Phlebotomy, or venipuncture, is the medical procedure of puncturing a vein to collect a blood sample, most often for laboratory testing. The standard technique involves using a straight needle, which is part of an evacuated tube system. This system utilizes the vacuum inside a collection tube to draw a specific volume of blood directly from the vein. Adherence to protocol is important to ensure patient safety and maintain the integrity of the collected sample for accurate diagnostic results.
Preparation and Site Selection
Before beginning the procedure, all necessary equipment must be assembled and placed within reach. Supplies include a safety-engineered straight needle, a tube holder (hub), the correct blood collection tubes, a tourniquet, an antiseptic skin preparation (such as 70% isopropyl alcohol), sterile gauze, and a sharps container. Proper patient identification is the first safety measure, requiring the healthcare professional to confirm the patient’s identity using two unique identifiers, such as their full name and date of birth, and verifying the requested tests.
The patient’s arm should be extended and supported comfortably for a thorough inspection of the venipuncture site. The preferred location is the antecubital fossa (the area inside the elbow), because it contains the median cubital vein. This vein is the first choice as it is well-anchored, large, and less likely to roll than other veins.
Site selection requires visual inspection and careful palpation to feel for a vein that is soft, bouncy, and refills quickly. A tourniquet is applied three to four inches above the chosen site to engorge the veins, making them more visible and palpable. The tourniquet must be snug enough to impede venous blood flow but not arterial flow. It should not remain on the arm for more than one minute, as prolonged application can alter test results.
Once the optimal vein is located, the site must be cleaned with an antiseptic solution to minimize the risk of infection. If using an alcohol wipe, the area is cleaned using a circular motion, moving outward from the center. The antiseptic must be allowed to dry completely for 30 seconds or more, depending on the product. This drying time is necessary for the antiseptic to be effective and prevents a stinging sensation upon needle insertion.
Performing the Venipuncture and Blood Collection
With the site prepared, the straight needle, attached to the tube holder, is uncapped, ensuring the bevel (the slanted opening) is facing upward. Anchoring the selected vein is important to prevent it from rolling away during insertion. This is achieved by gently pulling the skin taut below the puncture site with the thumb of the non-dominant hand. The needle is then inserted smoothly and quickly into the vein at a shallow angle, typically between 15 and 30 degrees.
Once the needle is positioned within the vein, the first collection tube is inserted into the tube holder, pushing it firmly onto the back end of the needle, which pierces the stopper. The vacuum inside the tube draws blood automatically, and the tube should be allowed to fill completely until the flow stops. If multiple tubes are required, they must be collected in a specific sequence known as the “Order of Draw” to prevent additive carryover, which could lead to inaccurate laboratory results.
Order of Draw Sequence
The standard Order of Draw begins with sterile blood culture bottles to prevent contamination. This is followed by the light blue-top tube (sodium citrate for coagulation studies). The ratio of blood to anticoagulant in the light blue tube is important, so it must be filled completely. Subsequent tubes follow a general sequence to prevent contamination from additives:
- Serum tubes (red or gold tops).
- Tubes containing heparin (green tops).
- Tubes with EDTA (lavender tops).
- Tubes with glycolytic inhibitors (gray tops).
Between tube changes, the holder must be stabilized to keep the needle steady within the vein, often by bracing the index finger against the flange. If the blood flow slows or stops before a tube is full, it may indicate the needle has shifted or the tube has lost its vacuum. A new tube should be tried before making subtle adjustments to the needle position. Complete the collection of all required tubes while maintaining the proper order and ensuring they are adequately filled.
Safe Completion and Specimen Handling
As the final tube is nearing completion, or immediately after the last tube is filled, the tourniquet must be released from the patient’s arm. Releasing the tourniquet before the needle is withdrawn prevents excessive pressure and potential hematoma formation. Once the tourniquet is off, a sterile gauze pad is placed over the insertion site, and the needle is swiftly withdrawn at the same angle it was inserted.
Immediately after withdrawal, the needle’s safety feature is engaged to shield the sharp tip and prevent accidental needlestick injury. The entire needle-holder assembly is discarded as a single unit into a sharps container. Pressure is applied to the puncture site with the gauze for a minimum of two minutes, or until the bleeding has stopped, before a clean bandage is applied. The patient should be instructed to avoid bending the arm and to keep the bandage on for at least 15 minutes.
The collected tubes must be gently inverted a specific number of times (typically five to eight) to ensure the blood thoroughly mixes with the tube’s additive. Shaking the tubes vigorously must be avoided, as this can cause hemolysis, which breaks red blood cells and compromises the sample quality. Every specimen tube must be accurately labeled immediately at the patient’s side with their full name, a second identifier, the date, and the time of collection. This labeling process ensures that the diagnostic results are correctly matched to the patient.