Define hepatic extraction ratio (E) and explain its implications for flow-limited versus capacity-limited clearance.

Define hepatic extraction ratio (E) and explain its implications for flow-limited versus capacity-limited clearance.

• A. E = CLh / Qh; high E indicates capacity-limited clearance; low E indicates flow-limited clearance.
• B. E = CLh / Qh; high E indicates flow-limited clearance; low E indicates clearance limited by enzyme capacity (CLint).
• C. E = Qh / CLh; high E indicates rapid elimination.
• D. E = CLint / CLh; high E indicates high intrinsic clearance.

Answer: (B)

Extraction ratio is the fraction of drug removed by the liver as blood passes through it. It’s defined as E = CLh / Qh, where CLh is hepatic clearance and Qh is hepatic blood flow. When E is high (near 1), the liver extracts most of the drug in one pass, so clearance is limited by how much blood can flow through the liver—this is flow-limited clearance. If E is low, only a small fraction is removed per pass, so clearance is governed by the liver’s metabolic capacity, i.e., intrinsic clearance (CLint). In that case, changes in enzyme activity or protein binding have a bigger effect on CLh. The best answer uses E = CLh / Qh and correctly links a high E to flow-limited clearance and a low E to capacity (enzyme)–limited clearance. The other formulations either invert the ratio or misstate what high E implies (e.g., simply “rapid elimination”) or use an incorrect relationship like E = CLint / CLh.

Extraction ratio is the fraction of drug removed by the liver as blood passes through it. It’s defined as E = CLh / Qh, where CLh is hepatic clearance and Qh is hepatic blood flow.

When E is high (near 1), the liver extracts most of the drug in one pass, so clearance is limited by how much blood can flow through the liver—this is flow-limited clearance. If E is low, only a small fraction is removed per pass, so clearance is governed by the liver’s metabolic capacity, i.e., intrinsic clearance (CLint). In that case, changes in enzyme activity or protein binding have a bigger effect on CLh.

The best answer uses E = CLh / Qh and correctly links a high E to flow-limited clearance and a low E to capacity (enzyme)–limited clearance. The other formulations either invert the ratio or misstate what high E implies (e.g., simply “rapid elimination”) or use an incorrect relationship like E = CLint / CLh.