Hepatic extraction ratio ER is defined as CLh / Qh. Which statement correctly describes high ER vs low ER drugs?

• A. ER = CLh / Qh; High ER drugs are flow-limited, with clearance approaching hepatic blood flow; Low ER drugs are capacity-limited by enzymes.
• B. ER = Qh / CLh; High ER drugs are capacity-limited by enzymes.
• C. ER = CLh / Qh; All drugs have the same clearance regardless of ER.
• D. ER does not influence hepatic clearance.

Answer: (A)

Hepatic extraction ratio tells you how much of a drug is removed by the liver in one pass relative to the blood flow through the liver. It’s defined as ER = CLh / Qh, so the hepatic clearance equals ER times hepatic blood flow (CLh = ER × Qh).

When ER is high (close to 1), the liver extracts most of the drug during its single pass, so clearance is limited by how much blood flows through the liver. In other words, clearance approaches hepatic blood flow. When ER is low, only a small fraction is extracted per pass, and the rate is limited by the liver’s enzymatic capacity (intrinsic clearance), not by blood flow.

So the best statement: the extraction ratio is CLh divided by Qh, high ER drugs are flow-limited with clearance approaching hepatic blood flow, and low ER drugs are limited by enzyme capacity.

The other ideas don’t fit because they invert the ratio, claim universal clearance regardless of ER, or deny ER’s influence on hepatic clearance.