Contraindications are relative and should be considered in terms of the risks to the patient under the circumstances and the importance of obtaining the sample. Areas of skin infection should be avoided because of the risks of inoculating the blood with bacteria. Patients on anticoagulants and those with coagulopathies are at risk for severe bruising and hematoma formation so blood gases should only be done when absolutely necessary and the operator should take extra care to apply pressure to the area to reduce bruising. Anytime an arterial puncture is made there is a risk of causing an obstructing thrombus in the artery. Patients with poor collateral flow to an area, for example a positive Allen test (discussed later) at the wrist, should not have an arterial puncture to that site. Finally, if it seems likely that a patient will require frequent arterial blood gas samples, consideration should be given to starting an indwelling arterial line. Arterial lines are beyond the scope of this module and will not be discussed further.