To determine if the patient has respiratory acidosis, which test must be performed?

• A. Arterial blood gas evaluation
• B. Chest X-ray
• C. Serum lactate
• D. ECG

Answer: (A)

To tell if respiratory acidosis is present, you need to assess ventilation and acid-base status with an arterial blood gas. This test provides the pH and the partial pressure of CO2, along with bicarbonate, so you can see if CO2 is elevated and the blood is acidotic due to hypoventilation. If the PaCO2 is high and the pH is low, that pattern indicates respiratory acidosis, with possible bicarbonate changes depending on whether the situation is acute or chronic. Other tests mainly help identify causes or related issues—Chest X-ray can reveal lung problems but not diagnose acid-base status; serum lactate points to metabolic acidosis; ECG assesses the heart, not gas exchange. Thus, arterial blood gas evaluation is the definitive test to determine respiratory acidosis.