Lab Report: Dose Calculations in Pharmacology

1. Introduction

The term "posology" originates from the Greek words "posos," meaning "how much," and "logos," meaning "science." Posology is a vital branch of medical science dedicated to determining the appropriate dose or quantity of drugs for achieving the desired pharmacological action in a patient. Drug dosage cannot be fixed rigidly due to various influencing factors, including age, sex, body weight, environmental conditions, emotional factors, and more.

Official doses listed in pharmacopoeias provide a general range suitable for adults when administered orally within 24 hours.

For alternative routes of administration, appropriate adjustments are made. It is important to distinguish between "dose" and "dosage." A "dose" refers to the specific amount of medication administered at a single time, e.g., a 450mg paracetamol tablet, while "dosage" relates to the size and frequency of a dose, e.g., "450mg of paracetamol three times a day."

Homeopathic posology, which focuses on minute doses, involves selecting the potency, application, and repetition of doses. Different doses of a drug can produce varying effects, and adjustments are essential to achieve therapeutic outcomes while minimizing adverse reactions.

Drug quantity plays a key role in classifying doses, including physiological, pathological, and minimum doses.

2. Formula for Dose Calculations

The calculation of drug doses is a fundamental aspect of pharmaceutical practice. Pharmacists often need to determine the size of a dose, the number of doses, or the total quantity of medication to dispense. Several formulas and equations are used for these calculations:

(a) Basic Formula

The basic formula for dose calculation is represented as:

D/H x V = A

Where:

• D = Desired dose (Drug dose ordered by the healthcare provider)
• H = On-hand quantity (Stock on hand/on label of container)
• V = Vehicle (Quantity/drug form in which the drug is available)
• A = Amount calculated to be administered to the patient

For example, if we have Paracetamol 500mg to be administered every 4 hours (Q 4 hour RTC) orally, and the stock on hand is 250mg/5ml in a 60ml bottle, the calculation would be:

500mg/250mg x 5ml = 10ml

(b) Ratio & Proportion

This method uses ratio and proportion to solve dose calculation problems.

The equation is:

H : V = D : x

Where:

• H = Drug on hand (available)
• V = Vehicle/drug form (e.g., capsule/tablet/liquid)
• D = Desired dose (as ordered)
• x = Unknown amount to administer

For instance, if we have Cefdinir (3rd gen cephalosporin) 100mg to be administered orally four times a day, and we want to determine how many milliliters (ml) the client should receive, we can calculate as follows:

250mg : 5ml = 100mg : x ml

x ml = 2ml

(c) Body Weight

Dosages of many drugs are expressed in terms of mg/kg. The equation for calculating the dose based on body weight is:

Patient’s dose (mg) = Patient’s weight (kg) x Drug dose (mg)/1(kg)

For example, if the usual initial dose of chlorambucil is 150mg/kg of body weight, and a person weighs 154lb, we can calculate the dose as follows:

Patient’s dose: 154lb x 0.15mg/2.2lb = 10.5mg chlorambucil

(d) Body Surface Area (BSA)

BSA is used for dosing chemotherapy in cancer patients and pediatric patients. The equation for calculating the dose based on BSA is the Catzel Formula:

Patient’s dose = Patient’s BSA (m²)/1.73 m² x Drug dose (mg)

For instance, if the adult dose of a drug is 100mg, and we need to calculate the approximate dose for a child with a BSA of 0.83 m², we can use the equation:

Child’s dose: 0.83 m²/1.73 m² x 100mg = 48mg

(e) Age

Age is a crucial factor in dosage calculation, especially for pediatric patients. Various formulae are available:

(i) Young’s Formula

This formula is used for children under 12 years of age:

Dose for children = Age in years/Age in years+12 x Adult dose

For example, if the adult dose of a drug is 500mg and the child is 5 years old:

Dose for a child: 5/5+12 x 500 = 147.05mg

(ii) Dilling’s Formula

This formula is used for children aged 4 to 20 years:

Dose for the child = Age in years/20 x Adult dose

For example, if the adult dose of a drug is 600mg and the child is 10 years old:

Dose for a child: 10/20 x 600 = 300mg

(iii) Fried’s Formula

Fried’s rule estimates a medication’s dose for a child based on age in months:

Dose for a child = Age in months/150 x Adult dose

For instance, if an 8-month-old infant is prescribed a drug with an average adult dose of 350mg:

Dose for a child: 8/150 x 350 = 18.66mg

(iv) Cowling’s Formula

Cowling’s rule calculates the dosage for children:

Dose for child = Age at next birthday (in years)/24 x Adult dose

For example, if a doctor prescribes 75mg of Oseltamivir Phosphate for an adult, the doctor would prescribe the following for a 10-year-old child:

Dose for a child: (10+1)/24 x 75 = 34.375mg

3. Factors Affecting Dose Calculations

Dose calculations are influenced by various factors, each of which can alter the optimal dosage of drugs. Understanding these factors is essential for safe and effective drug administration:

(i) Age

Age significantly impacts pharmacokinetics, affecting drug absorption, distribution, metabolism, and elimination. Children and the elderly often require lower drug doses due to differences in these processes.

(ii) Gender

Gender-related physiological differences can influence drug responses. Some drugs may produce different effects in men and women, particularly during menstruation, pregnancy, and lactation.

(iii) Body Weight

Body weight plays a crucial role in determining drug dosages. While many drugs use a one-weight-fits-all approach, individual variations, such as malnutrition or obesity, must be considered.

(iv) Time of Administration

The timing of drug administration, especially in relation to meals, can affect drug absorption. Administering drugs on an empty stomach may lead to faster absorption.

(v) Route of Administration

The route of drug administration influences dosage requirements. Intravenous administration delivers the full drug dose directly into the bloodstream, whereas oral administration may result in incomplete absorption.

(vi) Psychological State

The psychological state of a patient can alter drug reactions. Anxiety and placebo effects can impact drug efficacy and perception of therapeutic results.

(vii) Presence of Disease

Underlying medical conditions, such as renal disease, can affect drug metabolism and clearance, necessitating dosage adjustments to avoid toxicity.

(viii) Idiosyncrasy

Idiosyncratic reactions, characterized by heightened sensitivity to drugs, require caution and thorough patient history to prevent adverse events.

(ix) Tolerance

Tolerance can develop with prolonged drug use, leading to decreased drug effectiveness. Starting with the lowest effective dose can minimize tolerance development.

Conclusion

In conclusion, dose calculations in pharmacology are essential for safe and effective drug administration. Various factors, including age, gender, body weight, time of administration, route of administration, psychological state, presence of disease, idiosyncrasy, and tolerance, must be considered when determining the appropriate drug dosage. Different formulas and equations, such as those based on body weight, age, and body surface area, help pharmacists and healthcare providers calculate precise drug doses tailored to individual patient needs. Understanding these factors and applying the correct dosage calculations is critical in delivering optimal healthcare outcomes while minimizing the risk of adverse events.