Antibody tests for Covid-19: what use are they in practice?

12th November 2020 by Dr Robert Walton

Antibody tests for Covid-19: what use are they in practice?

Dr Robert Walton, a senior fellow in general practice at Cochrane UK, examines evidence from a new Cochrane Review on antibody testing for Covid-19, and asks how useful these tests might be for GPs.

Facing a new wave of the coronavirus, GPs may be better prepared this time for the onslaught. Among our new tools are a range of antibody tests for Covid-19 infection offering the prospect of quick diagnosis from routine blood samples. NHS labs have made these tests available to primary care remarkably quickly but it is not completely clear how they are best used and what the benefits of antibody testing might be when we already have a pretty reliable route to diagnosis using polymerase chain reaction (PCR) tests on nasopharangeal swabs.

What evidence is there for antibody testing?

The Cochrane Review examined data from 54 study cohorts with 15,976 samples and 8,526 of these were from people with Covid-19 infection.

As might be expected, the sensitivity of antibody testing was poor in the first week of the illness – only detecting 30% of people with Covid-19. This means that there would be a high number of false negative tests if used in the early stages of the disease. However the sensitivity of the test rose surprisingly quickly to 72% for the combination of IgG and IgM antibodies in the second week and 90% in week three. Unfortunately there were insufficient data at later time points so we don’t know how rapidly the antibody levels fall off.

It is surprising that the ‘gold standard’ in most of the studies included in the review was PCR alone rather than a composite of PCR and clinical data. So there may have been patients included who were PCR negative but who clearly had Covid-19 infection on clinical grounds. Thus the number of false positives for antibody tests in the studies may have been overestimated.

How useful is antibody testing in practice?

It seems clear that clinical assessment with a PCR test, if available, will remain the mainstay for diagnosis of Covid-19 infection in the first two weeks of the illness, given the low sensitivity of antibody testing. It is during this period that, at least in my experience, most people tend to present to GPs. So blood tests of any kind at this early stage seem to be irrelevant – indeed an unnecessary infection hazard for staff and other patients in the practice.

Since most people start to recover after the first week of infection the need for testing diminishes over time. But for those who remain unwell there may be uncertainty about the diagnosis. PCR testing at this stage may be less reliable so antibody testing could be of some use for those who did not have a PCR test initially or who were PCR negative. However many patients who are deteriorating clinically at this stage will need specialist assessment and antibody testing in primary care may simply introduce unnecessary delay.

People who remain unwell and who do not need specialist treatment will be managed in primary care and where there is diagnostic uncertainty then Covid-19 IgG and IgM antibody tests may be a useful addition to a routine panel of blood tests.

For people presenting later with ‘long Covid’ then antibody testing may still be of value in establishing whether the person has been infected. From the data in the review we don’t know how quickly levels fall after the first month. However it is likely that the IgM antibodies would fall more quickly whereas IgG antibodies would persist for longer. So for people presenting with persistent symptoms some weeks or months after the illness a positive antibody test may be helpful but a negative one would not rule out the diagnosis.

Similarly, since the studies in the Cochrane Review were mainly conducted in hospital patients who may have had more severe disease, we do not know how well the tests perform for people who have a milder illness and potentially lower antibody levels. In addition the authors had concerns about high risk of bias which means that the accuracy of tests in clinical care will be lower than reported in the review. Thus clinical assessment will continue to play an important role in diagnosis and management.

Key points on antibody testing for GPs

  1. A Cochrane Review has found that testing for a combination of IgG and IgM antibodies may be helpful for diagnosis in the third week of Covid-19 infection.
  2. Antibody tests may be most helpful where previous PCR test results were negative or unavailable.
  3. For people presenting after one month a positive test can indicate previous infection, but a negative test will not rule it out.

Dr Robert Walton is a GP and a member of the Berkshire Oxfordshire and Buckinghamshire (BOB) NASGP Locum Chambers. You can read more blogs about Covid-19 on Evidently Cochrane.

References
The Cochrane Review
‘Antibody tests for Covid-19’ was originally published on Evidently Cochrane.

NASGP e-Learning

Now you've read this article, record your learning here and we'll instantly send you an elegant PDF as a record of your learning.
  • Date Format: MM slash DD slash YYYY
  • Write at least two learning points.
  • One credit = one hour learning demonstrated by this reflective record; include the time it took to do this activity, plus the time to have a good old think about it and muse with colleagues, and to complete this form. e.g. 0.5 = half an hour.
  • Record any notes here for yourself or your appraiser - why not include a link to the blog, podcast or video for future reference.
  • About this article or form - this *won't* appear on the report we email back to you; it's to help _us_ improve.
    I give permission to allow the author, Dr Louise Hudman, access to my replies for the sole purpose of improving the quality of these articles.

"LocumDeck is so good and so intuitive that I am lost for words and utterly filled with glee.

It can send an email confirming the sessions that are booked and creates and emails invoices direct to the practice for you. These contain links to your beautifully laid out ‘credentials’ page with all your relevant documents, and a model contract that is all ready for you to accept or edit.

It automatically produces and sends the locum form A, and will produce locum form B. It also acts as the calendar... And it all happens automatically. It is really less than a minute to invoice and do the pension forms, with everything calculated perfectly."

Dr Annette Steele

See the full list of features within our NASGP membership plans

Membership

"LocumDeck is self-explanatory and having never made/sent an invoice before it's made my life a lot easier, and made me feel reassured I hadn’t forgotten anything important.

I’ve recommended it to my colleague/friend who is also starting out as a GP locum."

Dr Christine Vickery

Dr Christine Vickery

See the full list of features within our NASGP membership plans

Membership