Venous blood sampling is a diagnostic procedure in which a needle is inserted into a particular vein and a sample of blood is collected for laboratory analysis.
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.
Most of the routine laboratory tests use a venous blood sample. The blood is obtained either by direct puncture of a vein (venipuncture), or by using a vascular access device (VAD).
Most common location for venipuncture is the antecubital area of the arm or the back (dorsum) of the hand. VAD includes a central venous pressure line or Hickmann Catheter or an IV start.
Most laboratory reference ranges for blood analytes are based on venous blood. Abnormal levels of certain substances in the blood usually indicate the presence of disease in the organ or tissue that produces that substance.
Purpose of Venous Sampling
It 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, etc.
- measurement and morphological assessment of blood components like 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 for venous blood sampling
Veins of the upper limb are preferred for venous blood samples. Here are the commonly used veins for drawing venous blood or venipuncture.
There are three prominent veins in the antecubital area (front of the elbow) suitable for venipuncture. These are the three main veins to draw blood.
Median cubital vein
It is the first choice for blood collection. It serves as an anastomosis between the cephalic and basilic veins. It is a large, superficial vein and does not bruise easily. Care should be taken not to puncture too deeply otherwise there is risk of penetration of biceps tendon and brachial artery.
It is the second choice for blood collection. It runs superficially under the skin along the outer side of the arm and forearm. This vein is not as well anchored and is usually more difficult to find. It is better appreciated on the forearm rather than the arm. It can be used for venipuncture if the antecubital vein is not palpable or shows scarring from repeated punctures.
It is the third choice for blood collection. It is a superficial vein running along the inner side of the arm and forearm. It is a high-risk area due to the proximity of nerves. In addition, this vein tends to roll away and bruise more easily. It should only be considered if the median cubital and cephalic veins in both arms are inaccessible or scarred. Like cephalic vein, it is also commonly used for venipuncture on the forearm rather than the arm because halfway up the upper arm, it turns inwards to become a deep vein. Deep veins run along arteries and present a risk for a puncture to the artery.
Practical Tip – Use the most prominent and easily accessible vein.
Dorsal metacarpal veins
The veins on the backside (posterior surface) of the hand and thumb side (lateral aspect) of the wrist called dorsal metacarpal veins can be used for venipuncture if antecubital veins are not accessible. These veins are however not as large as the veins in the forearm and antecubital fossa.
The puncture on the dorsum of the hand is more painful due to the hand being a sensitive area.
Veins of the ankle and foot
The top of the foot and side of the ankle can be used for venipuncture. These sites should be used as a last resort because of higher chances of complications. There is an increased risk of clot formation and increased risk of infection (especially in diabetics).
Avoid Following Sites Venipuncture:
- Vein with the IV infusion line
- The blood sample may get diluted with IV solution causing inaccurate results.
- The arm on side of the mastectomy
- There is a risk of infection and lymphedema if arm on side of mastectomy is used.
- Hematomas or edematous sites or tattoo
- The blood sample may get contaminated with tissue fluid or with lead from tattoo ink.
- Fistula or shunt
- There is a risk of damage and infection in the surgically prepared access point for dialysis.
- Scarred areas from burns and surgery
- It is difficult to puncture the scar tissue and obtain a specimen.
Procedure of Venipuncture
Patients may be asked to discontinue medications or to avoid food (overnight fasting) for a period of time before the blood test.
The skin over the area is wiped with an antiseptic, and an elastic band/ tourniquet is tied around the arm/ site. The band/tourniquet, retains blood within the arm and makes the veins more visible.
The vein is felt by palpation so that an appropriate site for puncture is selected.
Once selected, 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 tourniquet should be released.
The appropriate amount of blood is drawn and the needle is withdrawn from the vein.
The amount of blood drawn depends on the purpose of the procedure. 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. Blood donors usually contribute a unit of blood (500 mL) in a session. In therapeutic phlebotomy, a larger amount of blood is removed.
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. Patients who experience swelling of the puncture site or continued bleeding after venipuncture should seek immediate medical treatment.
Most patients will have a small bruise or mild soreness at the puncture site for several days. As with any invasive procedure, infection is also a risk. This risk is minimized by the use of prepackaged sterilized equipment and careful attention to proper technique. There is no risk of HIV infection from this procedure since all needles are disposed of after a single use.
Properly performed, venipuncture does not carry the risk of mortality. It may cause temporary pain and bleeding, but these are usually easily managed.