Which diagnostic test is used to directly measure arterial oxygen levels?

• A. Chest X-ray
• B. ABGs
• C. Pulse oximetry
• D. Sputum culture

Answer: (B)

The key idea is that directly measuring arterial oxygen requires analyzing arterial blood. This is done with an arterial blood gas (ABG) analysis. In an ABG, blood is drawn from an artery and the sample is tested to determine the arterial oxygen tension (PaO2) along with other values like PaCO2, pH, and bicarbonate. PaO2 provides a precise, quantitative measure of how well oxygen is moving from the lungs into the bloodstream.

Other tests serve different purposes: chest X-ray shows lung structure and any obvious abnormalities but doesn’t quantify how much oxygen is in the blood; pulse oximetry estimates oxygen saturation (SpO2) noninvasively and can be influenced by perfusion, hemoglobin variants, or readings near abnormal levels, so it isn’t a direct PaO2 measurement; sputum culture detects infectious organisms and doesn’t assess oxygenation.