Laboratory testing for COVID-19
Antigen tests for SARS-CoV-2
Antigen tests for COVID infection are fast and suitable for point of care use as laboratory based tests as well as self-tests. Most of the currently authorized tests return results in approximately 15–30 minutes. The performance of the tests depends much on how they are used. Antigen tests are most effective when they are used as a diagnostic aid for persons who have recently become symptomatic and the virus load in the respiratory tract is high. Comprehensive evidence on usefulness in asymptomatic infected persons is not yet available.
What are antibodies?
When a foreign microbe such as a virus or bacterium invades the body, a defense reaction begins. The microbe is referred as antigen. During defending, white blood cells begin to produce antibodies that mark the intruder and also extensively activate the entire defense system, resulting in the destruction of the microbe. If the same microbe tries to re-enter a human, the microbe is already in the memory of the defense cells and the defense reaction occurs faster. When there are sufficient amounts of antibodies in the bloodstream, it is called immunity. In this case, the defense reaction starts quickly and the microbe cannot re-infect the person.
What does an antibody test tell you?
It can take up to weeks for antibodies to accumulate to a detectable level in the blood. In coronavirus disease, it has been observed that antibodies begin to form as early as a few days after the onset of symptoms. However, there seems to be considerable individual variation. Furthermore, you should not take an antibody test until at least 10 days have passed since the onset of symptoms.
Despite having coronavirus disease, the antibody test may be negative if performed too early when the level of antibodies has not yet elevated sufficiently. In the active and infectious phase of the disease, the best test is to detect the virus in the nasopharynx using molecular testing of the viral antigen either with a molecular or an antigen test. If the symptoms persist, detection of the virus from the nasopharynx may become unreliable, but then antibodies can provide evidence of the disease. After microbial infection, antibodies are likely to remain for months, perhaps years, but for this new coronavirus disease, it is not yet known.
How and when is an antibody test performed?
Antibody tests are intended for the detection of IgM or IgG class antibodies to SARS-CoV-2 virus. A blood specimen for the rapid antibody test is collected from fingertip, and the test is completed in about 15 minutes. A laboratory confirmatory antibody test uses venous blood and can take from several hours up to days.
The antibody test is recommended to be part of a doctor's assessment to support the diagnosis of prolonged and suspected coronavirus respiratory infections in which at least 10 days have elapsed since the onset of symptoms. A positive antibody test result in a patient with respiratory symptoms should always be confirmed with a nasopharyngeal sample to exclude coronavirus infection at the infectious stage. A positive antibody result from an asymptomatic patient taken as a fingertip examination can be confirmed by a laboratory test taken from venous blood.
Molecular coronavirus test
Molecular coronavirus tests detect viral nucleic acid, i.e., identifies certain virus-specific genes that indicate that the virus is present in the specimen. The molecular test is commonly performed on a sample taken from the nasopharynx with a flexible nasopharyngeal swab. The virus is present in the mucous membranes especially in the early stages of the disease. A positive molecular test result is a very reliable indicator of infection. However, a negative result does not completely rule out coronavirus disease, as the secretion of the virus into the mucosa varies between individuals and the success of sampling and/or laboratory analysis also affects the result. Antibody testing can provide more information in such cases.
Biomedical laboratory tests
C-reactive protein (CRP) - inflammatory marker
In contrary to the mild viral respiratory infections where CRP does not normally elevate, COVID-19 seems to increase CRP levels significantly to appr. 30-50 mg/l1-3. CRP correlates well with the severity of the symptoms of patients with COVID-19 and therefore it is a good marker in assessing patient´s condition together with other clinical findings4-5.
For more information how other biomedical laboratory tests support in COVID-19 diagnostics and treatment follow please check www.ifcc.org.
- Chen et al. 2020. Lancet 2020; 395:P507-513.
- Mo et al. 2020. doi: 10.1093/cid/ciaa270.
- Gao et al. 2020 doi: 10.1002/jmv.25770.
- Wang et al. doi: 10.1093/cid/ciaa272.
- Deng et al. 2020. doi: 10.1097/CM9.0000000000000824