The two-compartment model for extravascular administration represents a drug's absorption and distribution process. It features a central compartment, where the drug is first absorbed, and a peripheral compartment, which illustrates the drug's distribution throughout the body. The rate of change in drug concentration in the central compartment is calculated by three exponents: absorption, distribution, and elimination.

The absorption exponent (k_a) indicates the speed at which the drug is absorbed. The distribution exponent demonstrates how the drug is dispersed throughout the body, while the elimination exponent shows how the drug is removed from the body. These exponents can be determined using the method of residuals, provided k_a is significantly larger than the distribution and elimination elements.

In addition to the method of residuals, the Loo-Riegelman method can estimate k_a for drugs that follow two-compartment characteristics. This method requires data on plasma drug concentration over time after both oral and intravenous administration to the same subject at different times. Despite its intricacy, it can be applied to drugs distributed in any number of compartments. This method contrasts with the Wagner-Nelson method, typically used for drugs with one-compartment characteristics.