Question 71#

The specificity of the overall screen is approximately:

A. 67%

A. Specificity is defined as the test’s ability to exclude people who, according to the diagnostic (gold) standard, do not actually have the disorder (true negatives). Specificity = D/ (B + D) = 84/124 = 67.74%, i.e. 67.74% of the people who do not have depression will have a negative result on the two-question screen. Thus specificity is the proportion of true negatives among all non-diseased individuals. In other words, it is the ability of a test to rule out the disorder among people who do not have it.

Reference:

1. Streiner D and Geddes J. Some useful concepts and terms used in articles about diagnosis. Evidence-Based Mental Health 1998; 1: 6–8. 

Question 72#

The predictive power of a positive test using the overall screen is:

A. 49%

A. Not all of those people, who have been found to be ‘positive’ on the test, might actually have the disorder. Positive predictive value (PPV) gives the proportion of true positives among the test positives. It is calculated using the formula, PPV = A/(A + B) = 39/79 = 49.36%, i.e. 49.36% of people diagnosed with depression using the screening test actually have the illness.

Reference:

1. Streiner D and Geddes J. Some useful concepts and terms used in articles about diagnosis. Evidence-Based Mental Health 1998; 1: 6–8.

Question 73#

The predictive power of a negative test using the overall two-item screen is given by:

A. 49%

E. Not all of the people who have been found to be ‘negative’ on the test might actually be disease free. Negative predictive value (NPV) answers the question ‘Of those people who have been found to be ‘disease negative’ on the test, how many actually do not have the disorder?’ It is calculated using the formula, NPV = D/(C + D) = 84/88 = 95.45%, i.e. 95.45% of people diagnosed ‘normal’ on the test don’t have the disorder.

Reference:

1. Streiner D and Geddes J. Some useful concepts and terms used in articles about diagnosis. Evidence-Based Mental Health 1998; 1: 6–8.

Question 74#

The pretest probability of the overall two-item screen is:

A. 49%

D. The prevalence, also known as the pretest probability or base rate, refers to the proportion of people who have the disorder = (A + C)/N, i.e. 43/167 = 25.74%.

Reference:

1. Streiner D and Geddes J. Some useful concepts and terms used in articles about diagnosis. Evidence-Based Mental Health 1998; 1: 6–8.

Question 75#

The likelihood ratio of a positive test for the overall two-item screen is:

A. 2.8

A. PPV and NPV depend on the prevalence of the illness, and, as one can see, the prevalence of an illness can vary according to the population it tests. For example, the prevalence of depression is likely to be more in patients in a palliative care unit. Since the prevalence keeps changing with population, and hence the PPV and NPV, one way of summarizing the findings of a study of a diagnostic test where there is a different prevalence is to use the likelihood ratio. The likelihood ratio for a positive test (LR+) result is the likelihood that a positive test comes from a person with the disorder rather than one without the disorder. LR+ is calculated using the

formula, LR+ve = [A/(A + C)]/[B/(B + D)]

or simply

LR+ve = sensitivity/(1 - specificity)

So, (39/43)/(40/124) = 0.90/0.322 = 2.8. Since the specificity and sensitivity of a test are considered to be constant for any particular test, the LR is also constant irrespective of prevalence rates.

Reference:

1. Streiner D and Geddes J. Some useful concepts and terms used in articles about diagnosis. Evidence-Based Mental Health1998; 1: 6–8.