Pre and post-ductal blood gas studies are performed to evaluate which condition?

• A. Right-to-left shunting
• B. Left-to-right shunting
• C. Neonatal jaundice
• D. Coarctation of the aorta

Answer: (A)

Measuring pre- and post-ductal blood gases is used to detect mixing of blood across the ductus arteriosus and identify a ductal-dependent circulation pattern. The sample from the right hand (preductal) reflects blood before it crosses the ductus, while the sample from the leg (postductal) reflects blood after any mixing at the ductus arteriosus. If the postductal oxygenation is noticeably lower than the preductal value, this indicates a right-to-left shunt through the ductus arteriosus, allowing deoxygenated blood to enter the systemic circulation and causing differential cyanosis. This pattern helps diagnose conditions where the ductus arteriosus contributes to systemic desaturation. Left-to-right shunts typically do not produce this postductal–preductal disparity, and neonatal jaundice or coarctation of the aorta are not evaluated primarily by this differential gas testing.