Diagnostic tests

Question 1

What is a diagnostic test?

Answer 1

A test that provides information that aids in making a specific diagnosis

Used in clinical medicine, surveillance, international trade, and research

Question 2

What are dichotomous tests?

Answer 2

Tests that only have two possible answers - usually positive or negative

Ex - radiographs = presence or absence of a lesion

Question 3

What are continuous tests?

Answer 3

Tests that have a continuum of possible answers

Ex - serum chemistry

Question 4

T/F: A single hematology or biochemical diagnostic test will/can result in a diagnosis

Answer 4

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

Question 5

What do immunological tests look for?

Answer 5

Use antigens to measure antibodies

Question 6

What are some examples of pathogen detection tests?

Answer 6

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)

Question 7

What is the test value of a diagnostic test?

Answer 7

What ever is being measured

Question 8

How is the cut-off value for dx tests made?

Answer 8

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

Question 9

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

Answer 9

TRUE

This is why we will have some false positives and false negatives

Question 10

What is the definition of:
True positive
True negative
False negative
False positive

Answer 10

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

Question 11

Since gold standard tests are very accurate and reliable, why aren’t they used all the time?

Answer 11

They are often very labor intensive, impractical, highly invasive, slow, and or expensive

Question 12

How is the sensitivity of a test determined?

Answer 12

Sensitivity of a test is determined by using that test on a group of diseased animals

Question 13

How is the specificity of a test determined?

Answer 13

Specificity of a test is determined by using that test on a group of non diseased animals

Question 14

Sensitivity is the proportion of ______ animals that the test correctly classifies as _______

Answer 14

Diseased
Positive

The animals that do no test positive = false negatives

Question 15

Specificity is the proportion of ______ animals that the test correctly classifies as ______

Answer 15

non diseased
negative

The animals that do not test negative = false positives

Question 16

What kind of dx test should be used to rule a disease out?

Answer 16

A very sensitive test

A sensitive test, when negative, will rule the disease out

Question 17

What kind of test, when positive, will rule a disease in?

Answer 17

A very specific test, when positive, will rule a disease in

Question 18

When should you maximize sensitivity?

Answer 18

When you need to detect ALL diseased or infected animals and you don’t want any false negatives

*good to use when importing animals

Question 19

When do you want to maximize specificity?

Answer 19

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)

Question 20

When testing in a series, how should those tests be in regards to sensitivity and specificity?

Answer 20

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)

Question 21

What is the PPV and NPV?

Answer 21

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

Question 22

NPV and PPV are both dependent on what factor?

Answer 22

The prevalence of the disease in the population

and the sensitivity and specificity of the test