Blood sampling or blood collection method is an essential procedure in modern medicine. The blood can be taken from a vein where it is called venous blood collection or arteries where it is called arterial blood collection. Minute quantities of blood can be taken from various sites by pricking the skin.
Purpose of Blood Collection
Blood collection is carried out for the following reasons:
- To obtain blood for analysis and diagnosis of various disorders:
- analyzing and quantifying various biochemical parameters in the body like blood sugar, cholesterol, uric acid, blood gases, etc.
- measurement and morphological assessment of blood components (complete blood counts including red blood cells, white blood cells, and platelets).
- culture and antibiotic sensitivity of blood in case of septicemia.
- Therapeutic purpose: removal of blood for treatment of high levels of iron (as in hemochromatosis, thalassemia, etc) or red blood cells ( as in polycythemia)
- To collect blood for later uses, like blood donation.
Sites of Blood Sampling
Blood is most commonly obtained from the superficial veins of the upper limb. In case the veins of the upper limb are not available [trauma to both upper limbs or if previous sampling has rendered the veins fibrotic], other sites may be used for drawing blood.
The most common site is the median cubital vein, which lies within the cubital fossa in front of the elbow. It is the most preferred site because it is superficial and present in almost all people. The basilic vein on the dorsum of the arm or dorsal hand veins can also be used. Veins of the foot are the least preferred ones because they have a higher chance of complications.
Phlebotomy is the name of the procedure for drawing blood from the vein through an incision or puncture.
Puncture is via a needle attached to a syringe or vacutainer.
Incision is a surgical procedure that involves a cut section of the vein.
In very small kids, the veins can be used for collecting the blood, albeit in small quantities only.
Read more about Vacutainer and Their Use in Blood Sampling
Arterial Blood Sample
The artery is a less common site for blood collection. The most common reason for taking blood from the artery is arterial blood gas (ABG) analysis.
The most common puncture site is the radial artery at the wrist and the femoral artery in the groin. Blood can also be drawn from an arterial catheter.
In contrast to venous blood sampling, the withdrawal of a sample from an artery requires training.
Capillary blood sample is taken where only minute quantities of the blood are required as in blood glucose analysis by glucometer method.
The most common site is the finger. It is also done at the heel in neonates.
The best locations for fingersticks are the center of the finger pads of the 3rd (middle) and 4th (ring) fingers of the non-dominant hand. The tip of the finger or the center of the finger should not be used. The sides of the finger where there is less soft tissue should also be avoided. The 2nd (index) finger tends to have thicker, callused skin. The fifth finger tends to have less soft tissue overlying the bone. Fingers that are cold or cyanotic, swollen, scarred, or covered with a rash should also be avoided.
The recommended location for blood collection on a newborn baby or infant is the heel. Heel stick is a viable method of obtaining blood for many routine blood tests. Heel stick sampling can also help preserve venous access for future intravenous (IV) lines.
Venous Sample Collection Method
- Position the patient in a chair, or sitting or lying on a bed.
- An elastic band/ tourniquet is tied around the arm/ site. The band/tourniquet retains blood within the arm and makes the veins more visible
- Select the vein from where the sample is to be taken. Medial cubital vein is the most common vein but the sample can be taken from other sites also
- Cleanse the skin over the area with an antiseptic, in a circular motion, beginning at the site and working outward.
- Ask the patient to make a fist.
- Grasp the patient’s arm firmly using your thumb to draw the skin taut and anchor the vein.
- The needle is inserted into the vein keeping the beveled side of the needle up. As soon as blood enters the hub of the needle, the blood is sucked into the vacutainer. In the case of a syringe, the plunger is pulled back to allow filling of the blood.
- The appropriate amount of blood is drawn and when it is about to complete, remove the tourniquet.
- The needle is withdrawn from the vein.
Vein visualization device is available these days which displays veins beneath the skin surface. This greatly helps healthcare professionals in finding a good vein for venipuncture and eliminates the need for multiple pricks. It is especially useful in obese persons, dark-skinned individuals, small children, burn victims, or drug abusers where vein visualization remains a challenge.
After blood is drawn and the needle is removed, the pressure is placed on the puncture site with a cotton ball to stop bleeding, and a bandage is applied. The patient or an assistant is instructed to apply firm pressure on the site for sufficient time to stop the bleeding. After 2–3 minutes, the site should be examined to check whether bleeding has stopped. Patients having high blood pressure or bleeding disorder or those taking anticoagulants may require more time to stop the bleeding.
Patients who experience swelling of the puncture site or continued bleeding after phlebotomy should seek immediate medical treatment.
In cases when a large amount of blood is withdrawn (as in blood donation), patients are instructed to drink plenty of fluids and eat regularly over the next 24 hours to replace lost blood volume.
The best locations for fingersticks are the 3rd (middle) and 4th (ring) fingers of the non-dominant hand. Avoid sampling from the tip and sides of the finger. Also, avoid puncturing a finger that is cold or cyanotic, swollen, scarred, or covered with a rash.
- Massage the finger toward the selected site prior to the puncture.
- Clean the skin with antiseptic.
- Using a sterile safety lancet, make a skin puncture just off the center of the finger pad. The lancet should enter the finger perpendicularly other wise the blood may run along finger pad ridges.
- Wipe away the first drop of blood, which tends to contain excess tissue fluid.
- Collect drops of blood into the collection tube/device by gentle pressure on the finger. Avoid excessive pressure lest you may squeeze tissue fluid into the drop of blood.
- Place a small gauze pad over the puncture site for a few minutes to stop the bleeding.
Heel Stick or Lance Procedure
The recommended location for blood collection on a newborn baby or infant is the heel.
- Prewarming the infant’s heel increases the flow of blood for collection.
- Hold the baby’s foot firmly to avoid sudden movement. An assistant may be required to do that
- Clean the skin
- Using a sterile blood safety lancet, puncture the side of the heel in the appropriate regions shown above..
- Wipe away the first drop of blood.
- Newborns do not often bleed immediately, gentle pressure is required to produce the drop of the blood. Do not use excessive pressure otherwise, the blood may become diluted with tissue fluid.
- Fill the required tube or capillary as needed
- After it is done, elevate the heel, place a piece of clean, dry cotton on the puncture site, and hold it in place until the bleeding has stopped.
Arterial Sample Collection Method
For arterial samples, the syringe and needle is held like a dart. The index finger is used to locate the pulse. Then the needle is inserted at a 45-degree angle, approximately 1 cm distal to (i.e. away from) the index finger, to avoid contaminating the area where the needle enters the skin. The needle is advanced into the artery until a blood flashback appears, then the syringe is allowed to fill to the appropriate level. The syringe plunger should not be pulled back. Finally, the needle is withdrawn.
A cotton swab is pressed to the puncture site and kept for about a few minutes. Note for any hematoma formation.
How Much Blood is Withdrawn
The amount of blood drawn depends on the purpose of the phlebotomy. The volume of blood needed for laboratory analysis varies widely with the type of test being conducted. Typically one or several small (5–10 mL) tubes are drawn.
Order of Draw
The term order of draw is the sequence in which tubes should be filled. The order of draw is important in vacutainer as the needle which pierces the tubes can carry additives from one tube into the next, and cause cross-contamination.
The order of draw is as follows, considering that everything was used in sample.
- First – blood culture bottle or tube (yellow or yellow-black top)
- Second – coagulation tube (light blue top).
- Third – non-additive tube (red top)
- Last draw – additive tubes in this order:
- SST (red-gray or gold top). Contains a gel separator and clot activator.
- Sodium heparin (dark green top)
- PST (light green top). Contains lithium heparin anticoagulant and a gel separator.
- EDTA (lavender top)
- Oxalate/fluoride (light gray top) or other additives
Risks of Blood Sampling
Most patients will have a small bruise or mild soreness at the puncture site for a few days.
Areas to Avoid When Choosing a Site for Blood Draw
Certain areas are to be avoided when choosing a site for blood draw:
- Extensive scars from burns and surgery – Difficult sampling
- The upper extremity on the side of a previous mastectomy – test results may be affected because of lymphedema.
- Hematoma site may cause erroneous test results.
- Intravenous therapy/ blood transfusions sites – The sample may get diluted or mixed with transfuse.
- Edematous extremities – tissue fluid accumulation alters test results.
How to Prevent Hemolysis While Taking a Blood Sample
- Mix the blood gently. Do not shake vigorously
- If using a needle and syringe, avoid drawing the plunger back too forcefully.
- Avoid a probing, traumatic venipuncture.
- Avoid prolonged tourniquet application.
- Avoid massaging, squeezing, or probing a site.
- Avoid excessive fist clenching.
- If blood flow into tube slows, adjust the needle position to remain in the center of the lumen.