Testing for SARS-CoV-2 has attracted a tremendous amount of attention as a tool to manage the ongoing COVID-19 pandemic. Although diagnostic laboratory testing is utilized ubiquitously by physicians and encountered regularly by individuals receiving medical care, several aspects of test interpretation are incompletely understood by medical communities and the general population, creating a significant challenge in minimizing the damage caused by disease spread through informed decision making and proper testing utilization. Here, general principles of test interpretation are reviewed and applied to specific examples, such as whether asymptomatic individuals should be tested, what it means to test positive (or negative), and how to interpret tests for “immunity passports”. Unexpectedly, the answers seem to run contrary to many of the popular narratives about testing as a tool for managing COVID-19. Although testing is an important and essential part of managing diseases like COVID-19, improper utilization can potentially have unintended negative consequences.
The first major property needed to properly interpret a test result is sensitivity, which refers to the proportion of patients with the disease that the test correctly classifies as positive (Figure 1). Although the analytical sensitivity11 of RT-PCR can be quite good (i.e. it is capable of detecting very low quantities of viral RNA) the diagnostic sensitivity for a single test in clinical practice is believed to be much less good. This is an important point that is surprising to those who are familiar with RT-PCR as a highly analytically sensitive test in both research and clinical settings. For SARS-CoV-2, there have been many reports of frequent false negatives for RT-PCR testing of SARS-CoV-2,15-17 and reports suggest the sensitivity of RT-PCR in clinical practice could be in the range of 50-70%.16,18 This is a rapidly changing area where there is more uncertainty than would be encountered in more established areas of medicine. In one recent commentary, a laboratory director confided that he does not know what the clinical sensitivity of his tests is as a consequence of the rapid pace of COVID-19 diagnostics.19 Many reviews estimate a value of 70% for RT-PCR diagnostic sensitivity,6,14,20 which is approximately the same as previous RT-PCR tests for the original SARS-CoV.21 For our examples, we will estimate sensitivity at 80% to account for the widespread perception that tests are performing better than described in the literature. Use of this value does not change conclusions largely from an estimate of 70%, whereas use of a lower estimate might wrongly cause the reader to discount the examples as worst case scenarios.
There are many reasons why an infection may not be detected by RT-PCR, ranging from the whether the nasopharyngeal swab was performed properly, to the abundance of SARS-CoV-2 in the tested anatomical location, to when in the course of an infection the sample is obtained.3,7,22-24 False negatives can also occur due to clinical laboratory errors involving steps such as sample preparation, machine and/or operator error, and reporting errors. For example, during the early days of COVID 19 in the United States, a labeling error resulted in a patient receiving a false negative test result which in turn led to the patient being discharged from a hospital.Viral evolution could also result in reduced sensitivity.Additionally, the timing of the test relative to the course of the infection is an important variable as the tests have their lowest sensitivity both early and late in the infection.This relatively high rate of false negatives is problematic for several reasons. RT-PCR is less effective early in the course of a SARS-CoV-2 infection,7 which limits the potential value of RT-PCR for screening asymptomatic individuals before they exhibit robust viral replication. For individuals with symptoms and/or known exposures, false negatives may cause the individual to believe they are not infected and believe that they can return to work and other activities.13 It is therefore important for there to be a clear understanding that a negative result needs to be interpreted as “no virus detected” rather than “not infected with the virus.” The reported sensitivities for serological tests are typically higher (85%-99%).28 However, timing is also a very important variable due to the timing of the immune response to infection that produces IgM and then IgG antibodies.3 For this reason, serology is not as useful in the early stages of an infection where the virus is present but a significant antibody response has not yet been mounted. Multiple studies that compare serological assays have reported poor diagnostic sensitivity if they are used early in infection.
It is important to note that reported sensitivities for serological tests may not apply to all clinical scenarios encountered. For example, individuals with mild SARS-CoV2 infection have lower antibody titers.4 If the sensitivity reported for a serological assay was obtained by studying samples from hospitalized individuals with advanced COVID-19, the value reported may not reflect the sensitivity for detecting mild cases
The attention given to the importance of testing, to ongoing testing shortages, and to emerging problems with tests has achieved considerably more attention than proper test interpretation. These issues have been mentioned in passing within the academic medical literature as issues to be aware of, but detailed discussions have been relatively limited.14,20 This is problematic as there is abundant evidence that physicians struggle with proper probabilistic test interpretation.8-10 As government, business, and institutional leaders turn to physicians and health care providers for advice, it seems likely that a reasonable fraction may be giving incorrect advice based on these systemic weaknesses. The consequences of policies and practices that are based upon improper use of laboratory testing are potentially fatal during a pandemic
Many communities are limiting tests to those who are most in need. This is often presented as a problem. This is not necessarily a problem; it is actually consistent with best practices in laboratory medicine. One of the examples above highlighted the limited value of testing for those without a good indication for testing; positive results are likely to be false positives, and negative results do not provide much additional information and the patient is not at less risk from a future infection. The idea that anyone who wants a test can get a test is complicated because the results of a test are not absolute, and what they actually mean depends upon the history of exposures and symptoms of the person being tested and the community within which they live. This issue does not appear to have been as prominently discussed in ongoing COVID-19 public dialogues.
Not only are government and organizational leaders faced with an urgent need to consider aspects of medicine that they have rarely considered in such detail, many scientists and engineers have been mobilized to lend their expertise to the ongoing crisis. The limitations of testing is often surprising to them. Although there are many differences between best practice in a clinical laboratory and in a research laboratory, the lack of absolute certainty with any one test should not be a surprise. Scientists do not publish the results from a single iteration of a single experiment. Rather, they expect tests to be reproducible and therefore perform experiments multiple times to demonstrate that results were not a single outlier. Additionally, they support their conclusion with multiple lines of new experimental evidence and historical evidence from the literature that together support their finding. This is akin to a physician considering laboratory results on the basis of the patient history, physical examination, and any other imaging or laboratory test results.
With the reported sub-optimal sensitivities of RT-PCR tests in practice, and the reported poor performance of some serology tests that are flooding the market after emergency use authorizations reduced many of the regulatory burdens in an effort to expedite the development and delivery of tests,5,48,49 probabilistic test interpretation is going to play an important role in test interpretation. Additionally, as disease burdens shift through communities and the list of symptoms and syndromes associated with COVID-19 grow,50,51 assessments of pre-test probability will need to continually be adapted to current information. This is in contrast to traditional medical conditions where physicians may have a familiarity with test results developed over years and where the concepts of probabilistic test interpretation have been replaced by years of experience that influence judgment and interpretation
Reference & Source information: https://www.cell.com/
Read More on: