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  1. To obtain arterial blood for blood gas measurements.

  2. When blood is needed and venous or capillary blood samples cannot be obtained. Not preferred.

  3. To obtain ammonia levels. Venous blood can be used if it is collected, transported appropriately, and done quickly.

  4. To obtain lactate and pyruvate levels. Free-flowing arterial blood; stasis of blood increases lactate.


A 23–25-gauge scalp vein (butterfly) needle or a 23–25-gauge venipuncture needle (safety-engineered self-shielding), 1- or 3-mL syringe, povidone-iodine and alcohol swabs, 4 × 4 gauze pad, gloves, 1:1000 heparin or self-contained blood gas kit, high-intensity fiber optic light for transillumination, or a Doppler ultrasound (optional, can be useful to locate the artery). Smaller needles preferred (25 gauge for preterm infants, 23 gauge for term infants).


  1. For a blood gas, most hospitals have kits with 1-mL syringes coated with heparin. If this is not available, draw a small amount of heparin (1:1000) into the blood gas syringe (coat the surfaces and discard excess heparin from the syringe). The small amount of heparin coating the syringe is sufficient to prevent coagulation. Excessive heparin may interfere with laboratory results (see later). If any other laboratory test is to be performed, do not use heparin.

  2. The radial artery is the most frequently used site and is described in detail here. One of the advantages of the radial artery site is that the radial nerve does not lie close to the artery so there is no concern of nerve damage. Alternative sites are the posterior tibial (preferred second site) or the dorsalis pedis artery. Femoral arteries should be reserved for emergency situations. Brachial arteries should not be used (unless absolutely necessary) because there is minimal collateral circulation and a risk of median nerve damage. Temporal arteries should not be used because of the high risk of neurologic complications.

  3. Check for collateral circulation and patency of the ulnar artery by means of the Modified Allen's test. Elevate the arm and simultaneously occlude the radial and ulnar arteries at the wrist; rub the palm to cause blanching. Release pressure on the ulnar artery. If normal color returns in the palm in <10 seconds, adequate collateral circulation from the ulnar artery is present. If normal color does not return for >15 seconds or does not return at all, the collateral circulation is poor, and it is best not to use the radial artery in this arm. The radial and ulnar arteries in the other arm should then be tested for collateral circulation. Because of concern on the reliability of the modified Allen test, other methods such as the modified Allen test with Doppler ultrasound evaluation of collateral flow are being used (controversial). One study found that the best way to perform the Allen test was using the laser Doppler flowmetry method. Some are combining the use of pulse oximetry with ...

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