What is Sensitivity?
We hear so much about antigen test sensitivity and validity. What are they and how do they relate to COVID-19 testing? This stuff can be confusing, but we’ll try to explain it in as jargon free a blog as possible.
When it comes to controlling the spread of COVID-19, public health services need tests in order to separate healthy from infected individuals. One much talked about method is wide scale use of rapid antigen detection tests (RADT). These tests are simple and convenient point-of-care tests, administered and interpreted in a few minutes. Compared to arduous reverse transcriptase polymerase chain reaction (RT-PCR) tests, RADTs are considered a game changer if they are highly sensitive. Although they represent a potential breakthrough in pandemic control, RADTs have different sensitivity and specificity profiles to the tried and trusted “gold standard” rT-PCR. You see, rT-PCR assesses a static variable, the virus is either present or it is not. RT-PCR tests boast a very high sensitivity of more than 99.5%. Rapid antigen detection tests on the other hand assess a continuous variable, and in the case of COVID-19 infection test results are influenced by the amount of SARS-CoV-2 in an individual at the time of testing. Before developing any symptoms, infected individuals go through an incubation period where they may still have a “viral load” that can cause them to spread the infection to others. Understanding this difference forms the cornerstone of how these RADT tests are interpreted. With RADT, an infectious individual is highly likely to yield a positive result, provided the rapid antigen test is reasonably sensitive. The more virus in their cells, the more likely they are to be infectious and be positive. So testing with rapid antigen detection tests allow the identification of infectious people and isolate them from the community. This question is answered in under 15 minutes.
The test is set to read a certain viral threshold above which “symptom-free” individuals are classified as infectious and below which they are classified as non-infectious. Where we choose the threshold makes a difference. If the threshold is too high, the test may be positive in those with SARS-CoV-2 who are not infectious, for example those who recovered from recent infection. If the threshold is too low, a negative result may occur even if the patient actually has SARS-CoV-2 and is infectious. If a test can classify a large proportion of infectious and non-infectious correctly, it is said to have a high validity. Test validity has two major components; one is sensitivity and the other is specificity–these are quality criteria for the test.
Let’s look at an example for better clarity. Say we pick a population of 1000 people and let’s say that 100 of them have a high enough SARS-CoV-2 viral load to pose an infection risk to others. Now let’s also assume that 92 out of the 100 infectious people test positive with DropTech RADT. The sensitivity of DropTech test is therefore calculated as the number of infectious – that are correctly classified – divided by all infectious individuals, so 92 / 100 * 100 and that equals 92%. In other words, the sensitivity is the proportion of infectious individuals correctly identified, ie.92%. In case of the DropTech test, this exceeds the World Health Organization (WHO) recommended sensitivity of at least 80% for such tests to be useful.
REAL WORLD COVID-19 SETTING
Unlike in a laboratory setting, once the new test is used in the real world, we don’t know who’s infectious and non-infectious at the outset otherwise we wouldn’t be doing the test in the first place. In the real world, we end up with positives or negatives and we have to do something with them so sensitivity and specificity give us an indication as to how much trust we can put into a rapid antigen test. Infectious individuals who are tested positive are called “true positives” whereas infectious individuals who were tested negative are called “false negatives”. Ideally, we would like everyone to fall into the true positive or true negative groups, but no test is perfect and invariably we will end up with people in the false negative and false positive groups. If someone is falsely labeled as positive they’ll be sent for further testing with rt-PCR; a negative follow-up RT-PCR result indicates that the initial test was a false positive. On the other hand, someone who is falsely labeled as negative might interact closely with healthy individuals and unbeknownst to them silently spread SARS-CoV-2 to others.
Like all rapid antigen detection tests, DropTech is just one further weapon we can easily use to screen individuals for SARS-CoV-2. Infectious individuals are likely to have a viral load sufficient to yield a positive RADT. Although just a “snapshot” of the current status of an individual, serial testing using RADT is a valuable way to identify infected people, even if they are asymptomatic.