Pawan Acharya
The terminology RDT (Rapid Diagnostic Test) is frequently heard nowadays. A rapid test is used because it enables testing a large number of individuals in a short time period, it is relatively cheaper, and it does not require sophisticated technology or specialized skills. These features make a rapid test useful in the time of rapidly spreading epidemic situation like COVID-19. There is a heated discussion about the rapid test and its’ usefulness in the scenario of COVID-19 in Nepal.
Many of us may have a curiosity regarding why the RDT is being used and what does it mean to have a positive or negative test result from the rapid test. The main concern regarding the use of RDT is its validity; or in a simpler term, the accuracy of the test results. Understanding the meaning of two key terminologies: Sensitivity and Specificity would help us understand the validity or accuracy of a rapid test.
Sensitivity is the probability of getting a positive test result when the disease is truly present. The sensitivity of a test ranges from 0 to 100%. A test with 100% sensitivity truly classified all individuals with the disease as diseased individuals. However, the sensitivity of a rapid test is often less than 100%.
Let’s discuss, for example, what does a 95% sensitivity mean? It means if we test 100 individuals who truly have the disease, the test will show a positive result for 95 individuals. The test falsely shows negative test results for the remaining 5 individuals; therefore, those 5 cases are called false-negative cases. False negatives are the cases who in fact, have the disease but they are falsely tested negative by the rapid test.
Specificity is the probability of getting a negative test result when the disease is truly absent. Like sensitivity, specificity is also calibrated between 0 to 100%. As an example, let’s talk about what does a 95% specificity means? It means- if 100 healthy (disease-free) individuals are tested using a rapid test, the test will show negative results for 95 individuals. The test will falsely result positive for the remaining 5 individuals while in fact, those 5 individuals are disease-free. Therefore, these cases are called false-positive cases.
A perfect test has both sensitivity and specificity close to 100%. A perfect test correctly classifies all the diseased and disease-free individuals without any false-positive and false-negative results. We can say that a perfect test is 100% accurate. We may wonder, why don’t we always use a perfect test instead of a rapid test? This is because a perfect test is time-consuming, expensive and it demands a sophisticated technology and specialized skills. Therefore, when a perfect test-is not feasible, RDTs are selected as the best alternative. One question may arise- how do we know who truly has the disease and who doesn’t? In the process of assessing the sensitivity and specificity of an RDT, the result of the RDT is compared to the results from the perfect test (or a gold standard test). Researchers use specialized study designs to assess the accuracy of the RDT. For COVID-19 Polymerase Chain Reaction (PCR) is considered as the gold standard or perfect test for the final diagnosis of the disease.
A rapid test kit often has sensitivity and specificity both less than 100% and it can have different values for sensitivity and specificity. It expectedly produces both false-positive and false-negative results. In the epidemic situation like COVID-19 for the rapid test, a highly sensitive rapid test is preferred because we don’t want to miss the people who have the disease in the initial screening. There is a willingness to accept a few false-positive cases in order to avoid any false-negative result, if necessary. Positive cases detected by a rapid test that has high sensitivity and somehow low specificity would need to go through confirmatory testing by a perfect test. The confirmatory test will separate the true positive cases from the false-positive cases. A confirmatory test should have 100% specificity.
In the case of COVID-19, the confirmatory test, also called the gold standard is PCR. When a person who is tested positive by the RDT is tested positive in the PCR, the person truly has the disease. On the other hand, if a person who was tested positive by the rapid test but tested negative by the PCR (PCR has 100% sensitivity), the person is considered false positive and the person is disease-free.
-(Acharya is Student of PhD in Epidemiology, University of Oklahoma Health Sciences Center)
