Venipuncture in Medical Laboratory – Arteries or veins

According to Wikipedia, venepuncture or venipuncture is the puncture of a vein as part of a medical procedure, typically to withdraw a blood sample or for an intravenous injection.

In order to carry out this process, it is essential to follow a Standard Operating Procedure (SOP) for the collection of blood specimen to get the accurate laboratory results for any test or diagnostic to be performs. Any kind of error in blood collection or filing the test tubes may lead erroneous laboratory results.

Quality Assurance and Control in Venipuncture

This means that, even Quality assurance has it part to play during blood collection by ensuring the following

• Right patient
• Right time to collect the sample
• Right sample
• Right container with the right additive
• Right test
• Right Result
• Right physician

Reasons for Carrying out Venipuncture

• to obtain blood for diagnostic purposes;
• to monitor levels of blood components
• to administer therapeutic treatments including medications, nutrition, or chemotherapy;
• to remove blood due to excess levels of iron or erythrocytes (red blood cells)
• For biological researches
• Preparation of culture media such as Blood agar, diphasic broths etc
• To collect blood for later uses, mainly transfusion either in the donor or in another person.

Types of Blood used in Medical Laboratory analyses

Venous Blood

Venous blood is deoxygenated blood that flows from tiny capillary blood vessels within the tissues into progressively larger veins to the right side of the heart.

Venous blood is the specimen of choice for most routine laboratory tests. The blood is obtained by direct puncture to a vein, most often located in the antecubital area of the arm or the back (top) of the hand. At times, venous blood may be obtained using a vascular access device such as a central venous pressure line or Hickmann Catheter or an IV start. Most laboratory reference ranges for blood analytes are based on venous blood.

Arterial Blood

Deoxygenated blood is pumped from the right side of the heart to the lungs where it takes up oxygen. The now oxygenated blood is pumped through the left side of the heart via arteries.

The most common reason for collection of arterial blood is the evaluation of arterial blood gases. Arterial blood may be obtained directly from the artery (most commonly, the radial artery) by personnel who are trained to perform this procedure and are knowledgeable about the complications that could occur as a result of this procedure. Arterial blood may also be obtained from a vascular access device (VAD) inserted in an artery such as a femoral arterial line or Swan-Gantz catheter.

Capillary Blood

Capillary blood is obtained from capillary beds that consist of the smallest veins (venules) and arteries (arterioles) of the circulatory system. The venules and arterioles join together in capillary beds forming a mixture of venous and arterial blood. The specimen from a dermal puncture will therefore be a mixture of arterial and venous blood along with interstitial and intracellular fluids.

Capillary blood is often the specimen of choice for infants, very young children, elderly patients with fragile veins, and severely burned patients. Point-of-care testing is often performed using a capillary blood specimen.

Macroscopic Difference between Arterial and Venous blood

Firstly is important to distinguish between arterial and venous blood from their basic characteristics or appearance

• Arterial blood is Oxygenated and has a Bright red color while venous blood is deeper darker red and deoxygenated
• Arterial blood will pulsate or spurt into the collection tube while venous blood will flow evenly into the corresponding collection tube

Reason Why Blood is mostly Drawn from Veins and Not Arteries

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• Veins are comparatively easier to draw blood from, physically speaking, as the placement of veins is such that they are close to the surface of the skin. This makes the process easier by avoiding a deep needle plunge just to draw a bit of blood. On the contrary, arteries are located a bit deeper in the skin, so it doesn’t make much sense to make the process unnecessarily difficult – and potentially dangerous.

• The walls of veins are also thinner than arteries, which enables them to hold more blood (more volume). This quickens the process of blood collection and simultaneously results in more blood released into the sample tube.  It’s also easier to pierce a vein than it is to pierce an artery, so drawing blood from a vein is less painful for the subject.

• The pressure in the veins is less than that of the arteries, so there is a smaller chance of blood coming back through the spot where you were punctured by the needle before the tiny wound is healed. The blood collected from an artery is bright red in color, as compared to the darker shade of red that is observed in the blood collected from veins.

Sites for venipuncture in Medical Laboratory

Meanwhile, blood analysis is an important diagnostic tool available to clinicians within healthcare.

Blood is most commonly obtained from the superficial veins of the upper limb, which are

Here, there are three veins of choice for blood collection

• The median cubital vein is the first choice for blood collection because, this vein is usually large, visible, and well anchored and does not easily bruise.
• The cephalic vein becomes the second choice for blood collection because, this vein is not as well anchored and is usually more difficult to find.
• The basilic vein is the third choice for blood collection and should only be considered if the median cubital and cephalic veins in both arms have been ruled out.

Note that, Bacilic veins is a high-risk area due to the proximity of nerves. In addition, this vein tends to roll away and bruise more easily as compare to the Median cubital and cephalic vein.

• The backside (posterior) of the hand, and the thumb side (lateral) of the wrist.
• The backside (posterior) of the hand, and the thumb side (lateral) of the wrist.
• The recommended location for blood collection on a newborn baby or infant is the heel. Minute quantities of blood may be taken by fingerprick or finger stick sampling and collected from infants by means of a heel stick or from scalp veins with a winged infusion needle.

Tourniquet may be applied above collection site and below lock site provided there is a space of at least 6 cm between the tourniquet and lock site.  

Order of  Blood Draw in Medical Laboratory:

Blood collection tubes must be drawn in a specific order to avoid cross-contamination of additives between tubes and recollection of blood sample in case of errors. The recommended order of draw from vacutainer tubes is as follow

1. blood culture bottle or tube (yellow or yellow-black top)
2. Coagulation tube (light blue top).
3. non-additive tube (red top)
4. additive tubes in the following order of draw :
   • Serum separator tubes or SSTs (red-gray or gold top). Contains a gel separator and clot activator.
   • Sodium heparin (dark green top)

• Plasma separator tubes (PST) (light green top). Contains lithium heparin anticoagulant and a gel separator.

• EDTA Tubes (lavender top)
• Oxalate/fluoride (light gray top) or other additives

NOTE: Tubes with additives must be thoroughly mixed. Clotting or erroneous test results may be obtained when the blood is not completely mixed with the corresponding additive.

Areas to Avoid When Choosing a Site for venipuncture:

Certain areas are to be avoided when choosing a favorable site for venipuncture during blood draw:

• Extensive scars from burns and surgery – because it is difficult to puncture the scar tissue and obtain a specimen.
• The upper extremity on the side of a previous mastectomy – This may affect test results because of lymphedema.
• Hematoma – This may obviously cause erroneous test results. If another site is not available, collect the specimen distal to the hematoma.
• Intravenous therapy (IV) / blood transfusions – Don’t collect specimen from infusion vein,fluid may dilute the specimen, so collect from the opposite arm if possible.
• Cannula/fistula/heparin lock – blood should not be drawn from an arm with a fistula or cannula without consulting the attending physician.
• Edematous extremities – tissue fluid accumulation alters test results.

Techniques to Prevent Hemolysis

• Mix all tubes with anticoagulant additives gently (note that vigorous shaking can cause hemolysis) 5-10 times.
• Avoid drawing blood from a hematoma; select another draw site as earlier stated.
• If using a needle and syringe, avoid drawing the plunger back too forcefully.This can cause air bubble and hemolysis
• Make sure the venipuncture site is dry before proceeding with draw.
• Avoid a probing, traumatic venipuncture.
• Avoid prolonged tourniquet application (no more than 2 minutes; less than 1 minute is optimal).
• Avoid massaging, squeezing, or probing a site.
• Avoid excessive fist clenching.
• If blood flow into tube slows, adjust needle position to remain in the center of the lumen.

References:

• library.med.utah.edu
• wikipedia.org
• enotes
• calgarylabservices.com