uitwerking oef. Epidemiologie en volksgezondheid

Femke Van Nimmen S0213580

Diagnostic testing - Confounding

1. A physical examination was used to screen for breast cancer in 2,500 women
with biopsy-proven adenocarcinoma of the breast and in 5,000 age- and race-
matched control women. The results of the physical examination were positive
(i.e., a mass was palpated) in 1,800 cases and in 800 control women, all of
whom showed no evidence of cancer at biopsy.

Calculate
a) The sensitivity of the physical examination

Sensiviteit= TP/ TP + FN
= 1800/ 1800+700= 0,72 = 72%

b) The specificity of the physical examination

Specificiteit = TN/TN + FP
4200/ 4200 + 800 = 0,84 = 84%


c) The positive predictive value of the physical examination

PPV= TP/ TP + FP
1800/ 1800 + 800 = 0,692 = 69%

2. A screening test is used in the same way in two similar populations, but the
proportion of false- positive results among those who test positive in population
A is lower than that among those who test positive in population B.
What is the likely explanation for this finding?
a) It is impossible to determine what caused the difference
b) The specificity of the test is lower in population A
c) The prevalence of disease is lower in population A
d) The prevalence of disease is higher in population A
e) The specificity of the test is higher in population A


3. A physical examination and an audiometric test were given to 500 persons with
suspected hearing problems, of whom 300 were actually found to have them.
The results of the examinations were as follows:

Physical examination
Hearing problems
Result Present Absent
Positive 240 40
Negative 60 160


Audiometric test
Hearing problems

, Femke Van Nimmen S0213580
Result Present Absent
Positive 240 40
Negative 60 160

Compared with the physical examination, the audiometric test is:
a) Equally sensitive and specific
b) Less sensitive and less specific
c) Less sensitive and more specific
d) More sensitive and less specific
e) More sensitive and more specific


4. Two pediatricians want to investigate a new laboratory test that identifies
streptococcal infections. Dr. Weyler uses the standard culture test, which has a
sensitivity of 90% and a specificity of 96%. Dr. Deby uses the new test, which is
96% sensitive and 96% specific.

If 200 patients undergo culture with both tests, which of the following is
correct?
a) Dr. Weyler will correctly identify more people with streptococcal infection
than Dr. Deby
b) Dr. Weyler will correctly identify fewer people with streptococcal infection
than Dr. Deby
c) Dr. Weyler will correctly identify more people without streptococcal
infection than Dr. Deby
d) The prevalence of streptococcal infection is needed to determine which
pediatrician will correctly identify the larger number of people with the
disease

5. In the table below, the results of a new diagnostic test for cancer are compared
with the complete diagnostic package in current use.
Complete diagnosis (true disease status)

New test Present Absent
Positive 8 1000
Negative 2 9000

a. What are the sensitivity and specificity of the new test?

Sensitiviteit = 8/ 8+ 2 = 0,80 = 80%

Specificiteit = 9000/ 9000 + 1000 = 0,90 = 90%

b. Would you recommend its general use?

Nee, hoewel de test een hoge sensitivitet en specificiteit heft, is de positieve
predicitieve waarde PVV extreem lag.
PVV= 8/ 8+ 1000 = 0,79%
 Dit betekent dat van elke 100 positieve resultaten, meer dan 99 fout positief
zijn.