Diagnostic tests Flashcards
What is a diagnostic test?
A test that provides information that aids in making a specific diagnosis
Used in clinical medicine, surveillance, international trade, and research
What are dichotomous tests?
Tests that only have two possible answers - usually positive or negative
Ex - radiographs = presence or absence of a lesion
What are continuous tests?
Tests that have a continuum of possible answers
Ex - serum chemistry
T/F: A single hematology or biochemical diagnostic test will/can result in a diagnosis
FALSE
Usually a definitive dx will require more than just one of those blood tests - but they can give a PRESUMPTIVE dx and a limited list of differentials
What do immunological tests look for?
Use antigens to measure antibodies
What are some examples of pathogen detection tests?
Direct visualization - microscopy Immunological tests Virus isolation or bacterial culture PCR based tests *some tests used for epi investigations discriminate infected animals from vax animals (DIVA)
What is the test value of a diagnostic test?
What ever is being measured
How is the cut-off value for dx tests made?
The cut-off value is determined experimentally, as the value that minimizes false positive/false negative results
Animals with dz will be tested with the gold standard test and animals without dz will be proven to be dz free using a gold standard test. A cut-off value that best separates the two groups is used
T/F: When determining a cut off value - there usually is not a clear separation in the test values between diseased and non diseased animals
TRUE
This is why we will have some false positives and false negatives
What is the definition of: True positive True negative False negative False positive
True positive = diseased animal that tests positive
True negative = non-diseased animal that tests negative
False negative = diseased animal that tests negative
False positive = non diseased animal that tests positive
Since gold standard tests are very accurate and reliable, why aren’t they used all the time?
They are often very labor intensive, impractical, highly invasive, slow, and or expensive
How is the sensitivity of a test determined?
Sensitivity of a test is determined by using that test on a group of diseased animals
How is the specificity of a test determined?
Specificity of a test is determined by using that test on a group of non diseased animals
Sensitivity is the proportion of ______ animals that the test correctly classifies as _______
Diseased
Positive
The animals that do no test positive = false negatives
Specificity is the proportion of ______ animals that the test correctly classifies as ______
non diseased
negative
The animals that do not test negative = false positives
What kind of dx test should be used to rule a disease out?
A very sensitive test
A sensitive test, when negative, will rule the disease out
What kind of test, when positive, will rule a disease in?
A very specific test, when positive, will rule a disease in
When should you maximize sensitivity?
When you need to detect ALL diseased or infected animals and you don’t want any false negatives
*good to use when importing animals
When do you want to maximize specificity?
When the cost of a false positive is high
*Ex- when the prognosis of dz is very poor or the treatment for that dz is very expensive - you want to be sure the diagnosis is correct
*positive = rules the dx in
(but negative will not rule the dz out)
When testing in a series, how should those tests be in regards to sensitivity and specificity?
test one - high sensitivity
(all negatives will be true - but you may have some false positives)
test two - high specificity
(this will eliminate any false positives from the first test)
What is the PPV and NPV?
PPV = positive predictive value - the probability that an animal who tested positive actually has the disease
NPV - negative predictive value - the probability that an animal who tested negative for the disease is actually disease free
NPV and PPV are both dependent on what factor?
The prevalence of the disease in the population
and the sensitivity and specificity of the test
