Diagnostic Testing

Sensitivity (se)

the probability of infected or exposed animals that are correctly identified by a test (ID those with the disease)

Se equation

A/A+C = true pos/all diseased

something highly sensitive, has few false ____ and an inc change of false ____

neg; pos

when a sensitive test is negative, it rules disease ____

out

when would we select a sensitive test

when it's critical to detect every possible case (high contagious diseases)

a sensitive test reflects ____ tests

screening

Specificity

the probability of noninfected or nonexposed animals that are correctly classified by a test (ID those who DO NOT have disease)

Sp equation

D/B+D = true neg/all non-diseased

something highly specific, has few false ____ and an inc change of false ____

pos;neg

when a specific test is positive, it rules disease ____

IN

when would we select a specific test

when it's critical to confirm a disease (rabies)

a specific test reflects ____ tests

confirmatory

Negative predictive values (NPV)

the proportion of animals with negative test results that are truly noninfected or nonexposed (test neg truly do not have disease)

NPV equation

D/C+D = true neg/all neg tests

when a diseae is rare, most negatives are true = ____ NPV

high

if you have a low prevalence, how does it affect NPV

inc (high)

if you have a low prevalance, how does it affect PPV

dec (low)

Positive predictive values (PPV)

the proportion of animals with positive test results that are truly infected or exposed (test post truly has disease)

PPV equation

A/A+B = true pos/all pos tests

when a diseae is rare, have mnay false positivies = ____ PPV

low

if you have a high prevalance, how does it affect PPV

inc (high)

if you have a high prevalance, how does it affect NPV

dec (low)

how are specificity and sensitivity related

inversely

if you lower the cut-point/threshold, how is Se and Sp affected

increases sensitivity but decreases specificity

if you raise the cut-point/threshold, how is Se and Sp affected

Decreases sensitivity and increases specificity

what are the 4 diagnostic testing approaches

sequential, battery, series, and parallel

Sequential testing

screening test followed by a confirmatory test

Battery testing

multiple tests applied at one time

Series testing

two tests applied and BOTH have to be positive

Parallel testing

two tests applied and only ONE has to be positive

when do you use a series test

when you want to be certain about the diagnosis (rule disease IN)

a series test creates increased ____ and ____

Sp and PPV

a series test is best for _____ tests

confirmatory

a parallel test creates increaed ____ and _____

Se and NPV

a parallel test is best for _____ tests

screening

what 3 factors and impact diagnostic results

stage of disease, target of test, and age

Stages of disease (3)

Latent, subclinical, or clinical

Targets of test (2)

antigen vs antibody