Systems of Weights and Measures The Metric System This system is being accepted as the international standard. The basis of the metric system is an arbitrary unit of linear measure, the meter, from which other units of the system was derived: The litres (1 x 106 cubic centimetres) is the cube of one tenth of a metre, and is the unit of volume. The gram is the unit of weight, and is the weight of a cubic centimetre of water at 400C Weights (Mass) 1 kilogram (kg) = 1000 gram 1 gram (G) = 1000 milligrams The official abbreviation is g, but in prescription writing the symbol "G" should be used to avoid possible confusion with "gr." (grains). 1 mi1lgram (mg) = 1000 micrograms (mcg) The official abbreviation is "ug", but in prescription writing the symbol "mcg" should be used to avoid possible confusion with "mg" (milligram) Measures (fluid volume) 1 Litre (1) = 1,000 millilitres (ml) Length 1 meter (m) = 100 centimetres (cm) = 1000 millimetres (mm) 1millimetre = 1000 micrometers (micron) (u) 1 micron = 1000 millimicrons (mu) THE IMPERIAL SYSTEM This system is complicated by being divided into two separate systems with some units of common name but different value, particularly with respect to units of weight. The older Apothecaries System is still used in some places for pharmaceutical measures and prescription writing, while the Avoirdupois System is used for commercial transactions. Weights Both Apothecaries and Avoirdupois Systems have in common the grain (gr.) which is of equal value in both systems. Larger units however, have different values and different symbols. Apothecaries weights 20 grains (gr.) = 1 scruple 3 scruples = 1 drachm = 60 grains 8 drachms = 1 ounce = 480 grains 12 ounces = 1 pound = 5760 grains Note: In prescription writing the weight denomination is expressed by the symbol followed by the number in Roman numerals. These numbers have a line drawn above them and, above this a dot is placed above each unit numeral. normally. One half may be abbreviated as " ss" other fraction quantities are written normally. Avoirdupois Weights 1 pound (lb.) = 7000 grains = 6 ounces 1 ounce (oz) = 437.5 grains (gr.) Note: 1) Avoirdupois weights should be written in Arabic numerals followed by the unit symbol. 2) The Apothecaries' ounce (480 grains) is heavier than the Avoirdupois ounce (437.5 grains). Conversely the Avoirdupois pound (7,000 grains) is heavier than the Apothecaries' pound Fluid Measures Here both the Apothecaries' and Avoirdupois Systems have identical values, although different symbols are again used. The rules for the choice of numerals and positioning of symbols are the same as for the weights of the corresponding system. 1 minim is approximately equal to one drop of water 60 minims = 1 fluid drachm 8 fluid drachms = 1 fluid ounce 20 fluid ounces = 1 pint 2 pints = 1 quart 8 pints = 1 gallon Systems of Weights and Measures APPROXIMATE IMPERIAL-METRIC EQUIVALENTS Length 1 grain = 65 mg 100 mg = 1.5 gr. 1 ounce (avdp.) = 28G 1G = 15.4 gr. 1 ounce (apoth.) = 31 G 1 pound (avdp.) = 454G 1kg = 2.2 lb. Measures 60 minims = 3.5 ml. 1 ml = 16.9 m 1 fl. Oz = 28.4 ml 1 l. = 35 fl. oz. 1 pt. = 568 ml. 1 l .= 1.8 pt 1gal = 4.5 1itter. 1 l. = 0.22 gal. HOUSEOLD MEASURES A drop = 1 minim = 0.06 ml A teaspoonful = 60 minims = 3.5 ml A dessertspoonful = 120 minims = 7 ml A tablespoonful = 1/2 fl oz = 14 ml A cupful = 8 fl oz = 227 ml. * The American Standard pint contains only 16 fluid ounces (20 x 4/5). Accordingly American pints, quarts and gallon are four-fifths of the corresponding Standard Imperial measure. Calculation' of Drug Dosage Although there are examples to the contrary, recommended drug dosage is most commonly expressed, as units of drug weight per unit of animal body weight (e.g. mg/kg). Units of drug weight most commonly used are micrograms (ug), milligrams (mg), grams (gm), and grains (gr.). Units of body weight most commonly used are kilograms (kg) and pounds (lb.). Commercial drug preparations include tablets, boluses, and solutions or suspensions. The amount (by weight) or concentration of active drug in the preparation will, appear on the drug label. In the case of solutions or suspensions, drug concentration will be expressed in one of the following ways: 1. Units Weights per unit volume e.g. 10 mg/ml e.g. 25gm/l e.g. 3 gr./oz 2. Dilution (weight in gm: volume in' ml) e.g. l : l000 (this is equal to 1 gm/l, 1 mg/ml, or l x 10-3 gm/ml) e.g. 1: 50,000 (this is equal to 1 gm/50 1itter, 0.02 gm/l, 0.02 mg/ml, or 2 x 10-5gm/ml) e.g. l : 1 000,000 (this is equal to 1 gm/1OO0 1itter, 1 mg/litter, 1 ug/ml, or 1 x10-6 gm/ml) 3. Percent (Weight in gram/ Volume in ml) x 100 e.g. 100% solution contains 1 gm/ml 10% solution contains 0.1gm/ml or 100 mg/ml 1% solution, contains 0.01gm/ml or 10 mg/ml 0.1% solution contains 1 mg/ml 0.01% solution contains 0.1 mg/ml 0.001% solution contains 0.01 mg/ml Note: Do not confuse this expression of drug ,concentration as percent solution with the common clinical chemistry usage of mg% (mg/100 ml) to express concentration of a substance in body fluids. Drug concentrations in commercial drug preparations are almost never expressed as mg%. In calculating drug dosage, you will often be faced with a situation like the following example: Recommended dosage of Drug X: 22mg/kg body weight Concentration of drug solution: 100 mg/ml Weight of animal: 50 lb. Dose? In ml (recommended dosage x body weight) divide (by concentration of drug) Many therapeutic misadventures are due to goofs, in calculating drug dosage. Since both the therapeutic range and the margin of safety are quite narrow for many drugs, accurate calculation of drug dosage can often mean the difference between success and disaster. .