Capillary blood sampling is an essential method of blood collection performed by medical professionals of all skill levels and disciplines with diverse titles such as phlebotomist, patient care technician, medical assistant, nurse, lab assistant, lab tech, med tech and many others. Accuracy of results greatly depends on education and standardization of the sample collection technique.
Capillary blood collection is just as much a part of patient care as a tonsil or gallbladder surgery or any other invasive or non-invasive procedure. The specimen is a part of the patient and should be treated as such. Collecting capillary blood specimens requires patience, education, and a good technique. When facilities provide continuing education, standard updates, and quality equipment, lab employees can work toward collecting high-quality specimens. Any new or modified policies or processes within a laboratory’s own workflow may require education and training for the staff.
Proper capillary blood collection and handling procedures are critical to accurately reflect a patient’s physiology. In September 2020, the Clinical and Laboratory Standards Institute (CLSI) published the updated capillary standards (GP42-Collection of Capillary Blood Specimens).1 This was the first revision in 12 years.2 The standards provide guidance for proper capillary blood collection procedures and processes to ensure the safety of the patient as well as the healthcare professional responsible for collecting blood specimens. Maintaining a standard collection procedure is important because it will help reduce pre-examination errors.
This article describes some of the best practices for capillary blood collection and handling included in the updated standards.
Reasons for capillary blood collection
Capillary punctures are better suited than venipuncture in certain situations. For example, performing a venipuncture on infants can be difficult and potentially hazardous because these patients have smaller veins and tend to move more than older patients during the procedure. With a capillary puncture, a small but adequate amount of blood for laboratory examinations can be obtained. Although a capillary puncture can limit iatrogenic anemia, it does not eliminate the need to monitor blood volume removed from pediatric patients. Age and weight should be considered when the appropriate site for capillary puncture is selected in pediatric patients. Capillary blood specimens should be collected into appropriate capillary collection devices and not collected and or transferred into venipuncture tubes.
Adult patients may require a capillary puncture collection due to fragile, superficial, or difficult to access veins, if they have undergone multiple unsuccessful venipunctures, or if the requested test requires a small volume of blood. Other patients that may require capillary blood collection include burn patients or patients with dermatoporosis, which refers to issues common to aging skin. Patients with veins that are being preserved for IV therapy or if they are receiving IV therapy in both arms or hands also could benefit from capillary puncture collection. In these situations, the sites that should be used are the palmar surface of the distal segment of the middle or ring finger.
Site selection
There are sites that must not be used, such as infected sites, because of the potential for altered examination results, aggravation of infection as well as patient discomfort. There also are sites that require a physician’s permission, such as limbs on the side of a mastectomy, due to the risk of lymphedema and potential for altered examination results. Sites that should be avoided include areas with extensive scarring, healed burns, inflamed sites, edematous sites, previous puncture sites, earlobes and thumbs.
Such terms as “needs to,” “must,” “require” and “should” are used to explain how medical professionals should perform capillary blood collection procedures. Some of these actions are not a choice. For example, punctures must not be performed on the posterior curvature of the heel or toes other than the great toe or the area of the arch.
A heelstick capillary blood collection also requires the collector to consider the clinical condition of the patient as well as age and weight when choosing this site and when choosing a lancet that offers the depth of the puncture. Punctures 2.0 mm deep or less will provide adequate blood flow without risking bone injury.
Following a guideline with proper locations offers the least risk of puncturing the heel bone. If such areas as the lateral and medial surfaces of the heel have been repeatedly punctured or if bruising is extensive in these areas, a venipuncture may need to be considered.
Capillary blood collection from fingers is acceptable for adults and older children. However, fingers of newborns and infants less than 6 months of age must not be used for capillary blood collection because the distance from the capillary surface to the bone in the thickest portion of the last segment of each finger in newborns varies from only 1.2 to 2.2 mm. In newborns, local infection and gangrene are also potential complications of finger punctures. For pediatric patients between 6 and 12 months of age, the decision to use the finger instead of the heel must be based on weight. In infants weighing more than 10kg (~22 pounds), the finger can be used if the lancet depth does not exceed 1.5 mm.
Lancet Selection
Care must be taken not to shorten the distance between the skin and bone by compressing the tissue before the spring-loaded lancet activation. This may be difficult to do depending on the lancet your facility has chosen to use.
The manufacturer’s instructions must be followed for orientation of the lancet. The puncture should be made across the prints, which allows large drops of blood to form. If the incision is made going with the print instead of across, the blood will run down the grooves of the print and this becomes messy and wastes blood that could be collected if done properly.
Site preparation
Warming the site increases arterial blood flow to the site up to sevenfold and will not burn the skin if warmed at a temperature no higher than 42 degrees Celsius. Although studies show that pre-warming might not be necessary when an incision device is being used, increasing capillary blood flow through pre-warming can minimize the necessity to exert additional pressure to the site.
Cleansing the site is performed to minimize microbiological contamination of the specimen and patient infection. Allowing the site to dry, without wiping it dry, enables optimal decontamination while reducing the potential to interfere with the specimen and prevents the patient from experiencing a burning sensation when the puncture or incision is performed.
Policies and procedures
Some medical professionals have been performing capillary collection procedures for their entire career. Even so, there are a few things that demand attention when updating a facility’s own policies and procedures. For example, information on positioning the patient received only one line on one page in the previous document, but the new standard awards this topic quite a bit of well-deserved real estate.
In the past, some of our blood collection procedures resulted in clots within the anticoagulant tubes. That is why an important addition to any laboratory’s guidelines or standard operating procedures (SOP) is a suggestion to carefully mix the specimen periodically during collection to avoid clotting.
Intermittent gentle pressure may be necessary to obtain an adequate specimen. Pressure should be released between drops to enable the capillary beds to refill and then be reapplied and repeated until the required specimen volume is reached.
Order of the draw
There are important reasons to follow the order of draw during capillary blood collection. For example, beginning with the EDTA capillary blood tube ensures that the blood will not begin to clot before the specimen is collected. Clots in this tube will certainly affect the accuracy of the blood count. For both glass and plastic microcollection tubes during a single capillary puncture, the order of the blood draw is as follows:
- Capillary blood gas (CBG)
- EDTA tube
- Other additive tubes
- Nonadditive tubes
- Filter paper for DBS collections
Collection tubes
Having a greater understanding on how to properly collect blood into a capillary tube – also referred to as a straw – is important for collecting a CBG or when using a microcollection tube that is devised with a straw for the collection. Holding the capillary tube at a slight angle upwards to prevent any air bubbles from entering the tube is an accurate and important detail to include in a standardized technique.
Post collection care
It is important to apply pressure to the site after collection is complete by slightly elevating the extremity until bleeding has stopped. Continue to observe the site and the patient to be certain no adverse effects need to be reported. Label the specimen immediately after collection and in the presence of the patient and by the same person who collected the specimen. Of course, gloves and any other required personal protective equipment must be worn during collection, labeling, and preparation for transport.
In conclusion, establishing a step-by-step, updated, standard procedure within a facility is essential to help eliminate collection errors as well as improve the quality of care for the patient.
References
- Collection of Capillary Blood Specimens GP42 7th ed. Wayne, PA: Clinical and Laboratory Standards Institute, 2020.
- Procedures and Devices for the Collection of Diagnostic Capillary Blood Specimens; Approved Standard-Sixth Edition. H04-A6. Wayne, PA: Clinical and Laboratory Standards Institute, 2008.
