Conceptualización clínica cognitiva multinivel (CCCM)

The metric system of measurements is based on the principle of multiples of 10 to define different ranges of quantities. Prefixes in the metric system indicates that the mentioned numeric value be multiplied by nth power of 10. For exam-ple, the represented multiplier for the common prefixes are: micro (prefix: μ) is 10⁻⁶, milli (prefix: m) is 10⁻³, centi (prefix: c) is 10⁻², deci (prefix: d) is 10⁻¹, deca (prefix: dk) is 10¹, hecto (prefix: h) is × 10², and kilo (prefix: k) is × 10³. Therefore, 1 kg = 1000 g = 1,000,000 mg = 1,000,000,000 μg.

54 Pharmaceutical Dosage Forms and Drug Delivery In addition, the avoirdupois system is the commonly used system in everyday life and the apothecary (meaning, pharmacy) system is used mainly by the pharmacists.

Weight is expressed in the avoirdupois system in grain (gr), ounce (oz), and pound (lb). The interconversions between these units and their relationship to the metric system are as follows:

1 kg = 2.2 lb 1 lb = 16 oz

1 oz = 437.5 gr = 28.4 g 1 gr = 65 mg

Volume is mostly expressed in the apothecary system in the units of fluid dram (dr), fluid ounce (oz), pint (pt), quart (qt), and gallon (gal). The interconversions between these units and their relationship to the metric system are as follows:

1 gal = 4 qt = 3785 mL 1 qt = 2 pt = 946 mL 1 pt = 16 oz = 473 mL

1 oz = 30 mL (more accurately, 29.57 mL) ʒ (fluid dram) = 5 mL

Therefore, the sign “ʒ” appearing in the signa of the prescription as “ʒi” indicates one teaspoonful (tsp) or 5 mL. Note that one tablespoon is 15 mL and is symbolized

“ ʓss” in the prescription. Similarly, the dispensing instruction “ʓV” means dispense 5 fʓ or 150 mL.

The laws of ratios and proportions can be used to interconvert units during calcu-lations. Therefore, to convert 2.5 mg/5 mL into g/gal

2 5

Ratio and proportion can also be used for the reduction and enlargement of formulas for dispensing the required quantity of a prescription. Also, a “conversion factor” can be derived, which then becomes the multiplier for every ingredient in the formula-tion to dispense a given quantity:

Conversion factor Volume to be dispensed Volume in the unit formu

= ( ) lla

For example, to dispense 200 mL of a prescription with a unit formula for 5 mL quantity, the conversion factor would be 200/5 = 40. Therefore, the quantity of every ingredient would be multiplied by 40 to make a 200 mL dispensed quantity.

4.2.1  v^olume anD w^eight interconverSionS

The interconversions of weight and volume are useful in pharmacy dispensing to aid accuracy of measurement. Interconversions of weight for volume of liquids can be done using their density, which is weight per unit volume. Therefore, 1 mL of glyc-erol, of density 1.26 g/mL at room temperature, is equivalent to 1.26 g of glycerol.

Alternatively, 1 g of glycerol is 1/1.26 = 0.79 mL of glycerol. Note that 1 cubic centi-meter (cc or cm³) = 1 mL.

Sometimes the information on specific gravity of a substance is available, which can be used to do similar calculations. Specific gravity is the ratio of weight of a substance to the weight of an equal volume of distilled water at 25°C. Therefore, it does not have any units. Since 1 mL of water = 1 g of water at 25°C, specific gravity represents the number of grams of a substance per unit volume of that substance in mL at 25°C. Density is usually determined at ambient temperature, or at the tem-perature at which measurements are to be made, and its units depend on how the measurement was made.

4.2.2  t^emPerature interconverSionS

Interconversions of temperature between the Celsius, Fahrenheit, and Kelvin scales can be carried out using the following equations:

C F

5

32

= −9 (4.1)

K C= + 273 15. (4.2)

Interestingly, −40°C = −40°F.

At temperatures >−40°C, °F < °C.

At temperatures <−40°C, °F > °C.

While the Celsius and Fahrenheit scales are more commonly encountered in rou-tine use, the Kelvin scale is used more commonly in the derivation and use of scien-tific equations.

4.2.3  a^ccuracy, P^reciSion, ^anD S^ignificant f^igureS

Accuracy represents the degree of closeness of a measurement to the desired, target, or actual quantity. Thus, if the target quantity to be weighed is 125 mg and the actual weighed quantities are 121 and 123 mg, the latter would be considered more accurate than the former.

Precision, on the other hand, represents the reproducibility or repeatability of a measurement. It represents the relative closeness of individual measurements to the average of these measurements if the measurements were to be carried out more than once. For example, if the weight of 121 mg represents an average of 121, 120, 122, 121, 121 mg, while the weight of 123 mg represents an average of 120, 121, 129, 128, 126 mg—the former would be considered more precise over the latter. In pharmacy practice, both accuracy and precision are needed.

The significant figures or digits represent the precision of a measurement by indi-cating the least amount that could be measured. For example, a weight of 1.0 kg represents a weight of 0.9–1.1 kg and a precision of 0.1 kg, while a weight of 1.000 kg represents a weight of 0.999–1.001 kg and a precision of 0.001 kg. Thus, the weight

56 Pharmaceutical Dosage Forms and Drug Delivery of 1.0 kg could have been measured on a scale that can measure as low as 100 g but not lower, while 1.000 kg could have been measured on a scale that could measure as low as 1 g.

The concept of significant figures is utilized in rounding considerations.

Numerical calculations of quantities, for example, can introduce additional digits at the tailing end of the calculated number. These numbers should then be rounded off to the significant digits of original measurement when communication of precision is important. For example, splitting a 125 mg tablet in half would result in two halves of 125/2 = 62.5 mg if the splitting is accurate. However, the latter number (62.5) does not represent the precision of dose measurement.