epidemiology : MC9 - sensitivity and specificity

What is the purpose of diagnostic tests in epidemiology?

To correctly identify sick and healthy animals and support disease control, screening, and treatment decisions.

What does test validity mean?

The ability of a test to measure what it is supposed to measure; determined by sensitivity and specificity.

What does test reproducibility (reliability) mean?

The ability of a test to give consistent results when repeated.

What does the 2x2 diagnostic table contain?

True positives, false positives, false negatives, and true negatives.

What is sensitivity?

The probability that a test correctly identifies sick animals as positive.

What is the formula for sensitivity?

Sensitivity = TP / (TP + FN)

What does high sensitivity mean?

Few false negatives; the test rarely misses sick animals.

When do we want a highly sensitive test?

For screening dangerous diseases where missing a case would be harmful.

What is the phrase "SNOUT"?

A Sensitive test when Negative rules OUT disease.

What is specificity?

The probability that a test correctly identifies healthy animals as negative.

What is the formula for specificity?

Specificity = TN / (TN + FP)

What does high specificity mean?

Few false positives; the test rarely labels healthy animals as sick.

When do we want a highly specific test?

For confirmatory testing or diseases where false positives have serious consequences.

What is the phrase "SPIN"?

A Specific test when Positive rules IN disease.

What is a true positive (TP)?

An animal that is sick and tests positive.

What is a false positive (FP)?

A healthy animal that tests positive.

What is a false negative (FN)?

A sick animal that tests negative.

What is a true negative (TN)?

A healthy animal that tests negative.

What are common causes of false positives?

Maternal antibodies, cross-reactions, nonspecific reactions, lab errors, poor test specificity.

What are common causes of false negatives?

Early infection, low antibody levels, immune tolerance, poor sampling, lab errors.

What is Positive Predictive Value (PPV)?

The probability that an animal is truly sick if the test result is positive.

What is the formula for PPV?

PPV = TP / (TP + FP)

What is Negative Predictive Value (NPV)?

The probability that an animal is truly healthy if the test is negative.

What is the formula for NPV?

NPV = TN / (TN + FN)

What happens to PPV when disease prevalence increases?

PPV increases (positive tests become more trustworthy).

What happens to NPV when disease prevalence increases?

NPV decreases.

What happens to PPV when disease prevalence decreases?

PPV decreases (more false positives).

What happens to NPV when disease prevalence decreases?

NPV increases.

Do sensitivity and specificity change with prevalence?

No, they remain constant.

Do predictive values change with prevalence?

Yes, PPV and NPV are strongly affected by prevalence.

When should a test with high sensitivity be used?

For screening—when it is important not to miss any sick animals.

When should a test with high specificity be used?

For confirmation—when false positives must be avoided.

What is a screening test?

A test with high sensitivity used to identify potential cases.

What is a confirmatory test?

A test with high specificity used to verify true disease.

A test finds 90 true positives, 10 false negatives, and 20 false positives. What is the sensitivity?

90 / (90 + 10) = 90%

Using the same test, what is the specificity if there are 180 true negatives and 20 false positives?

180 / (180 + 20) = 90%

What is the PPV with 90 TP and 20 FP?

90 / (90 + 20) = 81.8%

What is the NPV if you have 180 TN and 10 FN?

180 / (180 + 10) = 94.7%

A highly sensitive test gives many false positives. Why is this acceptable?

Because missing sick animals is more dangerous than falsely identifying healthy ones.

A Brucella confirmatory test must avoid false positives. What characteristic must it have?

High specificity.

A screening test for rabies must not miss cases. What characteristic must it have?

High sensitivity.

You use a test in a population with very low disease prevalence. What happens to PPV?

PPV decreases—positive results are less trustworthy.

In high-prevalence herds, what happens to NPV?

NPV decreases—negative results may be false negatives.

A test has high sensitivity but low specificity. What does this mean?

It catches most sick animals but produces many false positives.

A test has high specificity but low sensitivity. What does this mean?

It confirms disease accurately but misses many sick animals.

A cat tests positive on a low-prevalence disease test. Should you trust the result?

No—low prevalence reduces PPV; confirm with a specific test.

A cow tests negative on a highly sensitive test. What can you conclude?

It is very unlikely the cow has the disease ("SNOUT").

A goat tests positive on a highly specific test. What can you conclude?

The goat is almost certainly sick ("SPIN").