Review Question - QID 216700

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Testing and Screening Sensitivity, Specificity, Positive Predictive Value, and Negative Predictive Value Cut-off point for positivity may be adjusted to optimize sensitivity and specificity for different purposes, which are inversely related (cut-off point with decreased sensitivity is associated with increased specificity and vice-versa) QID: 216700 (M1.OMB.4738)

QID 216700 (Type "216700" in App Search)

A new strain of the influenza virus is circulating in the community. The current best-performing assay for this virus tests for the presence of viral genetic material in nasopharyngeal samples. A test is positive when the concentration of viral genetic material is above a cutoff value. Using this assay, 32.1% of individuals who ultimately receive a clinical diagnosis of influenza infection test positive (Parameter A). In contrast, 87.9% of healthy individuals test negative (Parameter B). How would these parameters change if the cutoff value is lowered (Figure A)?

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Parameter A denotes the percentage of individuals with the disease who test positive, or test sensitivity, and parameter B denotes the percentage of healthy individuals who test negative, or test specificity. As the cutoff value for a diagnostic test is lowered, the sensitivity increases and the specificity decreases.

Test sensitivity is calculated as the proportion of individuals with a disease who test positive and can be expressed in percentage terms: diseased individuals with positive test / (diseased individuals with positive test + diseased individuals with negative test). Test specificity is calculated as the proportion of individuals without a disease who test negative and can also be expressed in percentage terms: healthy individuals with negative test / (healthy individuals with negative test + healthy individuals with positive test). Every diagnostic test has a threshold or cutoff value that is used to classify positive and negative cases. When the cutoff value is lowered, more cases are considered to be positive (higher diseased and healthy individuals with positive tests), thereby catching more individuals with the disease while simultaneously lowering the specificity by classifying more healthy individuals as positive cases.

Figure A shows the answer choices.

Incorrect Answers:
Answers 1, 2, & 3: These answer choices are incorrect because they indicate no changes in parameters A and/or B.

Answer 5: Sensitivity decreases and specificity increases corresponds to the case where the cutoff value for a diagnostic test is raised such that more individuals are classified as negative.

Bullet Summary:
As the cutoff value for a diagnostic test is lowered, its sensitivity increases and its specificity decreases.

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