A new test is devised to detect HIV. It has a sensitivity of 90% and specificity of 85% compared to the gold standard. The test is used in 2 populations: a population in Africa with a 20% prevalence of HIV, and a population in Asia with a 1% prevalence of HIV. Which of the following is the most accurate statement concerning the new test?

Question

A new test is devised to detect HIV.  It has a sensitivity of 90% and specificity of 85% compared to the gold standard.  The test is used in 2 populations: a population in Africa with a 20% prevalence of HIV, and a population in Asia with a 1% prevalence of HIV.  Which of the following is the most accurate statement concerning the new test?

Quizplus · Accepted Answer

11ec25c8_d322_25e0_bc81_47ed0e7fc1bf_MD0003_00 The positive predictive value (PPV) of a diagnostic test answers the following question: what is the probability that a patient has the disease given a positive test result? PPV depends on disease prevalence in the population. If disease prevalence is high (eg, 20% prevalence of HIV in the African population), an individual in that population will be more likely to have the disease. Therefore, the probability that a patient from that population who tests positive actually has the disease is high (true positive result). The greater the prevalence, the greater the PPV. Conversely, if disease prevalence is low (eg, 1% prevalence of HIV in the Asian population), the probability that a patient who tests positive actually has the disease is low (false positive result). The negative predictive value (NPV) of a diagnostic test answers the following question: what is the probability that a patient does not have the disease given a negative test result? Like PPV, the NPV of a test depends on disease prevalence. However, unlike PPV, NPV decreases as prevalence increases and increases as prevalence decreases. If disease prevalence is low (eg, 1% prevalence of HIV in the Asian population), the probability that a patient who tests negative does not actually have the disease is high (true negative result) (Choice A). If disease prevalence is high, the probability that a patient who tested negative does not actually have the disease is low (false negative result). (Choices C and D) The sensitivity and specificity of a test do not depend on disease prevalence. (Choice E) The validity of a test refers to its ability to identify diseased and non-diseased individuals. The statement that the test is not valid in the African population is incorrect because the test has high sensitivity and specificity. Educational objective: Both the positive predictive value (PPV) and negative predictive value (NPV) of a test depend on the prevalence of the disease of interest in the population in which the test is applied. PPV increases and NPV decreases with an increase in prevalence.

A New Test Is Devised to Detect HIV