Table 1.

General characteristics required of an ideal substrate for studying dynamic liver function

Pharmacokinetic and metabolic aspects

Rapidly and consistently absorbed when administered orally

Primarily metabolized in the liver

Low (20–30%) hepatic extraction ratio (i.e. metabolism  independent from liver blood flow)

Clear metabolic pathway; simple pharmacokinetic; short  elimination half-life

13CO2 generated should be distributed in the body, not  compartmentalized

Methodological aspects

Safe

Simple to prepare and administer

No- or minimal interaction with extra-hepatic tissues  (i.e. adipose tissue or muscle)

Reproducible over time and repeatable (useful for follow-up)

Costs

Low-priced