7 Adult and Pediatric Dosage Calculations

Accuracy in Drug Calculations Accurately calculating drug dosages is an essential skill in health care. Serious harm to a patient may occur from a mathematical error during dosage calculation. Health-care professionals who administer drugs must follow medical orders precisely and efficiently.

Background Dose: amount of drug taken by patient for intended therapeutic effect Dosage regimen: schedule of taking drug dose Most drug doses are measured in units of weight (e.g., 500 mg). Many drug doses are general, not patient specific.

Dosage Calculation Terms Desired dose: amount to be administered at one time Amount to administer: volume containing desired dose Dosage ordered: total amount of ordered drug and its frequency of administration

Dosage Calculation Terms Dosage unit: volume of medication containing quantity of drug as listed on drug label Dose on hand: amount of drug in dosage unit Dosage strength: dose on hand per dosage unit

Calculating Dosages First step: make sure the strength of drug ordered and the strength of drug available are in same unit of measure If necessary, convert to a single unit. Then, use formula: D/H × Q = X, which can also be expressed as D × Q/H = X

Using the Formula D/H × Q = X Divide D (desired dosage) by H (dose on hand or dosage of drug available). Then multiply by Q (number of tablets, milliliters, etc., that contains available dosage). Equals X (amount to be administered)

Ratio Formula of Dosage Calculation Example: Amoxil 500 mg is ordered. It is supplied as 250 mg/5 mL liquid. 250 mg ÷ 5 mL = 500 mg ÷ X In this example, X = 10 mL (the dose ordered)

Oral Dosages Oral medications are either liquid or solid. Tablets are most common solid oral medications. Oral medications may be chewable, administered sublingually or buccally, swallowed, or dissolved in water. Check the label before administering.

Solid Medications Scored (notched) tablets can be broken only along the line of scoring. Unscored tablets must not be broken.

Calculating Solid Doses Dose on hand (H) divided by dosage unit (Q) equals desired dose (D) divided by amount to administer (X). H/Q = DX or D × Q/H = X, which can also be expressed as D/H × Q = X.

Zocor Calculations Ordered: Zocor 40 mg bid On hand: Zocor 20 mg tablets D × Q/H = X 40 × 1 tablet/20 mg = X X = 2 tablets

Figure 7-1 Drug label for doxazosin mesylate. (For educational purposes only) Figure 7-1 Drug label for doxazosin mesylate.

Cardura Calculations 2 tablets = X

Figure 7-2 Drug label for amlodipine besylate. (For educational purposes only) Figure 7-2 Drug label for amlodipine besylate.

Norvasc Calculations

Liquid Medications Liquid medications can be measured in small units of volume. Examples of liquid drugs are: Syrups Spirits Elixirs Suspensions

Measuring Liquid Doses Liquids can be measured by: Measuring cup: calibrated in fluidounces, fluidrams, milliliters, teaspoons, or tablespoons Medicine dropper or oral syringe: calibrated in milliliters, minims, or drops Calibrated spoon: calibrated in teaspoons and milliliters

Calculating Liquid Doses The physician orders 400 mg of the antibiotic cefdinir (Omnicef). How many milliliters will you administer?

Figure 7-3 Drug label for cefdinir.. (For educational purposes only) Figure 7-3 Drug label for cefdinir..

Calculating Liquid Doses Label on bottle indicates that 5 mL contains 125 mg of cefdinir.

Figure 7-4 Drug label for erythromycin ethylsuccinate. (For educational purposes only) Figure 7-4 Drug label for erythromycin ethylsuccinate.

Administering Medicine to Pediatric Patients Dosage cups: for children who know how to drink from a cup Syringes: for infants who cannot drink from a cup; can be squirted into back of mouth Droppers: safe for infants; measure at eye level Cylindrical dosing spoons: easily used by small children

Smart Bottles for Elderly Patients Bottles that release correct amount of medication dosages Self-locking feature to prevent accidental overdose System alerts pharmacist when medication supply is low

Parenteral Medications Injectable mixtures that contain drug dissolved in appropriate liquid Prescribed in grams, milligrams, micrograms, grains, or units Can be packaged as premixed solutions, or in powdered form that requires mixing with a solution

Intradermal Injection Usually used for skin testing to diagnose an allergy or for tuberculin test Tuberculin syringe with a 25-gauge needle Site of injection: inner portion of forearm or upper back A reddened or raised area within 48 to 72 hours indicates a positive reaction

Subcutaneous Injection Drugs that must be absorbed slowly are usually injected subcutaneously Tuberculin syringe or a 3-mL syringe is used Heparin is a drug commonly administered subcutaneously

Insulin Injection Injection sites usually rotated May be administered subcutaneously, IV, or with insulin pump Four types of insulin: Quick-onset, short-duration Intermediate-acting Long-acting Ultra-long-acting

Figure 7-5 Mixing regular and NPH (Neutral Protamine Hagedorn) insulins in one syringe.

Figure 7-6 Insulin syringes: 100 unit (A), 50 unit (B), and 30 unit (C).

Figure 7-7 Tuberculin syringe.

Figure 7-8 100-unit insulin syringe.

Figure 7-9 50-Unit insulin syringe.

Figure 7-10 30-unit insulin syringe.

Figure 7-11 A partially filled 50-unit insulin syringe.

Figure 7-12 A partially filled 100-unit insulin syringe.

Preparing Insulin Injections Use 30-unit insulin syringe for insulin doses less than 30 units. Use 50-unit insulin syringe for insulin doses less than 50 units if standard 100-unit syringe is not available.

Preparing Insulin Injections To remove insulin from vial, first inject same quantity of air as ordered insulin volume Then withdraw appropriate insulin Two types of insulins can be combined in one syringe; draw up shorter-acting insulin first.

Safe Insulin Practices Avoid keeping insulin on top of medication carts or counters because it can be confused with heparin. If insulin concentration is not 100 units/mL, apply bold warning labels. Order insulin cartridges for outpatients to help ensure correct dispensing.

Heparin Calculation Heparin is an anticoagulant that is measured in USP (United States Pharmacopeia) units. It can be administered IV or subcutaneously. The term units should always be written out, and the abbreviation U avoided.

Heparin Calculation With a premixed parenteral solution of heparin, you have to convert the physician’s order to the volume of solution that contains the amount of heparin ordered.

Heparin Calculation Ordered: heparin 5,000 units subcutaneously q8h On hand: heparin 10,000 units/mL Convert units to milliliters: If 10,000 units equal 1 mL, then 5,000 units equal 0.5 mL 0.5 mL should be administered

Intravenous Drugs IV fluid therapy: to administer fluids containing water, dextrose, vitamins, electrolytes, and drugs Medications for IV administration are usually available in small-volume vials. Amount of drug can be listed in terms of concentration (5mg/5mL) or as amount of drug in vial (80 mg in a 2-mL vial).

Dosage Calculation in Pediatrics Dosages for infants and children are usually less than adult dosages for same medication. Dosage calculations for pediatric patients must be precise. The most accurate methods of determining appropriate pediatric doses are by weight and body area.

Drug Dosage Calculation by Body Surface Area Body surface area (BSA) is determined by using a nomogram (a numerical relationship chart) and the child’s height and weight.

Figure 7-13 Pediatric nomogram used for determining body surface area.

Calculating Drug Dose by BSA First determine BSA using nomogram. Then use formula calculation:

Calculating Ampicillin Dose Child’s BSA is 0.4 m2; adult dose of ampicillin is 250 mg.

Pediatric Doses in Package Inserts Drug data for pediatric doses are usually supplied in the drug’s package insert. Pediatric dosages are often based on mg/kg, mcg/kg, or units/kg.

Finding Safe Pediatric Dosage Ordered: amoxicillin 60 mg PO tid On hand: 125 mg/5 mL Child weighs 12.5 lb. First determine child’s weight in kilograms: 12.5  2.2 = 5.7 kg Safe pediatric dosage is 20–40 mg/kg/day in 3 equal doses.

Finding Safe Pediatric Dose Find a range of lowest to highest safe dosages: 20 mg × 5.7 kg =114 mg/day; 40 mg × 5.7 kg =228 mg/day 60 mg PO tid = 180 mg/day, which falls in the safe range of 114 mg to 228 mg per day. Ordered dose is safe for this child.

Clark’s Rule Clark’s rule: based on weight of child, which is much more accurate than either Young’s or Fried’s rules

Young’s Rule Young’s rule: for children older than 1 year

Fried’s Rule Fried’s rule: for infants younger than 1 year: