A 38-year-old primigravid woman comes to the physician's office at 20 weeks gestation for prenatal counseling. She is concerned about her baby's risk for Down syndrome and asks about measures to diagnose it early. The physician explains that triple screening may detect up to 60% of cases of chromosomal abnormalities and that amniocentesis may detect approximately 90% of cases. The patient decides not to undergo any testing. When explaining that amniocentesis detected a higher percentage of cases compared to triple screening, the physician was referring to which of the following values?

Question

A 38-year-old primigravid woman comes to the physician's office at 20 weeks gestation for prenatal counseling.  She is concerned about her baby's risk for Down syndrome and asks about measures to diagnose it early.  The physician explains that triple screening may detect up to 60% of cases of chromosomal abnormalities and that amniocentesis may detect approximately 90% of cases.  The patient decides not to undergo any testing.  When explaining that amniocentesis detected a higher percentage of cases compared to triple screening, the physician was referring to which of the following values?

Quizplus · Accepted Answer

Explanation: 11ec2b69_c102_5a46_8f63_0bf5aebb8a94_MD0005_00 A true positive (TP) occurs when a patient who has the disease obtains a positive test result on a diagnostic test. A false positive (FP) occurs when a patient who does not have the disease obtains a positive test result. A true negative (TN) occurs when a patient who does not have the disease obtains a negative test result. A false negative (FN) occurs when a patient who has the disease obtains a negative test result. The most commonly discussed characteristic of a diagnostic test is its sensitivity. The sensitivity of a test reflects its ability to correctly detect cases with the disease. It is obtained by dividing the number of TPs by the number of people who have the disease: Sensitivity = TP / (TP + FN). Sensitivity is an intrinsic test characteristic that does not depend on disease prevalence. In this case, the physician explained to the patient that triple screening had a sensitivity of 60% in detecting chromosome abnormalities, while amniocentesis had a sensitivity of 90%. A higher sensitivity typically means more TPs and fewer FNs (Choices B and C). (Choice A) The negative predictive value (NPV) of a test answers the question: Given a negative test result, what is the probability that the patient does not have the disease? NPV = TN / (TN + FN). NPV increases as sensitivity increases (fewer FNs), but the percentages cited by the physician refer to sensitivity directly. NPV also varies with disease prevalence. (Choice D) The positive predictive value (PPV) of a test (which also varies with prevalence) answers the question: Given a positive test result, what is the probability that the patient has the disease? PPV = TP / (TP + FP). For example, if triple screening had turned out to be positive, the PPV would represent the probability of chromosomal abnormalities (given the positive test result). Educational objective: The sensitivity of a test is the probability of detecting the disease if it is present.

A 38-Year-Old Primigravid Woman Comes to the Physician's Office at 20