As already mentioned, there may be some underdosing initially. Perhaps, one tends to expect too much when the equations predict a serum level of 13.7 mcg/ml and then the actual measured level turns out to be ONLY 12.1 mcg/ml and, therefore, is incorrect!. Mathematical equations, by their nature, are very exact. Yet they are applied in the same manner to the many differences among our patients. When I use any pharmacokinetic dosing model in an empirical fashion I am simply looking to be in the correct ball park once I have an actual level with which to work. RxCalc helps me do this without spending a lot of time and effort coming up with a regimen that will most likely be changed after the first serum level is measured and reported. Not having many options is no reason not to use RxCalc. This is an iPhone app after all, not software to run on the hospital's mainframe! However, I am disappointed that when adjusting dosing based on measured levels I must have both a peak and a trough. Most of the time I only have a trough and am unable to use RxCalc to assist in adjusting the dosing. I have been assured by Apple Core Labs that making the entry of a peak level optional is currently being tested and should be expected in the next RxCalc release.
Robert Martin, PharmD