10.5 Practice problems

1. 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).

Sensitivity =
Specificity =

1. 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.
   1. How many of the known positives will test positive?
   2. How many of the known negatives will test negative?
2. 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%.
   1. What is the chance that if you test positive you actually have the disease?
   2. What percentage of the known negatives will test negative?
3. 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.