Capillary blood (fingerstick) sampling is increasingly being used worldwide due to the growing availability of point-of-care (POC) testing.
With anemia affecting about 25% of the global population and much higher prevalence in the developing countries, hemoglobin is the most frequently performed test in POC hematology, also being used routinely by blood collection services to ensure safe donations.
At the same time, hemoglobin values are among the parameters most prone to being affected by pre-analytical errors so, in order to avoid generating variant and misleading hemoglobin results, health care personnel drawing blood must adhere to strict and standardized blood sampling techniques to ensure accurate and consistent POCT results that are comparable to laboratory techniques.
This page aims to provide a quick guide to capillary sampling best practice to help health care professionals understand common causes of pre-analytical errors and reduce their impact on the hemoglobin result.
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Detailed capillary sampling guidelines have also been published by both the Clinical and Laboratory Standards Institute and the World Health Organization (WHO).1, 2
Variability in reported hemoglobin values can be caused by a number of physiological factors like dehydration, smoking behavior or altitude, and it can be significantly affected by pre-analytical errors arising due to incorrect capillary blood sampling technique. Detailed below are some of the most common sources of error that occur that healthcare workers should be aware of.
Ernst DJ, Balance LO, Calam RR, McCall R, Szamosi DI, Tyndall L. Procedures and Devices for the Collection of Diagnostic Capillary Blood Specimens. 6th ed. Approved Standard GP42-A6. Wayne, Pa: Clinical and Laboratory Standards Institute, 2008. Available at: https://clsi.org/standards/products/general-laboratory/documents/gp42/. Accessed July 6, 2017.
Dhingra N, Diepart M, Dziekan G, et al. “Capillary Sampling,” in WHO Guidelines on Drawing Blood: Best Practices in Phlebotomy. Geneva, Switzerland: World Health Organization, 2010. Available at: www.ncbi.nlm.nih.gov/ books/NBK138654. Accessed July 6, 2017.
Briggs C et al, Where are we at with point- of- care testing in haematology? British Journal of Haematology, 2012, 158, 679–690
Massimo Daves et al, Evaluation of capillary haemoglobin determination for anaemia screening in blood donation settings Blood Transfus. 2016 Sep; 14(5): 387–390.
Another key factor that influences hemoglobin measurement is capillary flow. Typically for hemoglobin, the first 1-3 drops after puncture show a higher degree of variability of the hemoglobin concentration independent of the analytical device used for the test. It is for this reason that these first few drops of blood should be wiped away.
The highest accuracy is generally reached from the 4th drop after puncture, with good capillary flow occurring for a period of 30-45 seconds. After this time, coagulation will occur where blood clotting would lead to inaccurate hemoglobin results if blood is sampled then.
The most important factor to reduce pre-analytical errors is the presence of a free spontaneous blood flow, especially as neither the size of the drop nor the time of collection following the puncture is well defined and manufacturers’ recommendations on this subject vary.
Figure 1 - The effect of time and capillary blood flow on hemoglobin results
The following steps demonstrate how to take a proper capillary blood sample to ensure accurate POC hemoglobin measurements.
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