In patients where the veins are difficult to locate, it may be necessary to use ultrasound to image the vessels. The ultrasound machine requires gel, which is applied to the skin. When this gel is punctured during venipuncture, it enters the tube and is mixed with the blood taken. In the blood smear, the gel is reflected as a purple, almost fungus-like structure.

Ultrasound-guided puncture is used in patients where the veins are difficult to see or feel such as in small children, obesity, edema, burns or scarred skin.