Review Question - QID 210446

If the cutoff value is increased in the case of a PPD it will make the test less sensitive since it would require a larger area of induration to support the diagnosis. Individuals with a lesion 10-15 mm who actually have tuberculosis will be missed by this change.

The sensitivity and specificity are innate properties of a test. A sensitive test is likely to pick up a disease. For this reason, a negative test result on a very sensitive test tends to rule out the diagnosis. Changing the threshold can alter the sensitivity and specificity. If the cutoff value is lowered for a diagnosis (meaning it takes less of a finding to make the diagnosis) then the sensitivity is increased while the specificity is decreased. On the other hand, if the cutoff value is increased (meaning it takes more of a finding to suggest a diagnosis) then the sensitivity decreases while the specificity increases.

Incorrect Answers:
Answer 2: Decrease the specificity is incorrect as raising the threshold from 10 to 15 mm would lower the sensitivity and increase the specificity.

Answer 3: Increase the precision is incorrect as precision is a measure of how reliable a test is. For example, if a PPD given to the same patient every time gave the same value that would be a precise test.

Answer 4: Increase the sensitivity is incorrect as this new higher threshold is likely to miss more cases of tuberculosis since there is a higher threshold to make the diagnosis.

Answer 5: No change to the sensitivity or specificity is incorrect as changing the threshold of a test will change the sensitivity/specificity.

Bullet Summary:
Increasing the cutoff value of a test lowers the sensitivity and lowering the cutoff value increases the sensitivity.