Now that we are expanding testing for Corona Covid-19 Virus, many patients are confused when they feel sick but get a negative test result. They have all the signs of Covid infection with fever, cough and malaise but the test came back negative. The Washington Postreported on a woman who was hospitalized with presumed Covid pneumonia but her test was repeatedly negative. What is going on?
Every clinical test we use in science and medicine has a sensitivity and a specificity. This tells us how good the test is in diagnosing who has the disease (true positive) and how many people with the disease we missed (false negative). Doctors know that tests with high sensitivity are pretty good at giving us accurate results for Covid-19 to show us the patients who have the virus.
With the Covid Virus we have many many tests that are being used and each of them, depending upon how they are testing and how they were developed in the Petri dish, have different values. None of these tests were previously tested on humans because THIS IS A NEW VIRUS.
If a test is very sensitive it means it will catch all of the true positives in people being tested (a true positive rate). We don't know the sensitivity of the myriad of tests being used but we think it is between 75-99%. That means 25-1% of the patients will have a false negative test and will test negative even if they have the Covid-19 Virus.
The specificity of the test tells us who tested negative for the disease but they don't actually have it (true negative).
We are most interested in patients who test negative but really have Covid-19 (A false negative), because we want them to really isolate and not spread the disease.
The results of the tests we are using now can be affected by a number of factors. Testing too early. Poor swab technique. Lab error. Transport error. Or just the test itself. We know we are going to be wrong a certain percent of the time.
If you think you have Covid-19 because you have the symptoms, and your test came back negative, you should presume you are positive and quarantine for 14 days. (from the beginning of your exposure or symptoms). This may sound extreme but it is the safest way to proceed.
There is no need to be re-tested if you are not worsening or needing hospitalization.
One thing that has held true in this pandemic is that the majority of patients who have severe illness or die are older or have other medical conditions or are healthcare workers. The response to the virus depends upon the viral load (amount of exposure) and a persons own immune defense.