Guest Column from Dr. Mark Attwood, Cleveland County Health Officer
There is a lot of confusion today about testing for COVID-19. There are three types of tests used to diagnose SARS-CoV-19 infection. Two of the tests are used to diagnose a patient for a current infection. One test is used to determine whether or not a patient has had a previous infection.
We will often hear news commentators and medical personnel refer to COVID-19 as the current pandemic. It’s important to understand the terminology used. SARS-CoV-19 is the actual name of the virus. COVID-19 is the name of the disease. Therefore, the SARS-CoV-19 virus will cause the COVID-19 disease upon infection.
Another important term is Sensitivity. Sensitivity measures how well the test being used detects the virus when it is present. A test that has a sensitivity of 98% means that if 100 people are currently infected with SARS-CoV-19, then the test will diagnose 98 of the 100 patients, but will be negative in 2 out of the 100 even though they have the infection. Those 2 patients will have a “false negative” test. The other term used to measure how well a test performs is Specificity. Specificity means that when a test is positive, it is positive only for SARS-CoV-19 and not any other viruses. If a test has a specificity of 90%, then out of a 100 positive tests, 90 patients will actually have the SARS-CoV-19 infection, but 10 patients with a positive test will not have the infection. These are “false positive” tests. The studies and the tests are being perfected. These are examples of why the science must continually evolve and the scientific information you receive must change as it moves forward.
The test used most often to diagnose current infection is the PCR, which stands for Polymerase Chain Reaction. It tests for the presence of genetic material that is found only in SARS-CoV-19 virus. This is the infamous nasal swab that a lot of people have already had or at least have heard about. It has good sensitivity and specificity. It’s the test most often used to detect current infection. False negatives do occur, especially if the nasal swab is not done properly. PCR tests are complicated to perform and must be sent to a reference lab for completion. Turnaround time depends on how busy the reference lab is. Results can take from 2-14 days to be reported. When getting a PCR nasal swab performed, ask for an approximate length of time for the results to come back.
Another test used to diagnose current infection is the antigen test. Antigen tests detect viral proteins present in the respiratory tract and are also obtained by a nasal swab. It is less sensitive (meaning more false negatives) than the PCR test. It can be done in the office or hospital and takes only a few minutes to do. It is much like the Flu or Strep Throat swabs you get at the Doctor’s Office. These tests are still being developed. Therefore, the data on their usefulness is limited. Antigen testing kits are in short supply and are used mostly in hospitals on patients admitted for symptoms and in cases when the Doctors can’t wait 2-14 days for a PCR test result. The antigen tests have great potential, but must be developed further to be of practical use.
The third test is an antibody test. This is a blood test that measures antibodies your immune system has developed to help the body rid itself of the SARS-CoV-19 infection. Antibody tests take 8-21 days to become positive and are used to detect prior infection. Sensitivity and Specificity are questionable. Currently, the main use for antibody testing is to detect whether or not a patient previously had COVID-19 but is now testing negative, since they are past the time when the PCR or Antigen test would be positive. It is not known, based on a positive antibody test, whether or not someone is immune to a second SARS-CoV-19 infection. Someone with a positive antibody test will need to take the same necessary precautions (such as masks, hand washing and social distancing) to prevent a SARS-CoV-19 infection as someone with a negative antibody test.
Once again, evidence based medicine is proving that these guidelines, recommendations and precautions are necessary for the virus to be contained.
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July 02, 2020 at 03:55AM
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Dr. Attwood Explains Current Testing For COVID-19 – Cleveland County Herald - Cleveland County Herald
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