confusing- even for your Vet- who I am sure is doing their level best to find out what’s up, so he can help you out, and find an effective treatment. I just think it will help if you know that all diagnostic tests are not created equal, and neither a positive, nor a negative test result will give you absolute certainty most of the time. This is why we sometimes need to do more tests,and sometimes what we don’t find in a test is as meaningful as what we do find. (So no test is wasted!- they all help by increasing our knowledge of the case…)

And it might help you understand just how hard it sometimes is for your vet to work out what is wrong with your pet.

Happy pets until next time, Dr Edward