Chapter 5- Assessing the Validity and Reliability of Diagnostic Screening Tests

Question 1

Bimodal curve

Answer 1

A graph with 2 peaks. However, most human characteristics are not bimodal

Question 2

Validity

Answer 2

The ability of a test to determine who has a disease and who does not. It has 2 components- sensitivity and specificity

Question 3

Sensitivity

Answer 3

The ability of the test to correctly identify who has a disease

Question 4

Specificity

Answer 4

The ability of the test to correctly identify who does NOT have a disease

Question 5

Risks of false positives

Answer 5

People who test positive may have to undergo more expensive and maybe more invasive testing. It may place unnecessary burdens on the healthcare system. Also, it causes stress for the patient who thinks that they have that specific disease

Question 6

Risks of false negatives

Answer 6

If the disease being screened for is serious and has effective intervention, false negatives can delay diagnosis and negatively impact the patient’s outcome.

Question 7

Cutoff level

Answer 7

This is necessary if a test doesn’t have a positive or negative result- there is no such thing as a “positive” blood pressure because it is a continuous variable. In this case, a unimodal curve is most common. It must be determined which blood pressure levels qualify as hypertension or hypotension. If the level is too high, people may be falsely considered negative when they actually do have hypertension. The cutoff level essentially determines sensitivity or specificity

Question 8

Trade-off between sensitivity and specificity

Answer 8

If we increase the sensitivity by lowering the cutoff level, we decrease the specificity. However, if the cutoff level is raised to increase the specificity, the sensitivity will decrease

Question 9

Sequential testing

Answer 9

Two stage testing, where tests are done one after the other. A less expensive, less invasive, or less uncomfortable test is conducted first. Those who test positive are recalled for another testing that might be more invasive but may have greater sensitivity or specificity. The goal in bringing people back for additional testing is to reduce the issue of false positives. These tests are frequently used for screening programs.

Question 10

Simultaneous testing

Answer 10

When both tests are conducted at the same time. This may be done with clinical workups, like when people are being admitted to the hospital. Results are interpreted as positive if the patient is positive on either test, and negative if negative on all tests

Question 11

Screening tests

Answer 11

Tests performed on asymptomatic
patients. Goal is to evaluate likelihood of
disease with the goal of early
detection

Question 12

Diagnostic tests

Answer 12

Tests performed on symptomatic
individuals –or– persons with
abnormal screening tests. Goal is to establish presence or absence of disease

Question 13

Dichotomous tests

Answer 13

Tests that have either a positive or negative result

Question 14

Impact of sequential testing on sensitivity and specificity

Answer 14

With sequential testing, there is a loss in net sensitivity. There is a gain in net specificity

Question 15

Impact of simultaneous testing on sensitivity and specificity

Answer 15

There is a net gain in sensitivity and a net loss in specificity

Question 16

Positive predictive value (PPV)

Answer 16

If the test results are positive for the patient, what is the probability that the patient has the disease?

Question 17

Negative predictive value (NPV)

Answer 17

If the test result is negative, what is the probability that the patient does not have the disease?

Question 18

2 factors that affect PPV

Answer 18

1. The prevalence of the disease in the population tested
2. The specificity of the test being used- in situations when the disease is infrequent

Question 19

Intrasubject variation

Answer 19

How an individual patient’s test results can vary over time. For example, a person’s blood pressure can be different depending on the time of day

Question 20

Intraobserver variation

Answer 20

Variation that occurs between 2 or more readings of the same test results made by the same observer. If a radiologist reads the same group of X-Rays at 2 different times, how will the results change?

Question 21

Interobserver variation

Answer 21

The variation between observers- two examiners may not give the same result

Question 22

Reliability

Answer 22

Ability to get the same results if the test is repeated. There are 2 measures- percent agreement and the kappa statistic

Question 23

Percent agreement

Answer 23

Does not account for agreement between observations due to “chance”

Question 24

Kappa statistic

Answer 24

Accounts for agreement between observations due to “chance”. Less than 40% is poor agreement, greater than 40% is intermediate to good

Question 25

When is disease detection worthwhile?

Answer 25

“Bear” diseases are dangerous, but move slowly enough to catch. These diseases benefit the most from screening programs. “Birds” move too quickly to be caught, and it will be too late to treat. “Turtles” move too slowly to ever be dangerous, and don’t need treatment.

Question 26

Lead-time bias

Answer 26

Occurs when a disease is detected by a screening or surveillance test at an earlier time point than it would have been if it had been diagnosed by its clinical appearance. This time lag or “lead time” during which the disease is asymptomatic is not taken into account during the survival analysis