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How To Do Venous Blood Sampling

By

Yiju Teresa Liu

, MD, David Geffen School of Medicine at UCLA

Last full review/revision Oct 2020| Content last modified Oct 2020
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Topic Resources

In venous blood sampling, a needle is inserted into a vein to collect a sample of blood for testing.

Peripheral veins, typically the antecubital veins, are the usual sites for venous blood sampling.

Ultrasound guidance, when equipment and trained personnel are available, can facilitate blood sampling from deep, nonpalpable veins.

(See also Vascular Access.)

Indications

  • Need for a venous blood sample for laboratory study

Contraindications

Absolute contraindications

  • None

Relative contraindications

  • Infection or hematoma at a prospective venipuncture site

  • Injured or massively edematous extremity

  • Thrombotic or phlebitic vein

  • Intravenous catheter distal to a prospective venipuncture site (may affect laboratory results if IV fluids or drugs are infusing distal to the venipuncture site)

  • Ipsilateral mastectomy

In the above situations, use another site (eg, the opposite arm) for venipuncture.

Coagulopathy is not a contraindication, but sites must be compressed for a longer time after venipuncture.

Complications

Complications are uncommon and include

  • Local infection

  • Arterial puncture

  • Hematoma or bleeding

  • Damage to the vein

  • Nerve damage

  • Vasovagal syncope

Equipment

  • Skin-cleansing materials: Alcohol, chlorhexidine, or povidone-iodine swabs or wipes

  • Nonsterile gloves (sterile gloves if blood cultures are being obtained)

  • Tourniquet, single-use

  • Needle system (eg, needle and syringe, or needle and vacuum tube, typically 21-gauge needles for adults; 22- or 23-gauge for neonates, small children, and sometimes older patients)

  • Blood-collection tubes and blood-culture bottles, as appropriate

  • Dressing materials (eg, tape, gauze, bandages)

Optional equipment includes

  • Vein-finder device (eg, infrared vein viewer, ultrasonography device)

  • Topical anesthetic (standard for children): Needle-free lidocaine gas-injector, lidocaine-epinephrine-tetracaine mixture, or lidocaine-prilocaine cream 

Additional Considerations

  • Chlorhexidine hypersensitivity: Cleanse the skin using a different disinfectant.

  • Latex hypersensitivity: Use latex-free gloves and tourniquet.

  • Efficient, expeditious performance of the procedure is required to avoid prolonged tourniquet placement and blood stasis, which can cause artifactually abnormal laboratory results (eg, hemolysis and hyperkalemia).

  • If povidone-iodine is used to cleanse the skin, allow it to dry and then remove it with isopropyl alcohol, to avoid erroneous blood test results (eg, hyperkalemia, hyperphosphatemia, or hyperuricemia) and also to avoid skin irritation.

  • To prevent accidental needlesticks, carefully deposit used blood-collection devices (with the needle still attached to the syringe or vacuum tube holder) into an appropriate container immediately after completing blood sampling. Do not recap standard (ie, nonsafety) needles prior to disposal unless a container is not immediately available.

Relevant Anatomy

  • The median cubital and cephalic veins are preferred for blood sampling, but other arm and hand veins may be used.

  • The cephalic vein is located on the lateral (radial) side of the arm, and the basilic vein is located on the medial (ulnar) side. These veins extend through and branch within the antecubital fossa, creating the large antecubital and proximal forearm veins.

  • The brachial artery (blood supply to the forearm and hand) lies deep to the basilic vein in the upper arm and bifurcates into the radial and ulnar arteries in the antecubital fossa or proximal forearm. Caution must be taken to avoid arterial puncture.

  • Leg veins (eg, dorsal digital veins and the greater saphenous vein inferoanterior to the medial malleolus) or external jugular veins may be used if upper extremity veins are inaccessible.

Positioning

  • Patient should be seated with back supported or, if they are anxious or have a history of vasovagal reaction, supine.

  • Support the supinated forearm (or other chosen site) on a comfortable surface.

  • To access the external jugular vein, place the patient in Trendelenburg position with the head tilted slightly contralaterally.

Step-by-Step Description of Procedure

Identify and prepare the site

  • Do a preliminary inspection (nonsterile) to identify a suitable vein: Apply a tourniquet, have the patient make a fist, and palpate using your index finger to locate a large-diameter vein that is nonmobile and has good turgor.

  • To help distend and locate veins, tap a potential site with your fingertips. It may help to allow the arm to hang down, increasing venous pressure. Use a vein-finder device if a suitable vein is not readily seen or palpated.

  • After identifying a suitable cannulation site, remove the tourniquet.

  • Apply anesthetic if it is being used and allow adequate time for it to take effect (eg, 1 to 2 minutes for gas injector, 30 minutes for topical).

  • Cleanse the skin site with antiseptic solution, beginning at the needle-insertion site and making several outwardly expanding circles.

  • Wait for the antiseptic solution to dry completely. If applying povidone-iodine, wipe it off with alcohol and allow the alcohol to dry.

    If blood is being obtained for blood cultures, vigorously cleanse the site with alcohol for 30 seconds, allow the alcohol to dry, and then swab in outwardly expanding, overlapping circles using chlorhexidine or povidone-iodine. Wait for the antiseptic effect to occur (1 minute for chlorhexidine or 1.5 to 2 minutes for iodine). Wipe off povidone-iodine with alcohol and allow the alcohol to dry. For children, swab the site 3 times using only alcohol. After this point, do not touch the skin site with any nonsterile item.

Obtain the blood sample

Try to access the vein efficiently and collect the blood sample within 30 seconds after tourniquet placement. Do not leave the tourniquet on for > 1 minute.

  • Reapply the tourniquet proximal to the selected insertion site. Do not have patients make a fist or let their arm hang down during the blood sampling, because these maneuvers may cause various erroneous laboratory values (eg, increased potassium, lactate, phosphate).

  • Palpate with your gloved finger to locate the middle of the target vein.

  • Apply gentle traction to the vein distally using the thumb of your nondominant hand to prevent the vein from moving. Traction may not be necessary for larger veins of the forearm or antecubital fossa.

  • Tell the patient that the needlestick is about to happen.

  • Insert the needle proximally (ie, in the direction of venous blood flow), with the bevel facing up, along the midline of the vein at a shallow angle (about 10 to 30 degrees) to the skin.

  • Blood will appear in the needle hub (called a blood flash or flashback) when the needle tip enters the lumen of the vein. Stop advancing the needle, lower the needle to better align it with the vein, and advance it into the vein an additional 1 to 4 mm, to ensure that it stays in position during blood collection.

    If no flash appears in the hub after 1 to 2 cm of insertion, withdraw the needle slowly. If the needle had initially passed completely through the vein, a flash may now appear as you withdraw the needle tip back into the lumen. If a flash still does not appear, withdraw the needle almost to the skin surface, change direction, and try again to advance the needle into the vein.

    If rapid local swelling occurs, blood is extravasating. Terminate the procedure: Remove the tourniquet and the needle and apply pressure to the puncture site with a gauze pad (a minute or 2 is usually adequate unless the patient has a coagulopathy).

  • Keep the needle motionless.

  • Begin to withdraw the blood sample and, when blood begins to flow, remove the tourniquet.

    When using vacuum tubes, push each tube fully into the tube holder, use care to avoid dislodging the needle from the vein. Fill multiple collection tubes in the proper sequence.* After removing a tube from the holder, gently invert the tube 6 to 8 times to mix the contents; do not shake the tubes.

    When using a syringe, pull back on the plunger gently to avoid damaging the blood cells or collapsing the vein.

  • When blood collection is complete, gently hold a folded gauze square at the venipuncture site with your nondominant hand, and in one motion remove the needle and immediately apply pressure to the site with the gauze. Remove the tourniquet if you did not do so earlier.

  • Have the patient or an assistant continue to apply pressure to the site.

  • If you used a syringe to collect the blood, now transfer samples to collection tubes and bottles;* either insert the needle directly into the tops of the vacuum tubes, or remove the needle and attach a vacuum tube holder to the syringe. Do not inject blood into vacuum collection tubes; allow the vacuum to draw the blood into the tube. After blood has been added to a tube, gently invert the tube 6 to 8 times to mix the contents; do not shake the tubes.

  • Deploy safety cover over the exposed needle. Deposit used blood-collection devices (with needles) into a sharps container. Do not recap nonsafety needles prior to disposal unless a sharps container is not immediately available.

  • Dress the site with gauze and tape or a bandage.

* When multiple blood tests are to be done, blood should be allocated to the collection tubes in a proper sequence; first cultures, then tubes with anticoagulant, and then others.

Note that the rubber tops of blood-culture bottles must be properly disinfected prior to introducing the blood sample (eg, by scrubbing each top with separate 70% alcohol wipes for 30 seconds and allowing it to air-dry).

Warnings and Common Errors

  • Use only mild tension when applying the tourniquet; it is a venous, not an arterial, tourniquet. Remember to remove the tourniquet after the blood draw.

  • Take care not to puncture too deeply and go through the vein.

  • If the vein is not entered, do not try to reposition the needle by moving the tip to one side or another; this can push the vein out of the way and also damage tissue. Instead, withdraw the needle partway before changing the angle and direction of insertion.

  • It is no longer recommended to flex the elbow after antecubital venipuncture; this actually increases hematoma formation.

Aftercare

  • Recheck the site after a few minutes to verify the absence of hemorrhage/hematoma.

Tips and Tricks

  • Butterfly needles attached to a syringe may be preferred for difficult venipuncture (eg, small veins in neonates, fragile veins in older people).

  • If a suitable vein is difficult to locate, try lowering the extremity and/or applying warm compresses or nitroglycerin ointment to help dilate veins.

  • Well-fitting gloves make palpation of the vein easier.

  • Instruct patients to look away from the equipment and the procedure to help prevent a vasovagal episode.

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