Value of Diagnostic Tests in World of COVID-19

The sensitivity and specificity of any given diagnostic test are independent of the prevalence of the disease. However, in reality, the sensitivity and specificity varies depending on the prevalence of the disease. Variations in prevalence may occur due to the spectrum of the disease, verification bias, patient selection, and misclassification of reference standards. These factors are not easy to predict but in general, higher or lower prevalence usually does not lead to a change in specificity or sensitivity.

The COVID-19 pandemic has opened a whole new window into the world of diagnostic testing. From November 2019 when the prevalence of COVID-19 had moved from zero percent to a much more significant number, the infection has caused confusion and chaos in clinical medicine. What is known is that the prevalence of this disease changes all the time.

At present, clinicians are moving forward almost blindly when it comes to diagnosis and treatment because for them, the only way to manage this disease and the patients is to treat symptoms and situations as they happen. On the diagnostic side, questions that are constantly being asked include: who should be tested, when should the testing be done, how often should the testing be done, and what do the results mean? In each case, clinicians are forced to quickly adapt to the clinical scenario because there are no universal guidelines, which is understandable since this is a new disease.

The most important diagnostic test being used for COVID-19 is the reverse polymerase chain reaction using swabs taken from the oropharynx and/or nasopharynx. In patients with pneumonia, lower airway secretions can also be tested. However, the detection rate from each of these sites varies and may change during the course of the illness. Some reports indicate a sensitivity of throat samples to be 60% at initial presentation, but the numbers are higher in symptomatic patients and in those with severe disease. Therefore, it matters when and how the patient is tested.