Which laboratory test is crucial for assessing the severity of liver failure?

The International Normalized Ratio (INR) is a critical laboratory test for assessing the severity of liver failure because it measures the functionality of the liver in synthesizing clotting factors. In cases of liver failure, the liver's ability to produce these proteins diminishes, leading to an increased INR, which indicates a greater risk of bleeding due to impaired coagulation. This is particularly relevant in monitoring patients with liver dysfunction, as the INR helps guide treatment decisions and assess prognosis.

In contrast, Blood Urea Nitrogen (BUN) primarily reflects kidney function and is not specific to liver failure. The Complete Blood Count (CBC) provides information about blood cells but does not give a direct indication of liver function or the severity of liver injury. Arterial Blood Gas (ABG) assesses respiratory function and acid-base balance, which may be affected in critically ill patients but does not directly assess liver function or failure.