What blood tests require blood to be taken from the artery?is this procedure high risk?

This depends on the clinical situation. Tests like blood gasses require assessment of the blood content of enriched blood (arterial) to monitor lung efficiency, but is less critical settings capillary blood gives some valuable oxygen information. Safe blood draw from arteries require more training and experience than venous, but in well trained hands the risk becomes similar.

Agree with Dr. Ferguson. Arterial blood is needed for blood gases and for an accurate measurement of blood ammonia. Arterial puncture requires a different skill than usual blood collection and is a little more painful.