Sensitivity and Specificity

Question 1

Accuracy

Answer 1

Measure of how close a test result is to the truth

- proportion of all correct tests (out of total number of tests run)

Question 2

What are 3 things that affect accuracy?

Answer 2

• prevalence
• sensitivity
• specificity

Question 3

2 x 2 table: positives and negatives

Answer 3

Positive Negative
+ a (TP) b (FP)
- c (FN) d (TN)

Question 4

Accuracy formula

Answer 4

= a + d / n

Question 5

______ assumes the cost of mistakes in either direction are equal

Answer 5

Accuracy

Question 6

Precision

Answer 6

How close the measured values (test results) are to each other
- may not be accurate or correct, but they are close to each other

Question 7

What do accuracy and precision have in common?

Answer 7

• describes placement of data
• summarizes data
• determine exactness
• sometimes represents the same thing
• helps determine successfulness of results
• used in physics, engineering, stats, etc

Question 8

Accuracy - summary

Answer 8

• how close data or results are to the goal
• measure of quantity to reality
• takes into account the accepted value
• bad results would be far from the goal
• determined with a single measurement
• does not speak about quality

Question 9

Precision - summary

Answer 9

• how close data or results are to each other
• measures how well measurements can be reproduced
• doesn’t take into account accepted value
• bad results would be scattered
• needs several measurements to be determined
• speak about quality

Question 10

Sensitivity

Answer 10

Ability of a test to correctly identify those that have the disease
- of all animals that have the disease, what proportion test positive

Question 11

Highly sensitive tests will have a low number of _______

Answer 11

False negatives

Question 12

Specificity

Answer 12

Ability of a test to correctly identify those that do not have the disease
- of all the animals that do not have the disease, what proportion test negative

Question 13

Highly specific tests will have a low number of ______

Answer 13

False positives

Question 14

Concerned with the population of animals that have the disease

Answer 14

Sensitivity
- looking at the left side of the 2 x 2 table (a and c, TP/FN)
= a/a+c

Question 15

What are reasons to get a false negative?

Answer 15

• natural or induced tolerance
• improper timing
• improper selection of test
• non-specific inhibitors
• antibiotic induced Ig suppression
• incomplete or blocking antibody
• analytically insensitive tests

Question 16

As sensitivity decreases, the likelihood of getting _________ increases

Answer 16

False negatives

Question 17

How to interpret sensitivity?

Answer 17

TP = 93
FN = 7
- out of every 100 cats, the test will correctly identify 93 cats that have the disease, and will falsely identify 7 cats as negative that actually have the disease

Question 18

Concerned with the population of animals that do not have the disease

Answer 18

Specificity
- looking at the right side of the 2 x 2 table (b and d, FP/TN)
= d/b+d

Question 19

Why might a result be a false positive?

Answer 19

• cross reaction
• non specific inhibitors
• non specific agglutinins
• contamination
• vaccination cross reaction

Question 20

As specificity goes down, the likelihood of _______

Answer 20

False positives increases

Question 21

What is expressed as a proportion?

Answer 21

Epidemiologic sensitivity

- reliance on testing to identify ill animals

Question 22

How is analytical sensitivity different from epidemiologic sensitivity?

Answer 22

Analytical sensitivity expresses the detection limits of a test
- ability to detect very low levels of a target

Question 23

Gold standard

Answer 23

Required to calculate sensitivity, need samples from animals that we know the true disease status of

• are properly classified as either positive or negative
• run some developed gold standard test and compare results to determine the other tests’ accuracy

Question 24

Epidemiologic specificity

Answer 24

Ability of a test to correctly detect (classify) non-diseased animals

Question 25

Analytical specificity

Answer 25

Ability to measure the correct substance

- not measuring particles or molecules other than the target

Question 26

Proportion of all tests, both positive and negative, that are correct

Answer 26

Accuracy

Question 27

Is accuracy a good way to assess a test?

Answer 27

No, it is easily manipulated

- is used in some marketing advertisements

Question 28

Sensitivity of a test may be different if comparing _______

Answer 28

Populations in early vs late stages of disease

- false negatives may be due to very early infections