Question 76#

The likelihood ratio of a negative test (LR–) for the overall two-item screen is:

A. 0.14

A. The LR– represents the likelihood that a negative test comes from a person with the disorder rather than one without the disorder. LR– is calculated using the formula LR–ve = [C/(A + C)]/[D/(B + D)], or simply LR–ve = (1 – sensitivity)/specificity.

So, (4/43)/(84/124) = 0.10/0.67 = 0.14

Similar to LR+ve, LR–ve 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 Health 1998; 1: 6–8.

Question 77#

Using the nomogram below, calculate the post-test probability of a positive test when using the two-item depression screening test in the palliative care unit using the figures indicated at the beginning of Question 70.

Fagan TJ. Nomogram for Bayes theorem (letter). New England Journal of Medicine 1975; 293: 257.

A. 1

E. The post-test probability is the probability that a patient, scoring positive on the test, actually has the disorder (PPV). It can be calculated using the nomogram that is provided. Since we know the pre-test probability (prevalence) and the likelihood ratio, we should be able to fi nd the post-test probability from the chart. A straight line drawn through the pre-test probability (25) and the likelihood ratio +ve (2.8) should yield a post-test probability of about 50. 

References:

1. Streiner D and Geddes J. Some useful concepts and terms used in articles about diagnosis. Evidence-Based Mental Health 1998; 1: 6–8.
2. Fagan TJ. New England Journal of Medicine 1975; 293: 257.

Question 78#

Using the nomogram in Question 77, calculate the post-test probability of a negative test when using the two-item depression screening test in the palliative care unit:

A. 1

B. In this case, since the question is about post-test probability of a negative test, the likelihood ratio –ve (0.14) and the line would pass through 4.

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 79#

What is the false positive rate for the overall 2-items screening test?

A. 32%

A. False positive (FP) is the number of people diagnosed to have a condition with the new test when they actually do not have the condition according to the gold standard. In this case, the percentage of people falsely identified by the test as depressed. Using the 2 × 2 table, false positive is calculated FP = B/B+D = 40/124 = 32%.

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 80#

What is the false negative rate for the overall two-item screening test?

A. 32%

B. False negative (FN) is the number of people not diagnosed with a condition with the new test when they actually have the condition according to the gold standard. In this case, the percentage of people among the depressed group falsely identified by the test as not depressed, i.e. C/A +C; 4/43 = 9.3%.

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.