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.