Diagnostikk (in English)

Laboratory testing for COVID-19

Coronavirus can be tested either by examining whether the virus is present in the nasopharyngeal mucosa or by measuring whether the body has produced antibodies to the coronavirus. Careful consideration is required in selecting the test and especially in interpretation of its result due to benefits and limitations of different methods. In symptomatic patients, the primary test is a molecular coronavirus test from a nasopharyngeal specimen. However, negative molecular COVID-19 test result does not completely rule out coronavirus infection. Antibody tests from blood sample support other diagnostics especially when the respiratory symptoms are prolonged. Furthermore, a positive coronavirus antibody test result can be confirmed with a complementary method that yet increases the reliability of the results. However, the immunity against coronavirus or the duration of immunity cannot be directly assessed on the basis of the antibody test.

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Antigen tests for SARS-CoV-2

The antigen tests for COVID infection are relatively inexpensive, fast and suitable for Point-of-Care use. The performance of the tests depends much on how they are used. The antigen tests are most suitable 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.

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Antibody testing

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 virus. If the same virus tries to re-enter a human, the virus is already in the memory of the defense cells and the defense reaction occurs faster. When there are sufficient amount of antibodies in the bloodstream, it is called immunity. In this case, the defense reaction starts quickly and the virus 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 amount 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 viral antigen. 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?

The antibody test contains IgM or IgG antibodies to COVID-19 coronavirus. The 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 testing

Molecular coronavirus test

The molecular coronavirus test detects viral nucleic acid, i.e., identifies certain virus-specific genes that indicate that the virus is present in the specimen. The molecular test is most 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 sure 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.

  1. Chen et al. 2020. Lancet 2020; 395:P507-513.
  2. Mo et al. 2020. doi: 10.1093/cid/ciaa270.
  3. Gao et al. 2020 doi: 10.1002/jmv.25770.
  4. Wang et al. doi: 10.1093/cid/ciaa272.
  5. Deng et al. 2020. doi: 10.1097/CM9.0000000000000824