The 13C-Methacetin breath test is a rapid, reproducible, point-of-care test of liver metabolic function. After oral or nasogastric tube administration, the 13C labeled Methacetin is O-demethylated by cytochrome P4501A2 in the liver and further biotransformed into 13CO2, which is expired in breath. The BreathID® MCS device captures and quantifies expired 13CO2 and standardizes recovery against expired 12CO2 through a nasal cannula (in conscious patients) or an adaptor connected to the ventilator line (for intubated patients). The results obtained from the device are expressed as delta over baseline (DOB), which expresses the change in 13CO2/12CO2 ratio in comparison to the baseline measurement. It can be transformed into the percentage of 13C dose recovered over time (PDR) after the ingestion of Methacetin, and the cumulative PDR (CPRD), the rate at which 13C substrate is metabolized, derived from the breath 13C/12C ratio.