Capillary Blood Collection - POCD Learning [PDF]

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Course Description. A capillary sample is a blood specimen collected by pricking the skin and accessing the capillaries, tiny blood vessels near the surface of the skin. Capillary sampling from a finger, heel or (rarely) an ear lobe may be performed on patients of any age, for specific tests that require small quantities of blood.
Capillary Blood Collection Course Description A capillary sample is a blood specimen collected by pricking the skin and accessing the capillaries, tiny blood vessels near the surface of the skin. Capillary sampling from a finger, heel or (rarely) an ear lobe may be performed on patients of any age, for specific tests that require small quantities of blood. Acquiring a good quality capillary blood sample is essential to ensuring results are precise and accurate when conducting point of care tests. This training module will educate you about the correct way to collect blood, the common pitfalls to avoid and ensure that you get the most from your point of care testing equipment. Work your way through the lessons and complete the quiz at the end. When you achieve a score above 80 % you’ll earn a certificate of competency. Please note you must be registered on the site in order to enrol on this course.

What is Capillary Blood? Collecting capillary blood involves accessing the capillary beds which run through the subcutaneous layer of the skin by puncturing the dermis layer. Blood obtained via skin puncture is a mixture of undetermined proportions of blood from arterioles, venules, capillaries, plus interstitial and intracellular fluids. The proportion of arterial blood is greater than that of venous blood, due to the increased pressure in the arterioles leading into the capillaries versus the pressure in the venules exiting the capillaries.

How blood travels through blood vessels Heart➞ Artery ➞ Arteriole ➞ Capillary ➞ Venule ➞ Vein ➞ Heart Because of the way blood travels it is important that consistent technique based on best practice is employed, to ensure the quality of examination results which typically require small quantities of blood (e.g. point of care testing).

Limitations There are instances when capillary blood collection may be inappropriate, for example: Patients who are severely dehydrated. Individuals with poor circulation Tests that require large volumes of blood (e.g. > 500 µL) It is important to understand that there are differences between some analytes in capillary blood as compared to venous or arterial blood specimens. For example glucose, potassium, total protein, and calcium have been reported to show statistically and/or clinically important differences. With the exception of glucose, the concentration of these analytes is lower in capillary blood. Similarly, when undertaking an INR test where prothrombin (clotting) time is measured there is increased imprecision due to pre-analytical factors such as the length of time it takes to collect the sample and initiate a test. Limitations such as this are not a factor when a laboratory tests venous samples.

Device Selection The Clinical Laboratory Standards Institute (CLSI) recommend that a skin puncture device is a sterile, disposable, single-use device with a permanently retractable blade or needle to reduce the possibility of accidental needlestick injuries and reuse. There are two types of lancing devices that are used for collection of capillary blood: puncture devices and incision devices. Puncture devices such as the Unistik 3 Extra puncture the skin by inserting a needle vertically into the tissue and are preferable for sites that are repeatedly punctured (i.e. blood glucose monitoring). Incision devices such as the Unistik 3 Neonatal slice through the capillary beds, are less painful than puncture devices and require fewer repeat incision/shorter collection times. These are the main reason they are recommended for infant heel sticks or larger volumes that may be required by laboratories. A lancet slightly below the estimated depth required should be selected because the pressure applied when performing the fingerstick compresses the skin, causing a puncture depth slightly deeper than the lancet length. Naturally thicker lancets with higher penetration depths are more painful, however the blood volume obtained will also increase which can in many cases be more beneficial than having to perform repeated fingersticks. The following table provides an example of the variety of styles, sizes and depths of lancet that are commercially available.

Device

Width × Average Depth Blood Device Type (mm) Volume Intended Use

Unistik 3 Comfort

Puncture (needle)

28 G × 5 µL – 1.8  30 µL mm

Fingerstick, low flow (single drop)Less painful

Unistik 3 Normal

Puncture (needle)

23 G × 30 µL – 1.8  75 µL mm

Fingerstick, medium flow

Unistik 3 Extra

Puncture (needle)

21 G × 75 µL – 2.0  125 µL mm

Fingerstick high flow (> 400 µl;L from single puncture)

Unistik 3 Incision 18 G x > 200 µL Heelstick high flow Neonatal & (asymmetrical 1.8 Newborn babies or Laboratory blade) mm laboratories where a larger volume of blood volume is required

Site Selection The patient’s age, accessibility of the puncture site, and the blood volume required should all be taken into consideration when selecting the type of lancet and puncture site. Select a site that is warm, pink and free of any calluses, burns, cuts, scars, bruises, or rashes. The site should not be cyanotic (bluish from lack of oxygen), edematous (swollen), or infected. Avoid skin areas that have evidence of previous punctures or are otherwise compromised. Callouses are more likely on the dominant hand and the on thumbs and index fingers. Furthermore the thumb has a pulse, indicating arterial presence and the index finger is potentially more sensitive to pain due to additional nerve endings. For these reasons the best sites are located on the third (middle) or fourth (ring) finger of the non-dominant hand. Selecting these fingers will also help to reduce discomfort caused by bruising as they are employed less frequently in day-to-day activities.

Recommended puncture sites for a fingerstick. Once a suitable finger has been selected the recommended site for capillary collection on adults and children over one year of age is the palmar surface of the distal (end) segment of the finger. The puncture should be made on the fingertip, slightly off centre from the central, fleshy area and if using a bladetype incision device, perpendicular to the fingerprint whorls. Puncturing along or parallel to the whorls may cause the blood to follow the pattern of the fingerprint, redirecting the flow and making it more difficult to collect. When considering ease of collection the pad of the fingers should not be lanced as it makes holding the collection tube at a perpendicular (right) angle to the puncture site and on a horizontal incline difficult without holding it in a vertical

orientation, which can impede filling of the vessel. The side and central tip of the finger should also be avoided, as the tissue is about half as thick as the central portion of the fingertip.

Order of Draw It is recommended to collect blood gas specimens first, followed by haematology and then chemistry specimens as the blood becomes increasingly more ‘venous’ if the collection is delayed following warming (arterialising) of the puncture site. Likewise, if collection of blood for a blood count (FBC) is delayed, there is an increased likelihood of erroneous cell counts due to platelet clumping so collecting these samples sooner minimises these effects. An easy way to remember the order of draw for capillary blood collection is that it’s the reverse of that used for venipuncture collection. The order of draw may therefore be summarised as follows: 1. Blood gas 2. Haematology 3. Chemistry

Performing a Fingerstick The following steps are based on best practice principles and should be performed in accordance with the facility’s recommended procedures. 1. Review the test requirement(s). 2. Gather the appropriate supplies (lancing device, gloves, gauze, alcohol, bandages, etc…). 3. Positively identify the patient. 4. Verify diet restrictions (fasting required, etc.) and any latex sensitivity (if products containing latex are being used). 5. Wash hands and put on gloves. 6. Ensure the patient is sitting or lying down. 7. Select appropriate puncture site. 8. Warm the puncture site (e.g. ask the patient to rub their hands together briskly, or rinse under warm water). 9. Clean the puncture site with 70% isopropyl alcohol and allow to air dry. The site must be allowed to air dry in order to provide effective disinfection. 10. Notify older children and adults of the imminent puncture. 11. Puncture the skin with the disposable lancing/incision device. 12. Collect the specimen in the appropriate receptacle, according to the manufacturer’s instructions and the tips provided below. For most tests you should wipe away the first drop of blood with a dry gauze pad because it is rich in interstitial fluid, platelets and thromboplastins that cause accelerated clotting (refer to each point of care device manufacturer’s instructions). Pistol grip the finger – position your thumb along the length of the finger as shown in the photograph. This will allow you to apply a squeeze and release technique that will ensure a good flow of blood, without causing haemolysis or excessive milking which can introduce tissue fluid into the sample. Hold the collection tube horizontally to the blood droplet, but do not allow it to touch the finger. Performing the ‘squeeze’ with your non-dominant hand will allow you to hold the collection tube to the puncture site with greater dexterity (using your dominant hand). For capillary collection devices never attempt to draw blood into the collection tube manually by squeezing a pipette bulb. When held

13. 14. 15. 16.

correctly (horizontally) it will fill naturally by capillary action. Apply direct pressure to the wound site with a clean gauze pad and slightly elevate the extremity. Properly dispose of the lancet/incision device in a puncture-resistant disposal container. Properly dispose of any other contaminated materials (gloves, gauze, etc…) in a container approved for their disposal. After removing gloves, wash hands.

Illustrated example of the recommended steps to follow when to collect a capillary blood sample.1

1. WHO guidelines on drawing blood: best practices in phlebotomy (http://www.who.int/injection_safety/phleb_final_screen_ready.pdf)

Tips to Obtain a High Quality Specimen The following tips will aid in obtaining a good quality blood sample.1 Positively Identify the Patient.  Positive identification of the patient is the most important step in specimen collection. Patient misidentification can lead to incorrect diagnosis, therapy and treatment. The consequences can be serious, even fatal to the patient. Puncture Site and Lancing/Incision Device Selection.  Determine the appropriate puncture site and lancing/incision device for the patient and the tests requested. Using the wrong size lancet/incision device may result in excessive squeezing, prolonged or incomplete collection, poor specimen quality (heamolysis, clotting) and possible redraws, as well as injury to the patient (mainly children). Warming the Puncture Site.  Only a limited amount of blood will easily flow from a capillary puncture. Warming the puncture site will increase blood flow up to seven times and is critical for the collection of blood gases and pH specimens. CLSI guidelines recommend warming the skin puncture site for three – five minutes with a moist towel or commercially available warming device at a temperature no greater than 42°C. Cleaning the Puncture Site.  Allow the alcohol to air dry. Performing skin puncture through residual alcohol may cause heamolysis and can adversely affect test results. It also may cause additional discomfort for the patient. Do not use povidoneiodine to cleanse the puncture site. Povidone-iodine interferes with bilirubin, uric acid, phosphorus and potassium. Wipe Away the First Drop of Blood.  Immediately following skin puncture, platelets aggregate at the puncture site to form a platelet plug, initiating the clotting process. Without wiping away the platelet plug, bleeding may stop prior to completion of the blood collection, resulting in insufficient blood volume and redraws. In addition, the first drop of blood contains tissue fluid, which can cause specimen dilution, heamolysis and clotting.NOTE:  For point-of-care testing (e.g. INR testing), use of the first drop of blood may be appropriate. Refer to the manufacturer”s instructions for use. Avoid Milking, Scooping or Scraping of the Puncture Site.  It is recommended to touch the collector end of the container to the drop

of blood. After collecting 2 or 3 drops, the blood will freely flow down the container wall to the bottom of the tube. Excessive squeezing (milking), scooping and scraping may cause heamolysis and/or tissue fluid contamination of the specimen. Using a “scooping” or scraping motion along the surface of the skin can also result in platelet activation, promoting platelet clumping and clotting. Collect Specimen Quickly.  Puncturing the skin releases thromboplastin, which activates the coagulation process. Specimens must be collected quickly to minimise the effects of platelet clumping and microclot formation (heamatology testing). Specimens also should be collected quickly to avoid exposure to atmospheric air and light (blood gases and bilirubin testing).

An example of the pistol-grip technique used to collect capillary blood. 1. Nancy Niwinski, Capillary Blood Collection: Best Practices, Lab Notes 20 (1), 2009.