Ever since COVID 19 started in late 20219, scientists have kept busy with test design, modalities and their yielded results, including coming up with 2 awesome vaccines.
Fast forward to 2021, there are different COVID 19 tests available and various ways to acquire a sample for COVID 19 testing. They include nasopharyngeal secretions, nasal secretions, saliva and a blood sample.
Not all COVID 19 tests are created equal, they have different mechanisms of detecting the virus, different modalities and clinical applications. Since they vary on the test itself, this can be very confusing,
We'll start off by discussing some background terminology.
Sensitivity: statistical measure of the proportion of positives that are correctly identified, or the true positives.
Specificity: statistical measure of the proportion of negatives that are correctly identified, or the true negatives.
Terms like "true positive", "false positive", "true negative" and "false negative" are referred to the result of a test, and their correctness of classification. For example, if we were to say that your test is a "true positive" that means that your were correctly diagnosed as diseased, or if your test is a "false positive" that means that you were incorrectly diagnosed as diseased, and so on.
Prevalence: statistical measure of the proportion of people burdened by a disease in a particular point in time.
Incidence: statically measure of the proportion or rate in which people develop a disease in a particular point in time.
Disease incidence refers to how many new people have the disease versus disease Prevalence that refers to how many people have the disease in general in a particular time.
Now that we've covered these terms, lets break down the different types of COVID 19 tests available, sample acquisition, result meanings and clinical application.
Prevalence: statistical measure of the proportion of people burdened by a disease in a particular point in time.
Molecular testing: can be either by PCR (Polymerase Chain Reaction) or NAAT (Nucleic Acid Amplification Test) which are just a fancy word for amplification and duplication of DNA material. It works by taking a small sample of DNA, isolating, and then amplifying it and duplicating it to make millions of copies. Essentially facilitating the identification or presence of an organism even if its in smaller quantities. It can be performed in nasopharyngeal secretions, nasal secretions and in saliva samples.
PCR is considered the gold standard for diagnosing COVID 19, but only by nasopharyngeal secretion sample. The sensitivity and specificity of this test is very high, deeming it a "true positive" when it detects the disease, and a "true negative" when the disease is not detected. There are some providers that follow viral shedding by testing after 14 days of disease with seriated molecular tests, until test is negative. This is not necessary if the patient is asymptomatic and has been in quarantine for 14 days, persistent positive test results after this period are considered non-infectious.
Antigen testing or Rapid testing: Antigens are molecules that are specific to a particular organism. They are sometimes located on the surface of the organism and serve as identifiers. Antigen testing works by the principle of recognition, identifying the presence of an antigen in your blood. Small blood sample is collected and the kit detects the presence of COVID19 antigens.
Antigen testing is used for COVID 19 exposure detection. It's unable to detect active COVID19 disease and it warrants a PCR test for diagnosis confirmation. Sensitivity for antigen testing is lower than PCR meaning there is a higher possibility of false negatives.
Antibody testing: Antibodies are cells produced by the immune system to help combat diseases. They are specific for particular organisms and serve as identifiers for exposure and immune response. A blood sample is collected and tested for presence of specific SARS-CoV-2 antibodies.
This test should not be used for diagnostic purposes of COVID 19, clinical use is for confirmation of COVID 19 exposure if molecular testing is delayed. It cannot detect active infection since Antibodies are usually produced from 1-3 weeks after exposure to pathogen. If results are positive for antibodies you were exposed to the disease and your body has created an immune response against it which can range from 1-3 weeks after exposure and it is still unclear how much it lasts. For this reason we are recommending vaccination with or without disease exposure. If you tested negative for antibodies this does not rule out disease exposure or active infection since antibody production is a delayed reaction. In high disease prevalence areas, test sensitivity is increased.
Hopefully after reviewing this you can better understand the meaning of the results and
along with your healthcare provider, choose the test that's right for you.
