Performance of COVID-19 RTK-Ag Rapid Tests

When a test is not fit for its role, what good does it have? NONE. It is worthless and harmful, and belongs to the dustbin... Read together with another article "Let us know which test kit brands are reliable" today by TAN SRI LEE LAM THYE https://www.thestar.com.my/opinion/letters/2022/02/14/let-us-know-which-test-kit-brands-are-reliable For healthcare professionals, this article in NEJM would be informative and enlightening https://www.nejm.org/doi/full/10.1056/NEJMcp2117115?query=clinical-medicine Some scientific details about diagnostic research: Sensitivity (Sn) and specificity (Sp) are test perfomance indicators that are only reliable if the test was properly conducted according to the principles of a diagnostic test research. These Sn and Sp tell about how good are the test kits after knowing the states of the diseased or non-diseased or healthy. Therefore, they are counter-practice in real life which the testing is done to know the state of the health of a person ie before knowing the diagnosis. The right tests performance indicators to use are PPV and NPV (positive and negative predictive values) when talking about the tests performance in real populations because the current prevalence of the conditions (disease vs healthy) affect the test performance, this does not rest or judged solely from the test's sensitivity and specificity values. PPV and NPV are tests performance to predict a condition, and these are more affected by the current incidence (or prevalence or pandemic vs endemic situation) of the disease or healthy than the Sp or Sn, respectively. Therefore, knowing the true prevalence (and thus the probability of disease or healthy) of the conditions measured by the established (referent or gold) standard is critical to choose the right test (between the 2 imperfect tests of either low Sn + high Sp or high Sn + low Sp because the perfect test with both high Sn and high Sp will perform well; the test with both low sp and sn should NOT has been allowed to exist at all in the first place) to use in order to maintain acceptable PPV and NPV. In the absence of the perfect test (high in both Sn and Sp) and in the pandemic situation when the probablity of a disease is high, use tests that are high in Sn and low in Sp is ok as both the PPV and NPV are likely to be within acceptable limit ie better than by chance. Using tests that are high in Sp will cause unacceptably high false negative results. It is vice versa in the endemic situation, choose test kits with high Sp. If used high Sn test, to expect high false positive results. So, what about now? With the Omicron around, use RTK with high Sn is preferred (if there is no high Sn and high Sp test kits around). If the test kit you use is high Sp only, then expect high false negative results. This is likely to be fine among vaccinated and boostered people because they are not likely to experience severe diseases causing hospitalisation and ICU admission/death. This will not be fine in the unvaccinated or sufficient to protect from situations that may cause serious COVID-19 in the vulnerables. Look out for emerging viral variants and the transition of the pandemic. Once the situation is 'cooling' down with more in the population are likely to be healthy (non-diseased), test kits with high Sn are to be avoided to save us from high false positive results and all its consequences of false alarms, inconveniences and unnecessary quarantines. Of course, in any doubt retest with RT-PCR, but bear in mind it may continue to be positive up to a month after symptom onset after you have recovered.