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.