Capillary blood sampling involves using a lancet to create a small puncture in the skin (usually on the finger) from which blood drops can be collected to perform medical tests. Capillary-sampling is widely-used for collecting blood samples where traditional blood draws from a vein (venesection) can’t easily be obtained. We commonly use it in hospitals to collect blood from children and babies (often via a heal-prick). Once taken, the capillary sample is analysed in the lab on the same instruments as used for a venous draw - it’s only the collection method that differs.
It’s important to note that capillary sampling is not the same as point-of-care testing (often called “rapid testing”). Capillary samples still go to a lab for analysis. Point-of-care tests include small devices that take a drop of blood and provide an immediate result without sending the sample to a lab. We have point-of-care tests for HIV for example. However, there are currently no accurate point-of-care tests for SARS-CoV-2.
The benefits of finger-prick sampling are that it is easier to perform and is associated with less pain than a traditional venous blood draw. Its drawbacks are that the volume of blood collected is smaller and some people can find it difficult to express blood from a finger.
Finger-prick sampling is used for the diagnosis of many diseases, such as HIV, hepatitis and coeliac disease. Studies have demonstrated that the accuracy of testing for antibody levels is not affected by the sampling method used15, 16. The NHS, clinics and companies have been using capillary sampling for years to allow people to do blood tests from home.
Clinical testing laboratories (such as The Doctors Laboratory whom we work with) carry out validation of new tests to ensure that it’s suitable to use with capillary sampling. These laboratories have not found any difference in the accuracy of the Abbott SARS-CoV-2 IgG antibody test with traditional venous blood samples compared to capillary blood samples.
The Medicines and Healthcare products Regulatory Agency (MHRA) regulate medical tests including the Abbott SARS-CoV-2 IgG antibody test. They approved the Abbott antibody test for use because it met their accuracy requirements. The data they based this award on was gathered using venous blood samples, not capillary samples. This is normal; it’s generally up to the lab performing the testing to ensure that the sampling method is suitable for a particular test. The lab we work with (TDL), amongst others, have performed these checks and are satisfied with the performance of capillary samples for the Abbott IgG antibody test.
However, in the case of the coronavirus antibody testing, the MHRA have specifically requested that further evidence is provided regarding the accuracy of the test with capillary samples. It’s not clear exactly why the MHRA have taken this view but we would speculate that it’s due to the public interest in coronavirus testing and the fact that large corporations, such as Superdrug, have begun offering capillary sampling kits on a large scale.
We don’t have any reason to expect capillary blood samples to perform any differently than venous samples with the IgG antibody test and major laboratories in the UK state that the two methods of blood collection are equivalent. However, for the present time, we have stopped offering the test in-line with MHRA guidance.
Testing is clearly important to get the country back to work and normal activity safely and the government has often lagged behind the general public in its response to COVID-19. It’s difficult to see how finger-prick blood collection could not form part of its testing strategy. Performing venous blood draws on millions of people would be very slow and expensive and of course risks unnecessary contact between individuals involved in the testing. Finger-prick sampling offers a much more efficient way to roll out mass testing and we hope it receives the necessary approvals soon.