Unique approach to calculation of first-order absorption rate constants from blood or urine data.

Several methods are employed to estimate an apparent first-order rate constant for absorption in a one-compartment model. A method for calculation of the absorption rate constant (Ka) has been derived based on the area under the concentration-time curve for blood data or the area under the excretion rate-time curve for urine data between the observed time of maximum concentration or rate (tmax) and the maximum concentration (Cmax) or maximum rate of excretion (Xu max). The method obviates the need for large numbers of samples in the absorptive phase. The method also avoids extensive calculation and is less influenced by errors in data points prior to Cmax or Xu max, where the rate of change is rapid and error is likely. The methods have been tested on theoretical data with and without error generated using a ragne of values for Ka and elimination rate constants (KE). Errors in the estimate Ka are proportional to error in the data. The method compares favorably with nonlinear regression analysis.