What does the hepatic extraction ratio (ER) indicate about high-ER versus low-ER drugs?

• A. ER is calculated as Qh / CLh; high-ER drugs are capacity-limited.
• B. ER = CLh / Qh; high-ER drugs are flow-limited and approach liver blood flow; low-ER drugs are capacity-limited.
• C. ER describes only the first-pass effects.
• D. ER has no relation to hepatic clearance.

Answer: (B)

The key idea is that the hepatic extraction ratio tells how much drug is removed by the liver in one pass relative to what enters with portal blood. It is defined as ER = CLh / Qh, meaning the hepatic clearance is a fraction of the liver blood flow. If ER is high (approaching 1), the liver extracts nearly all the drug per pass, so clearance is limited by how much blood is flowing through the liver—this is flow-limited. If ER is low, the liver’s ability to metabolize (intrinsic clearance) is the bottleneck, so clearance is capacity-limited and increasing liver blood flow won’t greatly raise clearance.

In this framework, the statement that ER equals CLh / Qh and that high-ER drugs are flow-limited while low-ER drugs are capacity-limited is the correct interpretation. The other options misstate the relationship (inverting the ratio) or mischaracterize what ER describes (not just first-pass effects, and it is related to hepatic clearance).