What is considered the best specimen for arterial blood gas (ABG) analysis?

• A. Venous blood collected with a tourniquet in a plastic syringe
• B. Arterial blood collected without a tourniquet in a 1–3 mL glass syringe
• C. Capillary blood from fingerstick collected in a plastic capillary tube
• D. Venous blood collected in a lithium heparin tube

Answer: (B)

Arterial blood is used for ABG because it provides the true gas tensions (PO2, PCO2) and pH as they leave the lungs, before tissues alter them. A tourniquet at the puncture site can distort these values by changing blood flow and local gas concentrations, leading to misleading results. Using a small-volume glass syringe helps preserve the sample’s gas content, since glass is less permeable to gases than some plastics and reduces diffusion of CO2 or O2 before analysis. So the best specimen is arterial blood drawn without a tourniquet into an appropriate syringe and analyzed promptly.