A few summary thoughts about test performance. 1/
First, keep in mind that no diagnostic test is perfect. Clinicians always have to interpret the test in context. Tests can be wrong for a number of reasons. 2/
False negatives can arise from, among other things, poor sampling (how the swab was collected), a small # of viruses present (an asymptomatic but infected person), or a faulty test. 3/
If a person has a false negative test early in the course of disease, a reason may be that the # of viruses present (or viral load) is relatively low. Later, during the pneumonia phase, you *might* miss the virus in the nose--the viral load might be higher in the lungs 4/
In general, if you test the general population, positive tests may be false positives, because the positive predictive value (the probability that a person has disease if they have a positive test) is lower with low prevalence 5/
And if you test someone with fever, cough, shortness of breath, and a classic X-ray for COVID-19, then a negative test may be false negative, because the negative predictive value is lower with high pre-test probability (i.e. high prevalence). 6/
So remember that how a clinician interprets a test must account for the probability of infection before the test was done: general population, asymptomatic exposed, some classic symptoms, and multiple classic symptoms all have implications for interpretation of + and - tests 7/7
