Immunodiagnostic Assays

Sensitivity

Proportion of infected animals the test classifies as positive

Specificity

Proportion of uninfected animals the test classifies as negative

What does diagnostic sensitivity indicates?

Ability of test to detect presence of infection when infection is present

What does diagnostic specificity indicates?

ability of test to indicate the absence of infection, when infection is absent

How is sensitivity calculated?

True positives / infected

How is specificity calculated?

True negatives / uninfected

False positive test result

Test is positive, but animal is not infected by the pathogen

False negative test result

Test is negative but the animal is infected by that pathogen

Reasons for false positives and negatives

• Testing errors

• Diagnostic test characteristics

• Biological differences

Diagnostic characteristics that could cause false results

low specificity or low sensitivity

Biological differences that could cause false results

animal condition or disease process

Positive predictive value (PPV)

Probability a POSITIVE test result is a true positive

Negative predictive value (NPV)

Probability a NEGATIVE test result is a true negative

PPV =

A/(A+B)

NPV =

D/(C+D)

Sensitivity =

A/(A+C)

Specificity =

D/(B+D)

A

True positive (infected & tests positive)

B

False positive (uninfected & tests positive)

C

False negative (infected & tests neg.)

D

True negative (uninfected & tests neg.)