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While medical doctors frequently prescribe medicine to their patients, it is up to the pharmacist, the last dispenser of the medicine to make sure that the patient receives the appropriate dosage. The design of a dosage regimen depends on an understanding of the drug use process (DUP), which asks ‘Is this patient suffering from a drug-related problem?’
Pharmacokinetics provides a mathematical basis to assess the time course of drugs and their effects in the body. It quantifies the following four processes: Absorption, Distribution, Metabolism, and Excretion.
There are a variety of available techniques for representing the pharmacokinetics of a drug. The most usual takes the view of the body consisting of compartments between which the drug moves and from which elimination occurs. The transfer of drugs between these compartments occurs by rate constants.
Some of the rate constants are determined by:
- The Volume of distribution (Vd); this connects the administered dose of medicine with the actual initial concentration.
- Clearance (Cl); this is the removal of the drug, which normally takes place through the liver.
- Half-life (t1/2); this is the time taken for a drug concentration to be eliminated in the plasma by a factor of 50%.
These factors are important for pharmacists to know and keep in mind when dispensing prescription medicines. Normally, at the pharmacy office, a computer system has programs that will allow the pharmacist to get the ideal dosage for the patient. However, in most cases the operation may be slow, or may involve having to maneuver though multiple screens in order to access the relevant information.
See Also: Genetic Testing for Predicting Drug Response
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RxCalc for iPhone App - Features
Having put the calculations on the table, now the iPhone app RxCalc allows the pharmacist to quickly access the information needed to get the ideal dosage for the patient. It lets you calculate the Ideal and Dosing body weight; it gives information on the Creatinine Clearance. The app also gives the elimination rate constant and Half-life of the drug. It also provides information about the volume of distribution and the Dosing interval (Tau) and the Estimated Dose.
The app allows simple calculations to be determined, such as a way to include providing the Ideal Body Weight for the patient and giving the Creatinine Clearance (Cockcrot-Gault) value. The app also has information about current regimens requiring post-level adjustment, these are the patient specific elimination rate constant; patient specific volume of distribution, patient specific dosing interval (Tau) and patient specific dosages. The quick access allows the pharmacist to provide a timely dispensing of the relevant medicine.
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- Pharmpress: http://www.pharmpress.com/files/docs/clinical_pharmacokinetics_samplechapter.pdf
- Pharmacokinetics Image:http://miro.ifsc.usp.br/pkdb/
- Image: http://www.nature.com/nrd/journal/v2/n3/box/nrd1032_BX1.html
- RxCalc for iPhone App - Features Image: http://medgadget.com/archives/2009/09/_rxcalc_for_pharmacokinetic_calculations_on_the_iphone_1.html