Skin puncture or capillary blood collection
involves puncturing the dermis layer of the
skin to access the capillary beds which run
through the subcutaneous layer of the skin.
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. Warming of the puncture site
further 'arterializes' the blood and increases
Capillary blood collection is the preferred
method of blood specimen collection for
newborns and infants. Clinical Laboratory
Standards Institute (CLSI) recommends
capillary blood collection via heelstick
for infants less than one year of age.1
For children older than one year, capillary
blood collection via fingerstick should be
considered, where appropriate.
Capillary blood collection may also
be used for adults under certain
Capillary blood collection is inappropriate for:
- Patients with fragile, superficial or difficult to access veins
- Patients where multiple unsuccessful venipunctures have already been
performed, especially if the test(s) requested requires only a small volume of blood
- Patients with burns or scarring in venous blood collection sites
- Extremely obese patients
- Patients requiring frequent blood tests
- Patients receiving IV therapy in both arms or hands
- Patients at risk for serious complications associated with venipuncture, venous
thrombosis, or deep venous puncture (e.g. deep vein puncture in infants, thrombophlebitis)
- Patients requiring only one blood test for which a capillary specimen is appropriate
- Patients whose veins are 'reserved' for intravenous therapy or chemotherapy
- Point-of-care testing where only a few drops of blood are needed
It is important to understand that there are
differences between some analytes in capillary blood
as compared to venous or arterial blood specimens.
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.
- Severely dehydrated patients
- Patients with poor circulation
- Coagulation studies requiring plasma specimens
- Tests that require large volumes of blood (i.e. Erythrocyte Sedimentation Rate (ESR)
and blood cultures)
The following tests are commonly performed
using capillary blood:
The recommended Order of Draw for capillary
blood collection is different from blood specimens
drawn by venipuncture. CLSI recommends the
following order of draw for skin puncture:1
- Point-of-Care testing (POCT, i.e. blood
- Complete Blood Count (CBC), hemoglobin
& hematocrit (H&H)
- Peripheral Blood Smear (manual slide for
white blood cell (WBC) differential)
- Neonatal Blood Gases
- Neonatal Bilirubin
- Neonatal Screening (filter paper or blood
After warming (arterializing) the site, it is
recommended to collect capillary blood gases first,
as the blood becomes increasingly more 'venous'
if the collection is delayed. Likewise, if collection of
blood for a CBC (K2EDTA tubes) is delayed, there is
an increased likelihood of erroneous cell counts due
to platelet clumping.
- Blood gases
- EDTA tubes
- Other additive tubes
- Serum tubes
There are two types of lancing
devices that are used for collection of capillary blood:
puncture devices and incision devices. Puncture
devices (e.g. BD Microtainer' Contact-Activated
Lancets) puncture the skin by inserting either a needle
or blade vertically into the tissue. Puncture devices
are preferable for sites that are repeatedly punctured
(i.e. blood glucose monitoring). Incision devices
(e.g. BD Microtainer' Quikheel' Lancets) slice
through the capillary beds. Incision devices are less
painful than puncture devices and require fewer
repeat incisions and shorter collection times, and
are therefore, recommended, especially for infant
heelsticks.2 Both types of devices come in a variety
of styles, sizes and depths.
According to CLSI, a skin puncture device should be a sterile,
disposable, single-use device with a permanently retractable
blade or needle to reduce the possibility of accidental
needlestick injuries and reuse.1 The use of manual lancets
or blades without a retractable feature is a violation of
The patient's age, accessibility of the puncture site, and the
blood volume required should all be taken into consideration
when selecting the skin puncture device type 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.
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 third (middle) or fourth (ring)
finger, ideally of the non-dominant hand. Fingers on the nondominant
hand are generally less calloused. The puncture
should be made slightly off center from the central,
fleshy portion of the fingertip and if
using a blade-type puncture 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.
The index finger is
often calloused and potentially more sensitive to pain due to
additional nerve endings. The thumb also may be calloused
and has a pulse, indicating arterial presence, and, therefore,
should be avoided. The distance between the skin surface
and the bone in the fifth finger also makes it unsuitable for
puncture. The side and tip of the finger should be avoided,
as the tissue is about half as thick as the central portion of
The recommended site for heel punctures
is the lateral (outside) or medial (inside)
plantar surface of the heel. In small or
premature infants, the heel bone (calcaneus)
may be no more than 2.0 mm beneath
the skin surface and no more than half
this distance at the posterior curvature
of the heel. Puncturing deeper than 2.0 mm on the plantar
surface of the heel of small infants may, therefore, risk bone
damage. When using incision devices, puncturing the heel at
a 90' angle to the length of the foot is recommended.5 Such
incisions create a 'gap' puncture (one which opens when
pressure is applied) and further enhance blood flow.
For infants, punctures must not be performed on:
- The posterior curvature of the heel.
- The central area of an infant's foot (area of the arch).
- Punctures to this area may result in injury to nerves, tendons, and cartilage.
- The fingers of a newborn or infant less than one year old.
The following steps should be performed in accordance with the facilities' recommended procedures.
- Review the test requisition(s).
- Gather the appropriate supplies (lancing device, gloves, gauze, alcohol, bandages, etc.).
- Positively identify the patient.
- Verify diet restrictions (fasting required, etc.) and any latex sensitivity (if products containing latex are being used).
- Wash hands and put on gloves.
- The patient should be sitting or lying down.
- Select appropriate puncture site.
- Warm the puncture site.
- 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.
- Notify older children and adults of the imminent puncture.
- Puncture the skin with the disposable lancing/incision
- Wipe away the first drop of blood with a dry gauze
pad (refer to each point-of-care device manufacturer's
- Collect the specimen in the appropriate container, and mix
according to the manufacturer's instructions.
- Seal the specimen container.
- Apply direct pressure to the wound site with a clean gauze
pad and slightly elevate the extremity.
- Label the specimen container in direct view of the patient
or guardian to verify identification, and record time of
collection. Label each container individually.
- 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
- After removing gloves, wash hands before proceeding to
the next patient.
- Bilirubin samples must be protected from light, both
during and after collection, as light breaks down bilirubin.
If collecting a capillary specimen from an infant in an
incubator, turn off the ultraviolet (UV) light source above
the infant during specimen collection.6 Collect capillary
blood specimen quickly to minimize exposure of the blood
specimen to light. Use amber collection containers or foil
to protect specimens from light. Ensure that the UV light
is turned back on before leaving the nursery.
For newborn screening (filter paper collection), gently
touch the filter paper against the blood drop in a single
step to allow a sufficient quantity of blood to soak
completely through the paper and fill the preprinted circle.
The paper must not be pressed against the puncture site
on the heel. If the circle does not fill entirely, wipe the
heel and touch a different circle to the blood drop until
the circle is completely filled. Blood must soak through
the paper within the circle to the other side, and must be
applied to one side of the paper only.
In summary, there truly is an 'art' to capillary blood
collection. These 10 steps provide guidelines to assist your
facility in selecting the appropriate lancing/incision device
and puncture site for a successful capillary blood collection'
the first time. A high-quality specimen minimizes errors and
possible re-draws, while enhancing customer satisfaction and
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 (hemolysis, 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.1
- Cleaning the Puncture Site. Allow the alcohol to air
dry. Performing skin puncture through residual alcohol
may cause hemolysis and can adversely affect test results.
It also may cause additional discomfort for the patient.
Do not use povidone-iodine 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, hemolysis and clotting.
- 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 hemolysis 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
- Collect Specimen Quickly. Puncturing the skin releases
thromboplastin, which activates the coagulation process.
Specimens must be collected quickly to minimize the
effects of platelet clumping and microclot formation
(hematology testing). Specimens also should be collected
quickly to avoid exposure to atmospheric air and light
(blood gases and bilirubin testing).
- Fill to the Correct Fill Volume. Fill containers to the
recommended fill volume (if indicated). Underfilled
containers will have higher concentrations of additives.
For K2EDTA, higher concentrations may cause erroneous
results for MCV and red cell indices and cause RBC and
WBC morphological artifacts. Consequently, overfilled
containers will have lower concentrations of EDTA and
may result in clotting.7
- Mix Specimen. Microcollection tubes must be inverted
the appropriate number of times to ensure that the
blood and anticoagulant are sufficiently mixed. Mixing
is essential to prevent the formation of microclots and
platelet clumps, which can cause inaccurate or erroneous
test results. Small clots can also occlude sample aspiration
probes or tubing in laboratory instruments, resulting
in instrument downtime and/or additional unscheduled
maintenance. While modern analyzers have sophisticated
detection systems to recognize platelet clumps, it is still
possible for platelet clumps to cause erroneous test results
(e.g. platelet count, platelet volume, red cell volume,
white cell count). Adequate mixing, both during and after
the completion of capillary blood collection, will help
minimize these occurrences.
- Properly Label Specimen. Each tube should be
individually labeled at the bedside prior to leaving the
area. Mislabeling of the specimen can lead to incorrect
diagnosis, therapy and treatment. The consequences can
be serious, even fatal, for the patient.