Powdered substances suspended in a liquid vehicle would be calculated as w/v. Percent Weight in Volume (w/v): Is the number of grams in 100 mL of solution and is expressed as %w/v. The following units express percent based on the nature of the ingredient: Percent strength represents the number of grams contained in 100 mL of product and is very useful in pharmacy calculations. Normal saline has a concentration of 0.90% w/v of NaCl in sterile water and therefore is an isotonic crystalloid. It is important to note that when two solutions have equal osmotic pressure and salt concentration, they are said to be isotonic. Thus, if a solution requires a dilution, the active drug in the solution will remain constant, but the volume will increase. The relationship between concentration and volume is inversely proportional. (See Figure 1 for a schematic of the inverse relationship between percent of concentration or ratio strength (decreases), and the total quantity of product (increases)). Total IV Volume/Time (hour or minute) = mL per hour or minute x Stock Volume = Amount of Solution to be given The following pharmacy calculations are used to determine the dose and dispense drugs in the correct concentrations :ĭesired Dose/Stock Strength = Number of Tablets If the specific gravity is known, the volume or weight of the desired quantity can be determined. Specific gravity = number of grams of a substance/number of milliliters of a substance. Specific gravity refers to the ratio of the weight of a substance to the weight of an equal volume of water at the same time. The following definitions can aid in preparing and administering pharmacological drugs.Ĭoncentration is the amount of active ingredient per total weight of a substance.Īlligation involves mixing solutions or solids of varying strengths to obtain another strength of the active ingredient in both. Health care professionals must dispense drugs in the accurate concentrations, calculate correct doses, and compound and prepare medications accurately to ensure that each particular drug is being administered correctly.ĭose refers to the amount of medication taken at a specific time, whereas the dosage of the drug refers to the administration of a frequency, amount, and the number of doses taken over a particular period of time. Utilizing pharmacy calculations to administer therapeutics to patients correctly is vital to patient treatment.
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