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Most POCT samples are
capillary blood in origin. However,
regardless of the type of biological specimen collected- whole blood,
urine, stool, wound swab, blood cultures or drainage fluid, to name but
a few- the lab results will be only as good as the quality of the
collected sample. The collecting of samples and all that goes with it,
is is known as the
pre-analytical
component of Quality Assurance.
Good quality capillary blood
collecting is an art and a skill. As usual with a skill, practice makes
perfect- it's not as easy as it looks! The importance of good technique
is fundamental- the lack of a good quality capillary blood sample is the
weakest link in the point of care testing chain.
The commonest sources of error are:
- shallow skin puncture leading to a slow blood flow
- failure to wipe away the first drop of blood *
- squeezing too hard
- scooping blood along the skin as it dribbles from the puncture site
- "milking" the heel or digit.
- puncturing in an awkward or incorrect spot
* Except for haemostasis testing,
e.g., Roche CoaguChek S or XS analysers where the first drop of blood must be included in
the collection (the first drop is rich in tissue fluid clot activators).
A good capillary blood collect will give very similar results to a
plasma or serum sample for most analyses. However whole blood analysis
does have its limitations.
Whole blood
glucose analysers, because of their ubiquity, deserve special mention.
Poor
peripheral flow can give poor results. A warmed towel may be wound around the limb for a few
minutes before collection is started. Ensure skin in
dry before proceeding with the capillary blood collect.
Here are the three Golden Rules for
successful capillary blood sample collecting:
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Make
sure the blood is flowing freely from a clean, warm, dry puncture site.
-
Always wipe away the
first drop of blood (except for haemostasis testing).
-
Squeeze slowly
and rhythmically, gripping the digit firmly between the base of your thumb
and first finger. This will allow a good solid squeeze without hurting
the patient or rupturing red cells. Allow enough time between squeezes for blood to perfuse back into the
digit or heel.
| Frequently
Asked Questions about Capillary Blood Collecting |
| Is this the correct patient? |
You must have at least 2 clear points of
ID, preferably the National Patient Number, e.g., ABC1234 and the
patient's name. Date of birth and temporary hospital ID are acceptable
also. |
| Is this the correct sample tube? |
- Biochemistry- Green Microtainer (heparin)
- Haematology- Purple Microtainer (EDTA)
- Serology, Immunology,
drugs, etc.- Pink or salmon Microtainer- plain (no additives).
- Blood gases- heparinised blood gas capillary tube.
|
| Blood gas collects |
-
Blood gas capillary
collection tubes contain heparin.
-
Mix your sample well as
you go.
-
Expose your sample to ambient air as little as possible.
Remember:
|
| Is the patient's skin area
clean and dry? |
The patient's site of puncture must be clean and dry.
Wash your patient's hand or foot before collecting your sample. DRY
thoroughly. It is important no residual moisture be present, to
contaminate or dilute the sample. |
| Am I puncturing the skin in the right
place? |
For fingers or thumbs, always puncture on
the sides of the digit parallel to the side edges of the nail. Try not to
use the tip or pad of the finger- there are more nerves there so it will hurt
more.
Similarly for a heel- always puncture along the sides of the heel,
never on the sole of the heel.
The ear lobe may also be used.
Make sure the ear lobe is warm and pink before puncturing the lower fleshy
rim of the lobe. |
| Are my hands clean
and dry? Should I be wearing gloves? (Yes- Hospital Policy says you
should). |
Always wash and dry your hands between patients,
even if you are wearing gloves. Follow the hospital protocols concerning
hand washing. This will help to prevent the spread of nosocomial infections and
will hinder any contamination of the blood sample |
| If this is a capillary collect, is the patient's perfusion good? |
Any capillary sample should be taken from
a puncture site on the patient, where the blood is free-flowing. The
patient's extremities should be warm and pink. |
| Should I warm the hand or foot to encourage a good peripheral flow? |
If the patient's perfusion is poor, you
may be able to wrap the limb in a warm blanket for a few minutes to
encourage the flow of blood to the area. You may be able to wet a
cloth with warm water and wrap that about the patient's limb.
CAREFUL:
-
Don't burn the patient!
Young skin is sensitive.
-
Make sure all skin is clean
and dry before you start collecting your sample. No residual water!
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| Am I squeezing too hard?
NOTE: this point and the next point are the commonest sources of
error.
|
It is important not to squeeze too hard when
collecting a capillary blood sample. Squeeze quite firmly, but never with
enough force that the patient complains! Excessive squeezing will cause
haemolysis of the sample, i.e., the rupture of red blood cells, thus releasing their content
(Hb and electrolytes) into the plasma. Haemolysis will alter potassium, ionised calcium (ICa), phosphate and enzymes. If gross haemolysis occurs, all results
may be affected, due
to a dilution of the sample by dissolved haemoglobin.
Tissue fluid (resulting in increased potassium levels) may also
contaminate the blood sample if you squeeze
too hard.
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| Is the patient's blood flow adequate? |
A capillary blood collect should flow
freely from the puncture site. Gently squeezing to encourage the blood
flow is acceptable. DO NOT "milk" the finger or heel by
pressure sliding your thumb along the skin.
This will cause excessive hydraulic pressure in the capillaries, resulting
in haemolysis and altered ICa. Correct technique is to compress the finger or heel for a second, then release for a few seconds. Repeat.
Don't be in too much of a hurry. Take your time. It is far
better to apply a second puncture to the same area, if the flow is too
slow.
|
| Have I wiped the first drop of blood away? |
For most capillary collects, the first drop
of blood must be wiped away and discarded. This is because the first drop
is often contaminated with tissue fluid (high potassium levels).
NOTE: one instance where you would not wipe the first drop away is if you were
using a coagulation meter, e.g., a Roche CoaguChek meter for PR/INR.
Always follow the manufacturer's instructions in this regard. |
| How is my technique? |
Never scoop up blood which has smeared or
dribbled away from the puncture site. There are two reasons for this:
- It may have started to clot
- Scooping partially coagulated blood may cause haemolysis.
Wipe the smear away with cotton wool and squeeze a fresh bead of blood
from the puncture site. Try to keep the bead rounded with surface tension
by aspirating each bead as it forms.
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| Have I got enough sample to satisfy the analyser requirements? |
A blood gas capillary sample must be
completely full with no air bubbles or clots. It is important to ensure
that a sufficient volume of blood is collected to satisfy your analyser
requirement.
Make sure the Glucocard
Test Strip has
aspirated sufficient sample and goes beep. It is possible to short sample
the Glucocard Test Strip, which may give you incorrect results.
|
| Am I taking too long to collect the sample? Is it going to clot on
me? |
Always try to be as quick and efficient as
possible when you are collecting capillary blood. If the flow is
insufficient, use your lancet a second time. It makes sense to do this-
you don't want to have to repeat the collect later because the answer was
unacceptable. It's much better to get it right the first time around. |
| Have I mixed the sample properly in the collection tube? |
Please make sure that your sample is
adequately mixed. This is important for all BD Microtainer collection
tubes and in particular, capillary blood gas samples. Always mix as you
go- after every few drops of blood collected, hold the collection tube
vertically and flick the bottom of the tube with your finger to cause the
blood to swirl in the tube. Don't wait
until you have enough blood before you mix.
Of course,
with blood gas capillary samples, try to keep air exposure to an absolute
minimum, to ensure your gas values don't change. (Rotate the tube back and
forth between the tips of your fingers, instead of moving the blood back
and forth along the tube).
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| If it is a capillary blood gas, is the sample bubble-free? |
No blood gas sample, whether capillary,
arterial or mixed venous, should contain air bubbles. Both the pO2 and
pCO2 will be
affected, but especially the pO2. With capillary samples, the blood
volume
is minimal at the best of times. Bubbles can cause sampling errors to
occur in the analyser. |
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One last point: is the patient
on any lipid supplements?
*****
(Not really capillary
blood-specific, but worthy of a mention).
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If the patient is being infused with
IntraLipid or other fat supplement, the lipids circulating in the blood
stream may cause havoc with blood tests- not just capillary samples but
venous and arterial samples as well. Always note on the request form,
whether the patient has infused lipids. |
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