Update on the use of metabolic probes to quantify liver function: caffeine versus lidocaine

The search for continues for a safe, accurate and reliable method to quantify liver function similar in principle to renal creatinine clearance or pulmonary function spirometry tests. When evaluating patients in the more advanced stages of chronic liver disease, one's clinical judgement regarding the degree of liver dysfunction usually suffices, but in patients with early or only intermediate disease, and estimate based on routine blood tests and/or clinical severity scores is often unreliable. A more quantitative approach under investigation at present has been to monitor specific pharmacokinetic parameters of 'probe' drugs metabolized primarily by hepatic cytochrome P-450. These parameters include the plasma or salivary clearance rate of the parent compound and/or the formation rate of its metabolites. Following a review of basic hepatic pharmacology relevant to this topic, we shall explore the advantages and disadvantages of two 'metabolic probes' that have shown the most promise to date, caffeine and lidocaine.