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# Calculating Drug Dosages Calculating Doses from Prepared-Strength Liquids, Tablets, and Capsules

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 Calculating Drug Dosages Calculating Doses from Prepared-Strength Liquids, Tablets, and Capsules Calculating With Proportions Steps: 1. Convert to a consistent unit of measure. 2. Set up a proportion: Original dose = Desired dose 3. Cross-multiply and solve for the unknown (x). Example 1: Tablets How many tablets are needed to deliver 500 mg of Tylenol? Original dose = 250 mg Per amount = 1 tablet Desired dose = 500 mg Per amount = unknown (x) 1. Set up a proportion: 250 mg = 500 mg 2. Cross-multiply and solve for the unknown (x): 250x = 500 x = 500/250 = 2 Answer: 2 tablets are needed to deliver 500 mg of Tylenol Example 2: Solutions How many milliliters of elixir are needed to deliver 15 mg of Phenobarbital? Original dose = 120 mg Per amount = 30 ml Desired dose = 15 mg Per amount = x ml 1. Set up a proportion: 120 mg = 15 mg 30 ml x ml 2. Cross-multiply and solve for the unknown (x): 120x = 450 x = 450/120 = 3.75 ml Answer: 3.75 ml are needed to deliver 15 mg of the elixir Drug Amounts in Units Some drugs are manufactured in units (U) rather than in grams or milligrams. Note: A unit does not have a standard metric equivalent. The metric equivalent (milligrams, etc.) varies for each drug. Examples are penicillin, insulin and heparin. Solving dose problems for these drugs is exactly the same as for other dosage units except that unit (U) is substituted for milligrams (mg). Example 3: Units How many milliliters are needed to deliver 500 U of heparin? Original dose = 120 mg Per amount = 30 ml Desired dose = 15 mg Per amount = x ml 1. Set up a proportion: 1000 U = 500 U 1 ml x ml 2. Cross-multiply and solve for the unknown (x): 1000x = 500 x = 500/1000 = 0.5 ml Answer: 0.5 ml are needed to deliver 500 units of heparin Calculations With a Dosage Schedule There are times when the dose of a drug must be obtained from a schedule, which may be based on the size of the person. The dose may be presented in mg/kg of body weight. This means that the dose must first be calculated after the body weight is obtained, and then the amount of the drug preparation needed can be calculated. Steps: Calculate the dose needed Convert to a consistent unit of measure Set up a proportion Original dose = Desired dose Per amount Per amount 4. Cross-multiply and solve for the unknown (x) Example 4: Dosage Schedule Using a dosage schedule of 0.1 mg/kg for albuterol syrup, and a prepared-strength mixture of 2mg / 5 ml, how much of the syrup is needed for a 30 kg child? Calculate the dose needed: Dose = 0.1 mg/kg x 20 kg = 3.0 mg Set up a proportion: Original dose = 2.0 mg Per amount = 5 ml Desired dose = 3.0 mg Per amount = x ml 3. Set up a proportion, cross-multiply and solve for the unknown (x): 2.0 mg = 3.0 mg 5 ml x ml 2x = 15 x = 7.5 Answer: 7.5 ml are needed to deliver 3.0 mg of albuterol syrup to this child Calculating Doses From Percentage-Strength Solutions Terms: Solute – the active ingredient (solid or liquid) Solvent – the liquid that dissolves the solute (sterile H2O, NS) Solution – a solute which is dissolved in a solvent Strength – the percentage of solute to total solvent and solute Percentage – parts of the active ingredient (solute) in a preparations contained in 100 parts of the total preparation (solute and solvent) Types of Percentage Preparations: Weight to Weight: Percent in weight (W/W) expresses the number of grams of a drug or active ingredient in 100 ml of a mixture: W/W: Grams per 100 g of mixture Weight to Volume (most common): Percent may be expressed for the number of grams of a drug or active ingredient in 100 ml of a mixture: W/V: Grams per 100 ml of mixture Volume to Volume: Percent expresses the number of milliliters of drug or active ingredient in 100 ml of a mixture: V/V: Milliliters per 100 ml of a mixture Calculating with Percent-Strength Solutions Steps: Convert the percent solution to metric units (mg/1ml) Useful Trick: Percent Solutions can be quickly converted to milligrams per 1 milliliter by moving the decimal one place to the right: Example: 2% solution, move the decimal one place to the right, 2 .0. mg/1 ml = 20 mg/1 ml Set up a proportion and substitute knowns Original dose = Desired dose Per amount Per amount Cross-multiply and solve for the unknown (x) Express answer in the units requested Example 5: Percent-Strength Solution How many milligrams of active ingredient are in 3 ml of a 0.083% of albuterol (unit dose)? Convert to metric units (mg/1 ml): 0.083% = 0.083 g = 83 mg = 0.83 mg 100 ml 100 ml 1 ml 2. Set up a proportion: 0.83 mg = x 1 ml 3 ml 3. Cross-multiply and solve for the unknown (x): x = 0.83 * 3 x = 2.49 mg Answer: There are 2.5 mg of active ingredient in a unit dose of albuterol. Solutions By Ratio Frequently when diluting a medication for use in an aerosol treatment, a solute to solvent ratio is given (1:200, 1:1000) Example 5: Solutions by Ratio How many milligrams of active ingredient are in 2 ml of a 1:200 solution of isoproterenol? 1. Convert to metric units (mg/1 ml): Convert to % solution 1:200 = 1/200 = 0.005 x 100 = 0.5% Then, convert to metric units (mg/1 ml) 0.5% = 0.5 g = 500 mg = 5 mg 100 ml 100 ml 1 ml 2. Alternate method to convert to metric units (mg/1 ml) a. 1:200 = 1 g/200 ml = 1000 mg/200 ml = 10 mg/2ml = 5 mg/1ml 3. Set up a proportion: 5 mg = x 1 ml 2 ml 4. Cross-multiply and solve for the unknown (x): x = 5 * 2 x = 10 mg Answer: There are 10 mg of active ingredient in a 1:200 solution of isoproterenol. Solving Dilutions With a Dilute Solute When the active ingredient, or solute is already diluted and less than pure, the following equation is used: C1 * V1 = C2 * V2 Solving for V2: V2 = C1 * V1 C2 Where: C1 = desired concentration (in decimals) V1 = desired total volume (mL) C2 = concentration of solution on hand V2 = volume of solution needed for dilution Steps: Substitute known variables: Concentration of the solution as a decimal 20% solution = 20/100 = 0.20 Volume (ml) Solve for the unknown variable (V2) Determine the amount of diluent (normal saline, sterile H2O) needed Total solution amount – V2 Example: Dilutions with a Dilute Solute How much 20% Mucomyst is needed to prepare 5 ml of a 10% Mucomyst? Substitute the known variables: C1 = 0.10 V1 = 5 ml C2 = 0.20 V2 = unknown 2. Solve for the unknown variable (V2): V2 = 0.10 * 5 = 0.5 = 2.5 ml 0.20 0.2 3. Determine the amount of diluent needed: Total solution amount – V2 = amount of diluent needed 5 ml – 2.5 ml = 2.5 ml diluent Answer: 2.5 ml of 20% Mucomyst and 2.5 ml normal saline are mixed to prepare 5 ml of a 10% solution of Mucomyst ***Note: Students are not responsible for calculating intravenous infusion rates.

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