How is extraction ratio (ER) defined in pharmacokinetics?

• A. ER = (Cin − Cout)/Cin
• B. ER = (Cout − Cin)/Cout
• C. ER = (Cin − Cout)/Cout
• D. ER = (Cout − Cin)/Cin

Answer: (A)

Extraction ratio is the fraction of drug removed from blood as it passes through an organ. It compares the entering concentration (Cin) with the leaving concentration (Cout). The correct definition is ER = ( Cin − Cout ) / Cin. For example, if Cin is 10 and Cout is 3, ER = (10 − 3)/10 = 0.7, meaning 70% of the drug entering the organ is extracted in one pass. This ratio links to clearance through CL = ER × Q, where Q is organ blood flow—high extraction (ER near 1) means clearance closely follows blood flow, while low extraction (ER near 0) means the organ removes little of the drug per pass. The other forms would not reflect the fraction of entering drug that is removed, since they use the wrong numerator or denominator.