Procedures for the Collection of Blood for Hemotological Investigations

The blood collection process plays a critical role in ensuring the accuracy and reliability of laboratory tests. Proper adherence to established protocols for blood collection methods is essential to obtaining the most trustworthy results. Blood can be collected through various techniques, with the most common being venipuncture and skin puncture, depending on the type and quantity of the sample needed. Understanding the appropriate blood collection protocol for each scenario helps avoid complications and ensures the integrity of the specimen.

Skin Puncture

The skin puncture method is predominantly used in infants, small children, or cases where only a small amount of blood is required. This technique is ideal for the estimation of hemoglobin, cell counts, hematocrit (HCT), or packed cell volume (PCV) using micro methods, as well as for the preparation of blood smears. Blood collected through skin puncture is known as capillary blood, comprising a mixture of arterioles, venules, and capillaries, along with trace amounts of tissue fluid.

In infants, the heel (specifically the medial or lateral aspect of the plantar surface or great toe) is the preferred site for blood collection. In adults, blood is commonly drawn from the side of the middle or ring finger (distal digit) or the earlobe. The puncture site should be disinfected with 70% ethanol or another suitable agent and allowed to dry. A sterile, disposable lancet is then used to create a puncture deep enough to facilitate free blood flow.

The first drop of blood, often containing tissue fluid, should be wiped away with sterile cotton. Subsequent drops are then collected. Excessive pressure on the puncture site must be avoided to prevent the dilution of the sample with tissue fluid. After collection, the site is compressed with sterile cotton until the bleeding stops. Skin puncture blood collection often yields slightly higher hemoglobin, red cell count, and hematocrit levels than venous blood due to factors like platelet adherence at the puncture site and the smaller sample size, which limits the possibility of repeat testing.

Blood should not be collected from cold, cyanosed skin, as this may lead to falsely elevated values for red blood cells, white blood cells, and hemoglobin.

Venous Blood Collection

Venous blood collection is employed when a larger quantity of blood is required for multiple tests. Various test tubes are filled based on the specific needs of each laboratory test, considering the anticoagulant-to-blood ratio. For serum-based tests, no anticoagulant is necessary.

Step-by-Step Method for Venous Blood Collection

1. The antecubital fossa veins are the preferred site for venipuncture. A rubber tourniquet is applied to the upper arm to make the veins more prominent, though it should not be too tight or remain in place for more than 120 seconds. The patient is then asked to make a fist to enhance vein palpability.
2. The puncture site is cleaned with 70% ethanol or a similar disinfectant and allowed to dry.
3. The chosen vein is stabilized by pulling the surrounding soft tissue below the puncture site.
4. Sterile, disposable needles and syringes should be used for collection. A 23-gauge needle is recommended for children, while a 19- to 21-gauge needle is appropriate for adults. The needle should be inserted along the vein's direction, with the bevel facing upward. Blood is drawn slowly to prevent hemolysis and vein collapse. The tourniquet should be released once blood begins flowing into the syringe.
5. After collecting the required volume of blood, the patient should relax their fist, and the needle is carefully removed from the vein. A sterile alcohol swab is then applied to the site, and the patient is instructed to press until the bleeding stops.
6. The needle is detached from the syringe before transferring the blood into the test tube containing the appropriate anticoagulant. Forcing the blood through the syringe without removing the needle can cause hemolysis. Containers may be glass or disposable plastic tubes, sealed with corks.
7. To mix the blood with the anticoagulant, the container should be gently inverted several times. Vigorous shaking can result in hemolysis and fizzing.
8. Ensure the patient is stable and bleeding has ceased. The puncture site should be covered with a bandage. Needles should not be recapped by hand to avoid needle-prick injuries. After use, needles should be destroyed using a needle destroyer, and the syringe should be discarded in a biohazard box. Proper labeling of the blood container with the patient’s details is essential, and the sample should be sent promptly to the laboratory with a completed requisition form.

Important Precautions During Venous Blood Collection

1. The tourniquet should not be too tight or left in place for over 120 seconds, as this can lead to hemoconcentration and skewed test results.
2. The tourniquet must be released before removing the needle to avoid hematoma formation.
3. Blood should not be drawn from an arm being used for an intravenous line, as this can dilute the sample.
4. Avoid collecting blood from areas with hematoma or sclerosed veins.
5. Use appropriate needle sizes, withdraw blood slowly, and always remove the needle before transferring blood to avoid hemolysis.
6. All blood samples should be treated as potentially infectious, and proper safety protocols must be followed.
7. Anticoagulated blood samples should ideally be tested within 1-2 hours. If this is not possible, samples can be refrigerated at 4-6°C for up to 24 hours. Before testing, samples should be brought back to room temperature and mixed thoroughly.

Potential Complications of Venous Blood Collection

Several complications may arise during blood collection, including failure to obtain blood (which is both common and painful for the patient), hematoma formation, abscesses, thrombosis, thrombophlebitis, or bleeding. Additionally, the use of improperly sterilized reusable syringes and needles can result in the transmission of infections such as HIV or hepatitis B.