Sampling blood using a cannula may seem like a convenient method that might avoid the pain of venepuncture for a patient, but it carries multiple risks. Reasons why drawing blood via cannula is not best practice include chance of skewing test results through sample contamination by flush or medicine, reducing the cannula’s patency, and risk of introducing infection via the wound. Expert nurses advise how to discuss the issue with cannulated patients who wish to avoid venepuncture and share tips on venepuncture best practice.