Hepatic extraction ratio ER is defined as

• A. ER = CLh / Qh
• B. ER = Qh / CLh
• C. ER = CLh × fu
• D. ER = fu × Clint

Answer: (A)

Hepatic extraction ratio represents the fraction of drug removed by the liver in a single pass through the organ. It is defined as CLh divided by hepatic blood flow Qh, so ER = CLh / Qh. This makes ER a dimensionless quantity between 0 and 1, indicating how efficiently the liver extracts the drug from portal blood.

If you relate it to the well-known liver clearance model, CLh can be expressed as Qh × ER, and in more detail CLh = (Qh × fu × CLint) / (Qh + fu × CLint). From this, ER becomes ER = (fu × CLint) / (Qh + fu × CLint). This shows how intrinsic clearance and the unbound fraction influence the extraction ratio alongside blood flow.

Why the other forms aren’t the definition: taking the reciprocal, Qh / CLh, gives the inverse relationship and isn’t the fraction extracted. Multiplying by fu (CLh × fu) or using fu × Clint describes factors that contribute to hepatic clearance, not the fraction of drug removed per pass through the liver.