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Capillary vs Venous Blood Testing

 

Many of you reading this have most likely have had a finger prick-based blood test in the past. You effectively pricked your finger and collected the drops of blood into a test tube.  That meant no needles put inside a vein, no medical professional required to take your blood for you (and therefore cheaper), less blood needing to be taken and it can be done in your own time, at your own convenience.  Brilliant, right?

Well, if you’re anything like me, this may take a few attempts.  It may require you pricking every finger that you have in order to collect enough blood, only to then be told that your sample clotted during the process and has to be redone!  I’ve had times where my hands are quite literally covered in blood, like something out of a murder scene.  After successfully managing to collect a sample, you may also be disappointed to find that your results come back with serum levels that are clearly well beyond, or below, the realms of normal. Annoying!

There are certainly scenarios where capillary tests do work, and many will attest that their finger prick tests have indeed proven effective and accurate. The picture isn’t always quite so clear, though.

 

What Does The Scientific Literature Say?

As you should know by now, no blog of mine is complete without a thorough scientific review of the literature!  The World Health Organisation provides guidelines on capillary sampling.  Generally, they state that capillary tests should only really be used with small volumes of blood, and are best suited for paediatric patients (for obvious reasons – kids and needles for blood samples aren’t always an easy combo) 1.  The same can also be true in patients with burns and any patient that can’t have venepuncture, for whatever reason.

This isn’t saying that capillary testing is not accurate.  They’ve proven to have a relatively good amount of variation with venous testing for hGH, at a coefficient of variation of 14.2% 2.  This isn’t actually a ‘very good’ variation score; generally anything less than <10% would be best; but it’s an adequate replacement.  However, considering that testing in that particular study was performed quickly after exercising, there was optimal blood flow and very little time for the samples to be affected by any external factors prior to processing.  There is also the important fact that the capillary samples were taken by trained professionals.

Translate this into a scenario where you’re attempting to carry out the procedure on yourself, you’ve not exercised and therefore less vasodilation often means less free flowing blood, potentially forcing you to ‘milk’ your fingers in order to collect enough in the tube, and increasing the chances of the sample clotting because you’ve taken too long (if this happens, stop bleeding your figures and just get a new test kit).  It is well known that milking the puncture site and then vigorously mixing the sample after collection can cause haemolysis, in addition to sample dilution with extracellular fluid which can further alter results and cause false-high positives 1,4.  Some Testosterone assays are prone to interference from certain blood components (binding proteins and heterophile antibodies) 3.  Lyse the blood sample and you’re more likely to have interference and cause false-high results.

 

Verdict

Capillary tests are cheaper and more convenient if you don’t end up looking and feeling like Dracula’s victim, like I usually do.  But to the untrained, and those who’s blood tends to clot quickly or not bleed well (thus needing to ‘milk’ their fingers), you’re best off avoiding it.  In clinic, we’ve identified many erroneous high false-positive results from capillary testing.  A venous sample will always provide a clearer picture, as it mitigates many of the issues associated with capillary testing, leading to far more confidence in the results.  It does cost more, but for good reason.  You should never attempt to take your own blood sample from a vein as the risk of seriously adverse outcomes is extremely high, you must always arrange for a qualified professional to do it for you.

Essentially, if you’re well versed with taking capillary blood tests, trained to do so, don’t clot easily, don’t need to milk your fingers (do plenty of gentle exercise before the procedure and warm your hands up), and know how to mix a sample without compromising it, then you should be okay with a capillary blood test.  You may also have a specific medical reason to avoid venous blood sampling.  However, we insist on venous blood testing with all patients, with capillary samples being an extreme last resort when the resources for venous testing are not easily available (i.e. during the recent pandemic).  That way, we can have full confidence in the results, and you won’t have to repeat the test three times and then end up using the venous method in the end anyway, like I did!

 

References

  1. Lenicek Krleza J, Dorotic A, Grzunov A, Maradin M. Capillary blood sampling: National recommendations on behalf of the Croatian society of medical biochemistry and laboratory medicine. Biochem Medica. 2015;25(3):335-358. doi:10.11613/BM.2015.034
  2. Godfrey RJ, Whyte G, McCarthy J, Nevill A, Head A. The validity of capillary blood sampling in the determination of human growth hormone concentration during exercise in men. Br J Sports Med. 2004;38(5):27. doi:10.1136/bjsm.2003.008714
  3. Zhang W, Zhang Y, Xie E, Ma J, Xu HG, Pan SY. False high testosterone of unknown reason in a clinically inconspicuous female. J Lab Med. 2019;43(1):41-43. doi:10.1515/labmed-2018-0327
  4. Dukić L, Kopčinović LM, Dorotić A, Baršić I. Blood gas testing and related measurements: National recommendations on behalf of the Croatian society of medical biochemistry and laboratory medicine. Biochem Medica. 2016;26(3):318-336. doi:10.11613/BM.2016.036