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Posology: Factors & Dose Calculations
Comprehensive notes on Posology, the 10 major factors affecting dose selection, and standard Pediatric Dose Calculation formulas (Young's, Dilling's, Clark's).
The word Posology is derived from the Greek words 'posos' meaning "how much" and 'logos' meaning "science or study".
Definition:
Posology is a branch of medical science which deals with the dose or quantity of drugs which can be administered to a patient to produce the desired pharmacological or therapeutic action.
The optimum dose of a drug varies from person to person. A physician or pharmacist must consider the following major factors before calculating a dose:
1. Age & Body Weight
Children and elderly patients require lesser doses than average adults because their liver and kidney functions (drug metabolism and excretion) are not fully developed or are declining. Also, a heavier person generally requires a higher dose than a lighter person.
2. Sex (Gender)
Women generally require smaller doses than men because they typically have a smaller body mass and a higher percentage of body fat. Special care is needed during pregnancy, menstruation, and lactation (to avoid teratogenic effects on the fetus).
3. Route of Administration
The Intravenous (IV) route provides 100% bioavailability. Thus, the IV dose of a drug is always much smaller than the Oral dose of the same drug (which suffers from first-pass metabolism in the liver).
4. Tolerance & Tachyphylaxis
Tolerance: When an unusually large dose of a drug is required to produce the normal pharmacological effect (e.g., Alcohol, Morphine).
Tachyphylaxis: Acute/rapid development of tolerance.
Other Factors: Time of administration (drugs act faster on empty stomach), Environmental factors (day vs night), Presence of disease (liver/kidney failure demands dose reduction), and Drug Interactions (Synergism vs Antagonism).
Children are not simply "small adults." Since their organ systems are still developing, their doses must be carefully calculated using specific mathematical rules based on Age, Body Weight, or Body Surface Area (BSA).
A. Rules Based on Age
| Rule | Formula | Application |
|---|---|---|
| Young's Rule | [ Age in years / (Age + 12) ] × Adult Dose |
Used for children between 1 to 12 years of age. |
| Dilling's Rule | [ Age in years / 20 ] × Adult Dose |
Used for children between 12 to 20 years of age. (Easier to calculate). |
| Fried's Rule | [ Age in months / 150 ] × Adult Dose |
Used exclusively for infants (Up to 1 year or 12 months). |
Why is BSA the Most Accurate Method?
Calculations based merely on Age or Weight can be inaccurate because children of the same age can have vastly different weights (e.g., obese vs malnourished).
The Body Surface Area (BSA) method (Catzel's Rule) considers both the height and weight of the child. Thus, it correlates directly with the physiological processes of the body like cardiac output, kidney function, and liver metabolism, making it the safest and most accurate method for drug dosing (especially for highly toxic drugs like chemotherapy).