capillary blood vs. plasma: how close is microsampling to the "gold standard?"
Even though there are specific situations in which plasma derived from venous blood is not the best option for diagnosis, treatment, or therapeutic drug monitoring (TDM) - some tests, such as blood gas analysis, are better performed on arterial or capillary blood specimen - venous plasma remains the "gold standard" for clinical applications. But it's no longer the only technique you need to know about.
With the numerous advantages of blood microsampling (many of them significantly benefiting the patient), venipuncture can and should be replaced in some situations. The question is not which blood sampling method gives the best results, but rather how close to the best are test results, obtained from capillary blood via microsampling technique.
It’s Not Just About the Blood Collection Technique
The quality of capillary vs. plasma test results is primarily determined by specimen composition rather than blood collection method. Regarding pH and gasses, capillary blood is actually closer to arterial than venous blood. Compared to a venous plasma, capillary plasma contains higher concentrations of proteins, calcium and chloride, and lower levels of potassium, sodium, and urea nitrogen.
When the correlation between test results obtained from capillary blood and venous plasma has been established, capillary blood samples can produce high-quality results. As demonstrated in a study published in European Journal of Clinical Pharmacology, the capillary blood piperaquine measurement could prove useful in field assessment studies of the therapeutic efficacy of malaria treatment. Similarly, a study in American Journal of Clinical Pathology demonstrates the applicability of point-of-care capillary blood testing in international normalized ratio (INR) monitoring programs for POC INR programs.
Best Practices for Obtaining High-quality Capillary Blood Samples
To avoid poor-quality specimens, it is crucial that the sampling is conducted in accordance with established best practices:
- Positive patient identification
- Selection of appropriate puncture site and sampling/collection equipment, depending on the requested tests
- Warming the puncture site before collection for increased blood flow
- Proper puncture site disinfection
- The first drop of blood should be discarded
- Quick collection without milking or scraping of the puncture site
- Proper sample labeling, storage, handling, and transport
With numerous advantages of blood microsampling, it is tempting to label the technique the best. The fact is, it can be the best in some situations, and a close second in others. However, to enter the debate of capillary blood vs. plasma, rigorous, ongoing quality assurance measures during all stages of a sample cycle are an absolute must.