A study is conducted to assess myocardial perfusion imaging performance using a new pharmacologic agent for stress testing. The test is performed on inpatients and outpatients with chest pain. Patients with positive stress test results underwent cardiac catheterization to diagnose flow-limiting coronary artery disease (CAD). Physicians interpreting the catheterization radiographs were blinded to inpatient/outpatient status. The following results are obtained:
Inpatients:
Number of patients with positive stress test: 122
Flow-limiting CAD on cardiac catheterization: 102/122 (84%)
Outpatients:
Number of patients with positive stress test: 115
Flow-limiting CAD on cardiac catheterization: 67/115 (58%)
Which of the following best explains the difference in the rate of flow-limiting CAD between the groups in this study?
Positive condition | Negative condition | ||
Positive test result | TP | FP | PPV = |
Negative test result | FN | TN | NPV = |
Sensitivity = TP / (TP + FN) | Specificity = TN / (TN + FP) | ||
FN = false negative; FP = false positive; NPV = negative predictive value; PPV = positive predictive value; TN = true negative; TP = true positive. | |||
The cardiac catheterization results given above are essentially equivalent to the positive predictive value (PPV) of the new myocardial stress test for diagnosing flow-limiting coronary artery disease. PPV is defined as the percentage of individuals that actually have the disease out of all positive test results. Unlike sensitivity and specificity, positive and negative predictive values are not intrinsic characteristics of the test. They are dependent on the prevalence of the disease in the tested population. In a patient population with low disease prevalence, the PPV is likely to be low due to a high number of false positives relative to true positives. In this example, outpatients are likely to have a lower prevalence of flow-limiting coronary artery disease than inpatients, as patients with more severe findings (eg, positive myocardial enzymes, ischemic changes on ECG) are more likely to be admitted to the hospital. Therefore, a larger proportion of false positives (ie, patients with a positive stress test but without flow-limiting CAD) are expected in the outpatient group compared to the inpatient group.
(Choice A) Blinding is helpful in reducing systemic biases (eg, placebo effects, observer bias) between study groups. In this case, physicians interpreting the catheterization results were properly blinded from knowing inpatient/outpatient status to prevent biased interpretations.
(Choices B and D) Likelihood ratios are useful for determining how a positive or negative result influences a patient's chances of having the disease. A positive likelihood ratio >1 implies that a positive test result increases the patient's odds of having the disease. A negative likelihood ratio <1 implies that a negative test result decreases the patient's odds of having the disease. Likelihood ratios are not affected by disease prevalence.
(Choice E) Sensitivity is the percentage of diseased individuals that are correctly identified as having the disease. Unlike PPV, sensitivity and specificity are intrinsic characteristics of a test and are not affected by disease prevalence.
Educational objective:
Positive and negative predictive values are used to describe the clinical usefulness of a respective positive or negative test result. However, they are not intrinsic characteristics of a test and can vary based on disease prevalence in the tested population.