A 28-year-old man presents to the clinic with complaints of a cough, high fever, myalgias, and fatigue that has persisted for the past 5 days. Many patients have been coming to the clinic with similar symptoms, and the rate of positive influenza tests has been relatively high in the community over the past two weeks. A rapid influenza diagnostic test (RIDT) is ordered and comes back positive. The patient says that he received the flu vaccine this year and doesn't believe the test result. He asks, “How often is the test correct when it comes back positive?” The sensitivity of the test is 50-70%, and the specificity of the test is 90-95%.
Which of the following statements is the most appropriate answer to the patient's question?
- A) The chance of the test being correct is higher given the prevalence of influenza in the community
- B) The chance of the test being correct is lower given the prevalence of influenza in the community
- C) The question cannot be answered with the information provided
- D) The result is difficult to interpret given the low sensitivity
- E) The result is most likely correct given the high specificity
A) The chance of the test being correct is higher given the prevalence of influenza in the community
The patient is posing a question that describes the positive predictive value (PPV) of a test. The PPV describes the fraction of patients who have the disease and test positive (true positives) compared with the total number of patients who test positive (true positive + false positives). In other words, among patients who tested positive, how often is the test correct? The PPV of a test is dependent on the prevalence of the disease. If a community has a high rate of a disease, the test is more likely to correctly identify patients with the illness than in a community where a disease is rare. Therefore, the numeric answer to the patient's question is dependent on the prevalence of influenza in the community.
Answer B, the chance of the test being correct is lower given the prevalence of influenza in the community, is incorrect. The question stem describes the prevalence of influenza to be high in this community. If the prevalence of a disease is high, then the PPV should be higher than in communities where the prevalence is low. This answer choice states the opposite, that a lower prevalence would have a higher PPV.
Answer choice C, the question cannot be answered with the information provided, is incorrect because enough information is provided in this question to answer the patient.
Answer choice D, the result is difficult to interpret given the low sensitivity, is incorrect. The sensitivity of the test describes the percentage of positive tests among all patients who actually have the disease. A test with a low sensitivity misses patients who actually have the disease (high number of false negatives). These tests are poor at confirming a diagnosis and does not answer the patient's question.
Answer choice E, the result is most likely correct given the high specificity, is incorrect. Specificity describes the fraction of patients without a disease who test negative for the disease. Given the high prevalence and positive result of the test, discussing the specificity does not answer the patient's question. A high specificity is desirable for screening tests since there is a low fraction of false positive results. However, this statistic does not answer the question of a patient who already knows they have a positive result.
Key Learning Point
The positive predictive value (PPV) of a test is dependent on the prevalence of the disease. If a disease is rare, the inherent errors of the test produce false positive results and will have a large impact on the number of false positives and thus the PPV. If a disease has a high prevalence, the inherent errors of the test occur at the same rate but have a smaller impact on the PPV.