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Test criteria for coronavirus

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Everyone with suspected COVID-19 should be tested. Sometimes it may be appropriate to test people without symptoms. With limits in testing capacity, there is a recommended order of priority.

Everyone with suspected COVID-19 should be tested. Sometimes it may be appropriate to test people without symptoms. With limits in testing capacity, there is a recommended order of priority.


Everyone with COVID-19 symptoms should be tested

The NIPH recommends that everyone with symptoms of COVID-19 should be tested as quickly as possible.

This includes everyone with newly arisen respiratory tract infections or other symtoms that a doctor suspects are caused by COVID-19.

COVID-19 usually has the following symptoms; fever, cough, shortness of breath, loss of sense of smell and taste, sore throat or feeling unwell. For residents in a nursing home, there should be a low threshold for suspicion of COVID-19.

For children under 10 years with mild symptoms of respiratory tract infection, their symptoms can be observed at home for a couple of days before testing. Children with a runny nose as the only symptom, who are otherwise in good health without other signs of a newly arisen respiratory tract infection, do not need to stay at home or be tested.

Everyone who is tested should stay at home until they get a negative test result AND they are symptom-free. 

In some cases, it may be necessary to test people without symptoms.

For deaths in healthcare institutions where a doctor suspects that the patient had COVID-19, post mortem tests for COVID-19 should be performed.

With a lack of testing capacity

- these groups should be tested in this order of priority

  1. Patient in need of hospital admission
  2. Patient / resident in a nursing home or other healthcare institution
  3. Employee in the healthcare service with work that puts them in the vicinity of patients
  4. Person in a risk group, see risk groups and their relatives
  5. Person in quarantine because of being in close contact with a confirmed case of COVID-19, or after travel
  6. Employee, child or pupil in a re-opened childcare centre, school or after-school programme
  7. Others with suspected COVID-19 disease
  8. Certain groups of people without symptoms, see below.

Category 1-3 should be tested on a broad indication. Category 6-7 should preferably monitor symptoms at home for 2 days before considering testing if symptoms continue. 

2020-07-01 Flytskjema_engelsk.png

In some cases, people without symptoms should be tested

Testing can have one or more purposes: 1) clinical, diagnostic, 2) infection control, 3) monitoring, 4) research and 5) practical reasons.

In the following situations, testing of people without symptoms may be necessary. There is usually no basis to force someone without symptoms to be tested.

  • With confirmed infection in a nursing home, where all employees and residents in affected units are tested.
  • Close contacts who shall not be quarantined should be tested as soon as possible, preferably on day 3 after the last exposure and once again 2-3 days after the first test, preferably on day 7.
  • When new residents move into a nursing home testing may be appropriate.
  • For certain stays or procedures in hospital. Considered to be less useful when there is little transmission in society. Hospitals make their own routines.
  • Non-clinical reasons: In some cases, study centres in other countries or employers require testing. This is not provided by the public sector.
  • Research: In some studies, all participants are tested, regardless of symptoms.

Positive test results among people without symptoms who have not been exposed should be confirmed with a new test to improve the positive predictive value, see below.

Interpreting the test results

The PCR-test for COVID-19 is a good test with very high specificity and good sensitivity. It is estimated to have a clinical specificity as high as 99.9 per cent and a clinical sensitivity of around 80 per cent. In the majority of cases, the test results are reliable. However, no test is completely accurate and the result must be interpreted according to the probability that the person is infected.

2020-07-01 Flytskjema testresultat_engelsk.png

Positive test result from people without symptoms

A person without symptoms who has NOT been exposed, has a very low probability of being infected. A positive test result should be confirmed with a new test. A new test will increase the positive predictive value significantly. While waiting for the confirmatory test result, act as if the result is positive. 

  • With a new positive result (and the person still has no symptoms), it can be assumed to be a true positive result. There are estimated to be fewer than 2 false positive results per 100 true positive results.
  • With a negative result from the confirmatory test, because of the low probability of a positive result, it is very likely that the negative result is correct. The conclusion is therefore that the result is negative and further testing is not necessary as long as the person continues to have no symptoms. 

With a positive test result for a person without symptoms who has been exposed to infection during the last 10 days, it is most likely that the answer is true positive. 

Negative test results from people with symptoms 

The probability of a false negative test should be considered based on whether the person is a known close contact, clinical suspicion, time since symptom onset and other test results such as CT-thorax.

  • If there is clinical suspicion of COVID-19 and a negative test response, the person should be retested. With a pre-test probability of infection of 30 per cent, a new test will reduce the probability of false negative test results from 8 per cent to 1.7 per cent (at 80 per cent sensitivity of PCR test).
  • Tests from other localisations such as BAL, saliva, should be considered.

COVID-19-related test results at helsenorge.no and in Summary care records

From 2nd July 2020, COVID-19-related test results from the newly established NIPH laboratory database will appear on Helsenorge.no as soon as the samples are analysed and reported. The test results are sent from all the country's laboratories to the laboratory database in the Norwegian Surveillance System for Communicable Diseases (MSIS) at the Norwegian Institute of Public Health, at the same time as they are sent to the doctor.

The patient must log on to Helsenorge.no with electronic ID. Information has been prepared that can be distributed to the patient during testing.

COVID-19 related test results appear in the summary care record as soon as they are analysed and released by the laboratory. Specimens that are not analysed in medical microbiological laboratories (for example, COVID-19 rapid tests) are not shown here. The following infectious agents are defined by the Ministry of Health and Care Services as COVID-19 related infectious agents. The list is updated as of 2nd July 2020, and will be continuously updated at helsenorge.no:

  • SARS-CoV-2
  • SARS-CoV-2 antibodies
  • Influenza A / Influenza B
  • Parainfluenza virus
  • RS virus (Respiratory syncytial virus)
  • Human metapneumovirus
  • Adenovirus
  • Rhinovirus
  • Mycoplasma pneumonia
  • Chlamydia pneumonia
  • Bordetella pertussis (whooping cough)

About the solution:

Why is testing of people without symptoms and without exposure not usually recommended?

The consequence of a false positive test response is significant, not just for the people who are tested, but for their close contacts who risk 10 days in quarantine. Similarly, false negative test results may result in infection control measures not being followed for someone who is actually infected. Even with unlimited access to tests, it will usually not be advisable to test people without symptoms and have not been exposed to infection.

This is because:

  • A negative test can give a false sense of security. The test result only gives a snapshot of the situation. A person with a negative test result may be in the incubation phase and not yet be sick.  
  • A positive PCR result may show infection that has passed and is no longer contagious. The PCR test detects the virus's genetic material and does not distinguish between viral fragments and an infectious virus. Virus fragments may be present in the respiratory tract for weeks after the infection has passed.
  • High probability of false positive results where there is low transmission in society. When testing people with a very low risk of being infected, the majority of positive results will be false positive. Therefore, the positive answers are not so reliable.
    With a prevalence of 0.01 per cent (as in Norway today), the positive predictive value would be around 7 per cent with today's PCR test (sensitivity 80 per cent and specificity 99.9 per cent). That is, 14 out of 15 who test positive are not infected with SARS-CoV-2. If a new test is taken, the positive predictive value will improve from 7.4 to 98.5 per cent.
Table: Significance of prevalence for how many need to be tested and expected results. Given clinical sensitivity of 80 per cent, specificity of 99.9 per cent.

Prevalence in the population to be tested

30 %

3 %

1 %

0.1 %

0.01 %
(as in Norway today)

Number who need to be tested to find a true positive

4

42

125

1 250

12 250

Probability for a true positive result (positive predictive value)

99.7 %

96 %

89 %

44.5 %

7.4 %

Number of false positive results expected per true positive

0.003

0.04

0.12

1.25

12.5

Figures 1 and 2 below show examples of testing of 100 000 individuals with a prevalence of 0.01 per cent and 3 per cent, respectively, and with clinical sensitivity of 80 per cent and specificity of 99.9 per cent.

Prevalence.png

Figures 1 and 2: The illustrations show that with low prevalence/pre-test probability, false positive results are the biggest problem, yet with high prevalence/pre-test probability, false negative results are the biggest problem. Source: Norwegian Institute of Public Health.

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Facts

Coronavirus

SARS-CoV-2 is the name of the virus that is causing the outbreak of COVID-19 disease.

The virus is related to another coronavirus that caused the SARS outbreak in 2002/2003 but is not the same virus.