# 10.5 Practice problems

- A total of 1,500 children had a rapid strep test (RST) done by a standardised culture technique. Of the 1,500 children, 1,338 have a negative RST and 162 have a positive RST. In addition, a backup throat culture (gold standard test) was done on all children. Of those children with a negative RST, 1,302 have a negative throat culture. In the group with a positive RST, 159 have a positive throat culture. Complete the table below and then calculate the sensitivity and specificity of the RST (provide answers to one decimal place).

Target disorder (strep tonsillitis) |
||||

Present |
Absent |
Totals |
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Diagnostic test result |
Positive RST | |||

Negative RST | ||||

Total |

Sensitivity =

Specificity =

- A diagnostic test is 92% sensitive and 94% specific. A test group comprises 500 people known to have the disease and 500 people known to be free of the disease.
- How many of the known positives will test positive?
- How many of the known negatives will test negative?

- A screening test for a particular disease has a sensitivity of 96% and a specificity of 92%. You plan to screen a population in which the prevalence of the disease is 0.2%.
- What is the chance that if you test positive you actually have the disease?
- What percentage of the known negatives will test negative?

- For the diagnostic test below (Figure 10.6) for COVID-19, would you advise shifting the threshold (A) to the left, or right? Explain your answer.

Solution to Practice Problem 10.3.