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The news has reported that a new serological test for the presence of anti-SARS-CoV-2 antibodies has received an emergency use authorization from the FDA, and notably has a higher specificity than other tests (99.8%).

The new test requires blood to be drawn from a vein, rather than a finger prick, and a quote from the test's manufacturer reads, "If you take blood from a finger prick, you will never be able to achieve the same level of specificity that you will achieve … when you take blood from the vein" (non-scientific news outlet source). Although the question is prompted by news related to COVID-19, I'm interested in the general case as well.

It is unclear to me why this would be the case, since antibodies should be present in capillary blood taken from the fingertip. I would suspect one of two reasons, but I don't have enough of a biology background to verify whether they make sense:

1) The volume of blood which can be drawn from a vein is much larger. This would make sense if it improved the sensitivity to a low-concentration antibody of interest, since there may not be enough antibodies in a fingertip sample to bind to antigens in the test. However, it's not clear why having a larger sample would help with specificity.

2) Fingertip blood samples are contaminated with lymph, matter from the skin surface, and so on. I'm not sure how it would affect the specificity of the test. In my naive understanding of serology tests, the reagent would consist of a synthetic antigen of interest, plus either anti-human-Ig or a second moonoclonal antibody for the antigen that would be used to detect any bound antibodies from the sample. It's not clear to me how these sorts of contamination could cause false positives for this sort of binding.

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This ref, https://academic.oup.com/ajcp/article/144/6/885/1761216 show that drops of finger prick blood vary more than venous blood.

The main reason for using venous blood is licensing. A test is licensed only for an exact procedure and materials. If you vary any conditions the license is invalid.

However, I would not expect a test for antibodies (present vs. absent) to show much variation except for detecting the lowest levels of antibodies. (Sorry I cannot find a ref for this).

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  • $\begingroup$ Thanks. I'm still a bit confused by the licensing aspect: it makes sense that a test is licensed for a certain procedure, but here the choice of procedure was justified by a difference in specificity. The reference about fingertip sample variation is nice, and I'm still trying to wrap my head around any mechanisms by which it can affect specificity. $\endgroup$ – nanofarad May 4 at 18:38

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