Should Asymptomatic and Low-Risk Individuals be Tested for SARS-CoV-2?
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J Gen Intern Med DOI: 10.1007/s11606-020-06000-z © The Author(s) 2020
INTRODUCTION
Because people with asymptomatic SARS-CoV-2 infections are an important source of transmission, widespread testing has the potential to identify and isolate such individuals, quarantine their contacts, and prevent further spread. However, testing for a condition in a group with low disease prevalence may lead to numerous false positive results. This study quantifies the tradeoff between reducing transmission by true positives and unnecessary isolation and quarantine due to false positives.
METHODS
The impact of testing was modeled based on the following parameters: 1% prevalence of active infection, test sensitivity 90% and specificity 95%, reproduction number 0.9, and 5 contacts per case.1–5 Because performance of SARS-CoV-2 tests is not well established, the values that correspond to PCR-based tests for other respiratory viral infections were used.2, 3 The reproduction number R is the median value in the USA.4 Because individuals with positive test results will be instructed to isolate, R following a positive test was assumed to decrease to 0.1.5 The number of people subsequently infected by each infected case disease was calculated using the formula R + R2 + R3 + … = 1/(1−R)−1 when R < 1. For R = 0.9, this sum of 9.0 represents the number of new infections in a transmission chain and was applied to individuals with false negative results and to infected individuals who were not tested. The corresponding sum for a true positive case is 1.0, which assumes a reproduction number of 0.1 for the first transmission and 0.9 for subsequent transmissions.
RESULTS
In the base case, testing will result in 5.5 times more false positive than true positive results (Table 1). Testing will result Received May 28, 2020 Accepted June 15, 2020
in 72,000 fewer infections. Of the 351,000 people who will be isolated or quarantined, a substantial majority (297,000) will be isolated or quarantined because of false positive tests. With a higher prevalence of active infection, there will be 180,000 fewer infections, but a large number of people will still need to be isolated or quarantined. A highly specific test yields fewer false positives. About half (54,000) of the 113,400 people who must be isolated or quarantined will be because of true positive tests. A test that is both highly specific and sensitive will also greatly reduce the number of false negatives. At very high levels of test specificity, the number of people needed to isolate or quarantine (NNIQ) per infection prevented decreases and is minimally affected by variations in test sensitivity (Fig. 1).
DISCUSSION
This analysis indicates that policy makers should avoid instituting mass testing of asymptomatic and low-risk individuals until a test with very high specificity becomes available. Determining test specificity will be challenging in the absence of a gold standard that definitively establishes the presence of absence of SARS-CoV-2. There are several possible approaches to clarify
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