Pharmaceutical Calculations Techniques

Pharmaceutical Calculations Techniques and Terminologies

• Pharmaceutical calculations are crucial for various pharmacy tasks, including dispensing prescriptions, preparing solutions, and filling capsules.
• Pharmacists need thorough knowledge of weights and measures.
• Two primary systems are used: Imperial and Metric.

Imperial System

• An older system of weight and measures.
• Consists of the Avoirdupois and Apothecaries systems.

Avoirdupois System

• The pound (lb) is the standard unit for weighing.
• All mass measures are derived from the Imperial standard pound.

Apothecaries System

• Also known as the Troy system.
• The grain (gr) is the standard unit.
• Other weights are derived from the grain.

Metric System

• Used in the Indian pharmacopeia for weight and capacity.
• Implemented in India since April 1, 1964.
• The kilogram (kg) is the standard unit of weight.
• All other weight measures are derived from it.

Metric System Conversion Table

• 1 kilogram (kg) = 1000 grams (g)

• 1 hectogram (hg) = 100 grams (g)

• 1 decagram (dag) = 10 grams (g)

• 1 decigram (dg) = 0.1 grams (g)

• 1 centigram (cg) = 0.01 grams (g)

• 1 milligram (mg) = 0.001 grams (g)

• 1 microgram (µg) = 0.000001 grams (g)

• 1 Liter (L) = 1000 milliliters (ml)

Conversion Tables

• The Indian pharmacopeia uses the metric system for formulae, but prescriptions are often in the imperial system.
• Conversion tables are used by pharmacists for converting imperial to metric units.

Weight Measures Conversion Table

• 1 kg = 2.2 lb (pounds)
• 30 g = 1 oz (ounce)
• 450 g = 1 lb (avoirdupois pound)
• 1 g (gram) = 15 grains (gr)
• 60 mg (milligrams) = 1 grain (gr)

Capacity Measures Conversion Table

• 1000 ml = 1 quart
• 500 ml = 1 pint
• 30 ml = 1 fluid ounce
• 4 ml = 1 fluid drachm
• 1 ml = 15 minims
• 0.06 ml = 1 minim

Domestic Measures Conversion Table

• 1 drop = 0.06 ml
• 1 teaspoonful = 4 ml
• 1 dessertspoonful = 8 ml
• 1 tablespoonful = 15 ml
• 2 tablespoonful = 30 ml
• 1 wine glassful = 60 ml
• 1 tumblerful = 240 ml

Calculations Based on Density

• Density is the mass of a substance per unit volume (mass/volume).
• Specific gravity is the ratio of the substance's mass to an equal volume of water.
• In the metric system, density and specific gravity are numerically equal.
• Weight = Volume × Density

Example Problem for Density

• Calculate volume of 2 kg of glycerin with a density of 1.25 g/ml.
• Solution: Volume = 2000 g / 1.25 g/ml = 1600 ml

Alcoholic Dilutions

• Calculate the volume of 95% alcohol needed to prepare 400ml of 45% alcohol.
• Solution: Volume of strong alcohol = (400ml x 45%) / 95% = 184.47 ml

Isotonic Solutions

• Isotonic solutions have the same osmotic pressure as equal solute concentrations.
• 0.9% sodium chloride solution is isotonic with blood plasma.
• Concentrations above 0.9% are hypertonic; below 0.9%, hypotonic.

Posology

• Posology is the calculation of appropriate drug doses for children.
• Several methods exist, including Young's, Dilling's, and Fried's formulas.

Young's Formula

• Dose for child = (Age/Age+12) x Adult dose

Dilling's Formula

• Dose for child = (Age/20) x Adult dose

Fried's Formula

• Dose for child = (Age in months/150) x Adult dose

Calculations Based on Body Weight

• Catzel rule: Dose for the child = (Surface area of child/Surface area of adult) x Adult dose

• Average body surface area for an adult = 1.73 m²

• Example: Calculate dose for a child with a surface area of 0.57m² and an adult dose of 50mg.

• Solution: Child dose = (0.57 m²/1.73 m²) x 50 mg = 1.65 mg

Clarke's rule

Dose = (wt in lb /150) x Adult dose(mg) or Dose = (wt in kg/70) x Adult dose(mg)

Calculation Based on Body Surface Area

• Mosteller rule: BSA(m²) = √(height(cm) x weight(kg) / 3600)

Temperature Measurements

• Temperatures in pharmacy are often measured in either Fahrenheit or Celsius.

• 9°C = 5°F – 160, or 5°F = 9°C + 160

• Conversion example: Convert 120° F to °C:

• Solution: 9°C = 5(120) – 160

• C = 440/9 = 48.9°C

Explore the essential techniques and terminologies in pharmaceutical calculations. This quiz covers both the Imperial and Metric systems used in pharmacy, highlighting their significance in dispensing prescriptions and preparing medications. Test your knowledge on weights, measures, and the conversion processes in pharmaceutical practices.