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Test criteria for coronavirus

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It is advised that everyone with suspected COVID-19 should be tested. In some situations it may be necessary to test people without symptoms. With limits in testing capacity, there is a recommended order of priority.

It is advised that everyone with suspected COVID-19 should be tested. In some situations it may be necessary to test people without symptoms. With limits in testing capacity, there is a recommended order of priority.


Who should be tested?

The Norwegian Institute of Public Health recommends that everyone with symptoms of COVID-19 should be tested. In some cases it may be necessary to test people without symptoms.

If there is a lack of testing capacity, groups should be tested in this order of priority

Prioritised indications for COVID-19 testing

  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. See the flowchart further down in the article.

With a negative test result but strong clinical suspicion of COVID-19, re-testing can be considered.

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

When is PCR-testing of people without symptoms appropriate?

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

As a general rule, only people with symptoms should be tested. 

In some situations it can be beneficial to test people without symptoms. This applies particularly to nursing homes because residents may have atypical symptoms, difficulty in expressing themselves or a high risk of a severe course of the disease. There is usually no basis to force someone without symptoms to be tested.

In the following situations, testing of people without symptoms can be appropriate: 

  • With confirmed infection in a nursing home, where all employees and residents in affected units are tested.
  • With contact tracing in healthcare institutions, it can be appropriate to test close contacts without symptoms. 
  • When new residents move into a nursing home.
  • 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. 

Why is PCR-testing of people without symptoms usually not recommended?

The consequence of a false positive test response is significant, not just for 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, especially if they have not been exposed to infection.

This is because:

  • A person with a negative test result may be not yet be sick. A negative test result during the incubation period can give a false sense of security. 
  • A positive PCR result may show infection that has passed and is no longer infectious. 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.

figur 1 PPV og NPV engelsk 0,01%.jpg
Figures show how low and high prevalence can show false positive or negative results.
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

How to interpret positive test responses from asymptomatic individuals?

  • As a rule, a random positive test response from a person without symptoms, who has not been exposed to infection, should be confirmed by a new test.
  • Most asymptomatic positive test results will either have a false positive result or be not very / not contagious. However, some will be in a pre-symptomatic, infectious phase.
  • If a person without symptoms who has been infected within the last 10 days has a positive test result, it is most likely that the result is true positive and that the tested person is contagious. Contact tracing can be started from 2 days before the test date.
  • Since a test result is likely to be false positive for a person without symptoms who has not been exposed to infection, a new test for confirmation is recommended before contact tracing begins. A new test will significantly increase the positive predictive value and fewer than two false positives can be expected per 100 true positives. While waiting for the new test, the tested person should be in isolation and household members should be in quarantine.
  • If the result of the confirmatory test is positive and the person still has no symptoms, it is most likely a true positive result, but it is difficult to know at which stage the patient is in the course of the disease. Therefore, contact tracing is not initiated. Household members are in quarantine for 10 days from the first test date.
  • If the result of the confirmatory test is positive and symptoms have appeared in the meantime, regular contact tracing begins from 2 days before the onset of symptoms.
  • If the result of the confirmatory test is negative, isolation of the tested person and quarantine of household members ceases.

How to interpret negative test results from people without symptoms

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 on PCR test).

Flowchart

Flowchart for COVID-19 testing, click on picture to enlarge: 

2020-05-09 Flytskjema_engelsk.png

The flowchart can be downloaded as a pdf file: 

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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.