Which of the following tests is used to diagnose fasting metabolic ketoacidosis?

The correct answer is Arterial blood gas.

Arterial blood gas (ABG) analysis is used to diagnose fasting metabolic ketoacidosis (FKA). ABG analysis measures the pH, partial pressure of carbon dioxide (PCO2), partial pressure of oxygen (PO2), and bicarbonate (HCO3-) levels in the blood. In FKA, there is a decrease in pH, PCO2, and HCO3- levels, and an increase in PO2 levels.