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