For point-of-care hemoglobin testing, good capillary blood sampling practices are vitally important
By Katja Lemburg
Capillary blood (fingerstick) sampling is increasingly being used worldwide, in part due to the growing availability of point-of-care (POC) testing—one of the fastest-growing areas in laboratory medicine. In addition to its use for POC testing, capillary blood sampling by skin puncture is also often used to obtain small blood volumes for laboratory testing because it minimizes pain, is less invasive, and can be performed quickly and easily.
Obtaining blood by skin puncture rather than venipuncture is particularly important for pediatric patients, enabling physicians to avoid the effects of blood volume reduction and possible iatrogenic anemia. Capillary blood samples are also advised for obese, older, or anxious patients, as well as those with severe burns, thrombotic tendencies, or veins that are superficial, inaccessible, or fragile.1
Due to the fact that anemia is one of the world’s most common blood disorders, affecting around 25% of the global population, hemoglobin testing is among the most frequently performed procedures that make use of capillary blood samples collected at the point of care.2 Hemoglobin is an iron-containing protein found in all red blood cells, enabling the cells to bind to oxygen in the lungs and carry it to tissues and organs throughout the body. However, hemoglobin values are also among the parameters most prone to being affected by preanalytical errors and physiological factors during blood sampling.
To avoid generating variant and misleading hemoglobin results, healthcare personnel drawing blood must pay strict attention to the use of correct blood sampling techniques. To ensure accurate and consistent POC results that are comparable to laboratory techniques, it is important to understand why errors may arise, and to follow standardized practices for capillary blood sampling.
Factors That Can Affect Hemoglobin Measurement
Variability in reported hemoglobin values can be caused by a number of physiological factors, in addition to preanalytical sampling mistakes. Such variability is a key factor in the need to establish standardized procedures when measuring hemoglobin on POC testing devices. Physiological factors that should be considered include:
- For a given fingerstick result, the expected venous hemoglobin value for women is 0.5 to 0.8 g/dL lower than for men.3 The reference ranges stated in the literature for hemoglobin vary due to a wide range of conditions, including diet and geography; however, the expected values among adults are generally 12.0–15.0 g/dL for women and 13.0–17.0 g/dL for men.2,4
- Hemoglobin concentrations decline during the first trimester of pregnancy, reaching their lowest point during the second trimester (diminishing by approximately 0.5 g/dL to around 11.0 g/dL), before they start to rise again during the third trimester.2
- Source of sample. Capillary blood has higher hemoglobin than venous blood, especially in women and in men with severe iron depletion (median +0.67 g/dL for iron-depleted women to –0.1 g/dL for iron-depleted men).3 Venous blood has a slightly higher hemoglobin concentration than arterial blood.
- Sampling site. Earstick sampling has been used in the past, but has been found to produce values that are higher than venous and fingerstick values. Fingerstick sampling has been shown to more closely approximate venous hemoglobin values.5
- Tourniquet use. Tourniquet use of longer than 30 seconds increases the venous hemoglobin value.3
- Body position. Hemoglobin is higher in blood samples from standing subjects than in samples from sitting or supine subjects. For example, an increase of up to 9% after 15 minutes of standing or a decrease of 2.4% to 2.7% when moving from a standing to a seated position can be seen in venous hemoglobin values.3
- Diurnal variation. Hemoglobin tends to be higher in the morning and decreases throughout the day.3
- Loss of plasma volume due to transpiration or insufficient fluid uptake, for example, causes increased hematocrit and hemoglobin values.
- Normal hemoglobin concentration increases at high altitudes (>1,500 m) to compensate for the lower concentration of oxygen in the air. This effect becomes more pronounced with increasing altitudes and should be corrected for when interpreting hemoglobin results.2
- Smoking is known to affect hemoglobin concentrations, an effect which is proportional to the amount of tobacco smoked.2
- The highest hemoglobin concentrations are found in neonates on days 1 to 3 after birth (mean, 18.5 g/dL; –2SD, 14.5 g/dL). Hemoglobin levels then decline, reaching the lowest levels at 3 to 6 months (mean 11.4 g/dL; –SD, 9.5 g/dL), when they gradually start to increase to adult levels.6
It should also be noted that reference ranges may vary among individual laboratories, instruments, and methods. Therefore, when undertaking comparative hemoglobin testing for studies or evaluations, samples should be taken under identical conditions to maintain consistency of factors among measurements.
Capillary Sampling Technique
Both the accuracy and reliability of hemoglobin measurement can be affected by preanalytical errors arising due to incorrect capillary blood sampling technique. Capillary sampling best practices for adult and pediatric patients are detailed in guidelines published by both the Clinical and Laboratory Standards Institute and the World Health Organization (see “Best Practices for Fingerstick Capillary Sampling“).1,7 But to appreciate the importance of adhering to standardized best practices, it is also worth understanding where and how possible sources of error may occur, as described below.
- Choice of lancet. The lancet must make a sufficiently deep puncture to ensure an adequate flow of blood. A penetration depth of 1.85 to 2.25 mm is recommended for adults, depending on the thickness of the skin. For children aged less than 8 years, the penetration depth should not exceed 1.5 mm.
- Selection of puncture site. The middle or ring finger should be used, ideally of the nondominant hand, as those fingers are generally less calloused and less sensitive to pain than either the index finger or thumb. The thumb should also be avoided due to its pulse (arterial presence). In the fifth finger the distance between the skin surface and the bone is too small. The puncture should be made slightly off center from the central, fleshy portion of the fingertip—near the side, where the skin is thinner, with fewer nerve endings and less pain sensation—but not on the very side of the finger.
- Cleaning and disinfection. After cleaning and disinfection, the puncture site must be dried completely. Any remnants of alcohol solution will dilute the blood sample and cause false low readings.
- The finger should be supported by the operator’s hand. It can be massaged gently before and after the puncture to stimulate blood circulation, but not going past the first knuckle. Maintaining a light pressure at the moment of puncture ensures effective penetration. However, the finger should not be pressed hard, as this will push fluid from the tissue into the blood and cause false low readings.
The Importance of Time and Blood Flow
As highlighted above, another key factor that can influence hemoglobin measurement is capillary flow. For hemoglobin, the first one to three drops after puncture typically show a higher degree of variability, 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 using a clean gauze pad. The highest accuracy is generally reached from the fourth drop after puncture, with good capillary flow occurring for a period of 30 to 45 seconds. After this time coagulation will occur, and if the blood is sampled then, blood clotting can lead to inaccurate hemoglobin results.
The ideal capillary blood sampling window accounts for several considerations (see Figure 1). It is most important to ensure 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. To achieve the most accurate result, the fourth drop of blood should generally be used to fill the cuvette for the hemoglobin measurement. The drop must be big enough to fill the cuvette completely, as incomplete filling or air bubbles can cause false results. The finger must not be squeezed hard or ‘milked’ to increase the size of the drop as this will dilute the sample with interstitial fluid, leading to false low results.
Operator Training and Practice
In addition to following a standardized documented procedure, it is essential to ensure that all personnel taking blood samples are properly trained. Effective and continuous operator training and practice must be in place to eliminate preanalytical errors and to obtain correct results for hemoglobin testing, especially from capillary sampling. Other aspects of operator training, particularly for safety reasons, should include an understanding of anatomy, the risks of blood exposure, and the consequences of poor infection prevention and control.7
How POC Test Devices Can Support Best Practices
Easy-to-use sampling and POC testing devices are also necessary to support hemoglobin measurement best practices and ensure the delivery of accurate results comparable to laboratory techniques.
POC analyzers possess the virtues of being small, portable, and fast, but the key to the effectiveness of any POC analyzer is simplicity. Ensuring that a POC analyzer is as straightforward as possible will minimize the opportunities for user error, and hence the need for retesting, with subsequent time and cost implications. An analyzer that is simple to use also ensures minimal training requirements, again contributing to affordability.
Other factors that can support best practices include an analyzer’s ability to minimize the number of steps in the preanalytical procedure. Such simplification not only reduces the opportunities for user error, but also helps to standardize any variation introduced by different operators, particularly when pipetting and mixing.
Some POC analyzers utilize novel sampling devices that eliminate the need for any premixing and pipetting, enabling blood samples to be drawn straight from the patient and inserted directly into the analyzer (see Figure 2). Such devices include microcuvettes that can be filled completely by simply touching the tip or corner to the capillary blood drop. Some cuvettes are specifically designed to ensure easy ‘bubble free’ sample uptake; however, a quick visual check of the clear cuvette should be made to ensure that the cavity is completely filled. Also beneficially, recent developments have led to the introduction of reagent-free cuvettes that can be stored, at ambient temperatures, unaffected by humidity, for up to 2½ years.
Finally and crucially, even if easy-to-use, POC devices must also be accurate and reliable, for which certification and quality control are essential. Most devices are factory-calibrated against reference methodologies and standards. This practice eliminates the need for recalibration and ensures a high correlation with standard reference and common laboratory methods. Integrated automatic self-checks between every measurement, and the availability of ready-to-use control materials, together eliminate the risk of incorrect result reporting.
In addition, given the growth in volume of POC testing in recent years, organizations such as the American Association for Clinical Chemistry have devised increasingly rigorous standards for POC testing. Such changes have placed demands on developers to embark on a process of continual improvement to ensure that they can deliver data that stand up to scrutiny. In turn, satisfying more rigorous standards removes barriers that hinder the integration of POC testing as an extension of a laboratory’s portfolio, and also encourages the use of POC tests for enhanced patient care—even when not used in conjunction with laboratory testing.
POC hemoglobin testing can deliver accurate results for hemoglobin in settings where use of a benchtop laboratory hematology analyzer is not practical. Such locations can include acute clinical situations where short turnaround time is needed, field settings where mobility and simplicity are essential, or resource-poor locations where dedicated laboratories are not available.
Healthcare professionals must be able to trust POC test results and must not need to request confirmatory laboratory tests, so POC hemoglobin results must be comparable to laboratory techniques. Therefore, traceability to established reference methods must be confirmed, and quality control schemes—including standardized, documented procedures and continuous operator training—must be implemented.
Hemoglobin measurement can be influenced by a number of physiological factors that need to be considered in order to apply correct reference ranges. In addition, potential preanalytical errors introduced during capillary blood sampling can be eliminated by operator training and good sampling practices. With all of this in place, hemoglobin testing can be reliably undertaken in a wide variety of POC settings.
Katja Lemburg is a product manager for hematology at EKF Diagnostics. For further information, contact CLP chief editor Steve Halasey via [email protected].
- 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.
- Haemoglobin concentrations for the diagnosis of anaemia and assessment of severity. Vitamin and Mineral Nutrition Information System. Geneva, Switzerland: World Health Organization, 2011 (WHO/NMH/NHD/MNM/11.1). Available at: www.who.int/vmnis/indicators/haemoglobin.pdf. Accessed October 11, 2017.
- Cable RG, Steele WR, Melmed RS, et al. The difference between fingerstick and venous hemoglobin and hematocrit varies by sex and iron stores. NHLBI retrovirus epidemiology donor study II (REDS II). Transfusion. 2012;52(5):1031–1040; doi: 10.1111/j.1537-2995.2011.03389.x.
- Dacie and Lewis Practical Haematology. 9th ed. Lewis SM, Bain BJ, Bates I, eds. London: Churchill Livingston, 2001.
- Wood EM, Kim DM, Miller JP. Accuracy of predonation Hct sampling affects donor safety, eligibility, and deferral rates. Transfusion. 2001;41(3):353–359.
- Marks PW, Glader B. Approach to anemia in the adult and child. In Hematology: Basic Principles and Practice. 5th ed. Hoffman F, Benz EJ, Shattil SJ, eds. Philadelphia, Pa: Churchill Livingstone, 2009.
- 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.