# Calculating Drug Dosages

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Calculating Drug Dosages
Chapter 9 Calculating Drug Dosages

Chapter 9 Lesson 9.1

Learning Objectives Use formulas to determine the dosages of tablets, capsules, or liquids Use formulas to determine the total number of tablets or capsules or the amount of liquid to be ordered for a specified time Use information about the apothecaries', metric, and household measurements systems to accurately calculate drug dosages

Learning Objectives (cont.)
Calculate dosages for parenteral injections, including those for special preparations such as insulin Calculate flow rates for infusions

Calculating Medication Dosages
Three Steps Verify the drug available is the same measurement system as the drug dosage desired (convert if needed) 2. Reduce to lowest terms 3. Calculate dosage quantity to be administered When the measurement systems of what the physician has ordered and what is available are different, it is more efficient to convert the measurement of what is available to what the physician has ordered. If a physician ordered a mediation in micrograms and the medication comes in milligrams, what measurement do you want to end up with to check for accuracy against the original order? The techniques previously learned, such as simplifying fractions, ratios, and proportions, will all be necessary to safely solve medication problems.

Drug Calculation Methods
Fraction Method 600 mg = mg x tablets tablet Solve for x Ratios or Proportion Method 600 mg : x tablets :: 200 mg : 1 tablet Although there are different methods for solving medication calculations, it is important to find a method you are comfortable with and stay with that method. Remember to label the measurement system of the numbers to avoid calculation errors. The fraction and the ratio or proportion methods are solved in a similar way. One is written as a fraction and the other in a ratio format.

Drug Calculation Methods (cont.)
Desired over Available Method Desired units (conversion factor) x Quantity of drug form = Quantity to give Quantity available (x conversion factor) The desired over available method combines the conversion of ordered units into available units and the computation of drug dosage into one step. For those of you who are not familiar with the ratio proportion or the fraction method, this method would be helpful in that it combines all steps into one process. Remember, all methods take practice to master.

Forms of Oral Medications
Capsules Cannot be broken or divided If amount to be given is more than 0.5, round to next whole number Tablets Only divide if scored Coated tablets are not to be broken Liquids May be measured in a medication cup, syringe, or calibrated dropper Capsules are not scored and therefore cannot be broken or divided. If you’ve calculated 1.75 capsules are to be given, 0.75 is greater than 0.5, so 2 capsules are administered. Drug manufacturers try to provide capsules in varying dosages to facilitate accuracy. Tablets that are highly irritable to the stomach lining are coated. Coated tablets do not dissolve until they enter the small intestine. Dividing a coated tablet destroys the protection against stomach irritation. The process and formulas used to calculate dosages of liquids are the same as those used to calculate dosages of capsules or tablets.

Scored and Unscored Tablets

Parenteral Medications
Medication available in three forms: Prefilled syringe labeled with specific dosage For example: meperidine (Demerol) 100 mg in 1 mL Single-dose ampule or multiple-dose vial labeled with a specific dosage per volume For example: epinephrine (Adrenalin) 1:1000 in 0.1 mL A vial with powder that requires a specific fluid be added to it to obtain a specific dosage (Reconstitution) Regardless of the form of parenteral medication available, the proportion method is the standard method of calculation. It is important to read labels carefully because multidose vials will have the total volume in the vial stated, as well as the dose in a specific amount drawn from the vial. It is easy to confuse these numbers. The process of reconstituting is similar to mixing powdered drinks (such as Kool-Aid) with water. If you add 10 mL to a powdered amount of a drug, will the volume yielded be more, less, or equal to 10 mL? This is an important concept to keep in mind when choosing a syringe that holds the needed volume.

Insulin A critical medication that replaces the insulin not being produced by the patient’s pancreas Insulin comes in a standardized measure called a “Unit” Smallest amounts may be given; errors are critical The “Unit” measurement is specific to insulin. There is no conversion needed for Units into any other system. What other measurement has no conversion? (mEq) In addition to the special measurement system, some forms of insulin come in vials of two different strengths, U-100 and U-500. Syringes calibrated in Units are also used with insulin.

U-100 Vial

Insulin (cont.) Strengths Syringe
U-100 (100 Units of insulin per 1 mL) U-500 (500 Units of insulin per 1 mL) Preparation 5 times stronger, rarely used Syringe Calibrated in Units also Tuberculin syringe used in emergency Minims used; 16 minims = 1 mL Over the years there were various concentrations of insulin: U-45, U-60, U-250, etc. To prevent errors, manufactures now make only U-100 and U-500. U-500 is used in dialysis and transplant settings when large quantities of insulin are needed. Insulin syringes are marked with the U-100 calibration, which matches the U-100 calibration on the vial. If 1 Unit of U-500 insulin is to be administered, how many Units are drawn up using the U-100 syringe? Tuberculin syringes are used only if no insulin syringes are available or if you are in an emergency situation. It is critical to avoid errors in calculations when using a tuberculin syringe. When using a tuberculin syringe, the number of minims that will equal the number of Units must be calculated. The most accurate conversion is 16 minims = 1 mL. If you are using a tuberculin syringe to draw up 35 Units of insulin, how many minims are drawn up?

U-100 Syringe

Tuberculin Syringe

Intravenous Medications
Medications administered into the vein IV push IV hanging by gravity (flow rate formula) IV pump (mL/min or hr) Nurses must be aware of the required procedures and administration guidelines when injecting medications directly into an IV or access device (saline lock, PICC line, etc.) IV infusions not attached to pumps must be regulated manually by the nurse. Drops are “counted.” The flow rate formula is used to determine how many drops equals the desired milliliters of volume per minute. IV pumps regulate or control the volume of fluid going into the patient. Although the drops are not “counted,” the nurse must monitor the accuracy of the pump. Pumps can malfunction. Equipment does not replace the critical thinking required by the nurse.

Flow Rate Formula Gtts/min =
Volume to be administered × gtt factor Time in minutes Drop factor of tubing: Macrodrip = 10, 15, or 20 gtt/mL Microdrip = 60 gtt/mL When using the flow rate formula, you can solve for any variable, as long as the remaining variables are known. The drop factor of the tubing is found on the tubing box. For calculations, it must be stated in the problem. Typically, institutions carry one size of macrodrip tubing. Microdrip, or minidrip, tubing is always 60 gtt/mL. Microdrip tubing is used when the volume of fluid delivered must be closely regulated (e.g., for pediatric or geriatric patients or in the delivery of critical care medications).

Chapter 9 Lesson 9.2

Learning Objectives List the rule used to calculate medication dosages for children Calculate flow rates for infusions for children

Clark’s Rule Formula Weight of the child
________________ x Adult dose = Child’s dose Weight of the adult One of the most popular methods for determining medication dosages for children. Based upon a child’s body weight and the assumption that the average normal adult weighs 150 pounds. Ratio proportions are used in this method. Practice: If the usual adult dose of a medication is 500 mg, what is the dose of medication recommended for a child who weighs 60 pounds? Clark’s rule should be used if no other formula is specified.

Body Surface Area Body surface area (BSA) = the total tissue area
A nomogram is used to easily calculate the BSA in square meters BSA formula Surface area of the child (M2) × Usual adult dose Surface area of an adult (1.73 M2) = Child’s dose BSA calculations are more accurate, because children have a greater surface area than adults in relation to their weight. Nomogram charts are standardized and must be available to reference. Standardized charts are also found within the nomogram for children of normal height for weight. Standardized charts are inaccurate when used for very young infants. Refer to Figure 9-5. A straight edge is placed from the patient’s height in the left column to the weight in the right column, and the intersection on the BSA column indicates the patient’s BSA. The BSA value is then entered into the above formula.

Dimensional Analysis Steps
Numbers in the dosage calculation problem are placed on a grid along with their labels The labels are cross-canceled to assure only one label is left (one for answer) Numbers in calculation are placed along grid next to their labels Provides a single method to use for all kinds of drug problems (even those with two to three steps). This method provides a visual guide used to construct a problem in an orderly, stepwise fashion. This method reduces the chance of incorrect placement or inversion of drug calculation factors and is especially helpful for complex problems or those that call for unit conversions.

Dimensional Analysis (cont.)
Numbers are cross-canceled Numbers are multiplied across the top and bottom of the grid to yield a fraction The fraction is divided, and the remaining label is applied to the answer Let’s walk through the following problem to create a dimensional analysis grid. It takes practice to master this technique. If you are comfortable with other methods, stay with the calculation method that is easiest for you. Now, take out a piece of paper, and following this example on the (board, overhead, transparency), we’ll make a dimensional analysis grid. (Read the problem on page 90, and follow the step-by-step procedure on pp as a class.)