Routine laboratory testing to determine if a patient has COVID‐19

How accurate are routine laboratory tests for diagnosis of COVID‐19?

What are routine laboratory tests?

Routine laboratory tests are blood tests that assess the health status of a patient. Tests include counts of different types of white blood cells (these help the body fight infection), and detection of markers (proteins) that indicate organ damage, and general inflammation. These tests are widely available and in some places they may be the only tests available for diagnosis of COVID‐19.

What did we want to find out?

People with suspected COVID‐19 need to know quickly whether they are infected so that they can self‐isolate, receive treatment, and inform close contacts.

Currently, the standard test for COVID‐19 is usually the RT‐PCR test. In the RT‐PCR, samples from the nose and throat are sent away for testing, usually to a large, central laboratory with specialist equipment. Other tests include imaging tests, like X‐rays, which also require specialist equipment.

We wanted to know whether routine laboratory tests were sufficiently accurate to diagnose COVID‐19 in people with suspected COVID‐19. We also wanted to know whether they were accurate enough to prioritize patients for different levels of treatment.

What did we do?

We searched for studies that assessed the accuracy of routine laboratory tests to diagnose COVID‐19 compared with RT‐PCR or other tests. Studies could be of any design and be set anywhere in the world. Studies could include participants of any age or sex, with suspected COVID‐19, or use samples from people known to have – or not to have ‐ COVID‐19.

What we found

We found 21 studies that looked at 67 different routine laboratory tests for COVID‐19. Most of the studies looked at how accurately these tests diagnosed infection with the virus causing COVID‐19. Four studies included both children and adults, 16 included only adults and one study only children. Seventeen studies were done in China, and one each in Iran, Italy, Taiwan and the USA. All studies took place in hospitals, except one that used samples from a database. Most studies used RT‐PCR to confirm COVID‐19 diagnosis.

Accuracy of tests is most often reported using ‘sensitivity’ and ‘specificity’. Sensitivity is the proportion of people with COVID‐19 correctly detected by the test; specificity is the proportion of people without COVID‐19 who are correctly identified by the test. The nearer sensitivity and specificity are to 100%, the better the test. A test to prioritize people for treatment would require a high sensitivity of more than 80%.

Where four or more studies evaluated a particular test, we pooled their results and analyzed them together. Our analyses showed that only three of the tests had both sensitivity and specificity over 50%. Two of these were markers for general inflammation (increases in interleukin‐6 and C‐reactive protein). The third was for lymphocyte count decrease. Lymphocytes are a type of white blood cell where a low count might indicate infection.

How reliable are the results?

Our confidence in the evidence from this review is low because the studies were different from each other, which made them difficult to compare. For example, some included very sick people, while some included people with hardly any COVID‐19 symptoms. Also, the diagnosis of COVID‐19 was confirmed in different ways: RT‐PCR was sometimes used in combination with other tests.

Who do the results of this review apply to?

Routine laboratory tests can be issued by most healthcare facilities. However, our results are probably not representative of most clinical situations in which these tests are being used. Most studies included very sick people with high rates of COVID‐19 virus infection of between 27% and 76%. In most primary healthcare facilities, this percentage will be lower.

What does this mean?

Routine laboratory tests cannot distinguish between COVID‐19 and other diseases as the cause of infection, inflammation or tissue damage. None of the tests performed well enough to be a standalone diagnostic test for COVID‐19 nor to prioritize patients for treatment. They will mainly be used to provide an overall picture about the health status of the patient. The final COVID‐19 diagnosis has to be made based on other tests.

How up‐to‐date is this review?

We searched all COVID‐19 studies up to 4 May 2020.