Calculation of doses: patient parameters

Calculation of doses: patient parameters



Lecture Five

Calculation of doses: patient parameters
Usual dose of a drug is the amount that ordinarily produces the desired therapeutic response in the majority of patients in a general, or otherwise defined, population group.
The drug’s usual dosage range is the range of dosage determined to be safe and effective in that same population group.
For certain drugs and for certain patients, drug dosage is determined on the basis of specific patient parameters. These parameters include:
1. The patient’s age
2. Patient’s weight
3. Body surface area
4. Nutritional and functional status.

Therapeutic index or window is a comparison of the amount of a therapeutic agent that causes the therapeutic effect to the amount that causes toxicity

Certain drugs with a narrow therapeutic window often require individualized dosing based on blood level determinations and therapeutic monitoring. Digoxin, for example, at a blood level of 0.9 to 2 ng /ml is considered therapeutic, but above 2 ng /ml it is toxic.










Patients requiring individualized dosage are:
• Neonates and other pediatric patients.
• Elderly patients with diminished biologic functions.
• Individuals of all age groups with compromised liver and/or kidney function (and thus reduced ability to metabolize and eliminate drug substances),
• Critically ill patients,
• Patients being treated with highly toxic chemotherapeutic agents.






Pediatric Patients: Pediatrics is the branch of medicine that deals with disease in children from birth through adolescence.

• Neonate (newborn), from birth to 1 month;
• Infant, 1 month to 1 year;
• Early childhood, 1 year through 5 years;
• Late childhood, 6 years through 12 years;
• and adolescence, 13 years through 17 years of age.
A neonate is considered premature if born at less than 37 weeks’ gestation.

In the neonate, these biologic functions and systems are underdeveloped. Renal function, for example, develops over the span of the first 2 years of life.



? Geriatric Patients: Although the term elderly is subject to varying definitions with regard to chronologic age, it is clear that the functional capacities of most organ systems decline throughout adulthood, and important changes in drug response occur with advancing age.
Geriatric medicine or geriatrics is the field that encompasses the management of illness in the elderly.

Most age - related physiologic functions peak before age 30 years, with subsequent gradual linear decline.
For example renal blood flow diminishes nearly 1 % per year after age 30, making the cumulative decline in most persons 60 to 70 years of age, about 30 % to 40 %, a value that is even greater in older persons. Because reduced kidney function increases the possibility of toxic drug levels in the body and adverse drug effects, initial drug dosing in the elderly patient often reflects a downward variance from the usual adult dose.


? Dose determinations for elderly patients frequently require consideration of some or all of the following:
1. Therapy is often initiated with a lower - than - usual adult dose.
2. Dose adjustment may be required based on the therapeutic response.
3. The patient’s physical condition may determine the drug dose and the route of administration employed.
4. The dose may be determined, in part, on the patient’s weight, body surface area, health and disease status, and pharmacokinetic factors.
5. Concomitant drug therapy may affect drug / dose effectiveness.
6. A drug’s dose may produce undesired adverse effects and may affect patient compliance.
7. Complex dosage regimens of multiple drug therapy may affect patient compliance.



• Dosage Forms Applicable to Pediatric and Geriatric Patients
Solid dosage forms are often difficult or impossible for the pediatric, geriatric, or infirm patient to swallow. In these instances, liquid forms are preferred, such as oral solutions, syrups, suspensions, and drops. An advantage of liquid forms is that the dose can easily be adjusted by changing the volume of liquid administered.
For systemic effects, injections may be used rather than the oral route of administration when needed for pediatric and elderly patients, with the dose or strength of the preparation adjusted to meet the requirements of the individual patient.