Which diagnostic test is used to evaluate right-to-left shunting in neonates?

• A. Pre and post-ductal blood gas studies
• B. Pulse oximetry on a single limb
• C. Complete blood count
• D. MRI of the heart

Answer: (A)

Right-to-left shunting changes how oxygenated blood mixes into the systemic circulation, so the test you use should directly compare oxygenation before and after the ductus arteriosus. By drawing blood gases or measurements from a pre-ductal site (typically the right arm) and a post-ductal site (such as a foot or another limb), you can detect a differential in oxygen content that indicates mixing due to a ductal-dependent lesion or persistent fetal circulation. If the post-ductal values are notably lower than the pre-ductal ones, that imbalance points to right-to-left shunting. This targeted approach provides concrete evidence of shunt physiology, guiding urgent management. A single-limb pulse oximetry lacks this comparative angle and can miss or undercharacterize the shunt, while a complete blood count or MRI doesn’t directly assess the shunt physiology in the acute setting.