A hollow needle is advanced through the skin until blood is aspirated. The color of the blood and the rate of its flow help distinguish it from arterial blood (suggesting that an artery has been accidentally punctured). Within North American and Europe, ultrasound use now represents the gold standard for central venous access and skills, with diminishing use of landmark techniques.[20][21] Recent evidence shows that ultrasound-guidance for subclavian vein catheterization leads to a reduction in adverse events.[22]