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Adult and Pediatric Dosage Calculations

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.

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Adult and Pediatric Dosage Calculations

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  1. 7 Adult and Pediatric Dosage Calculations

  2. 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.

  3. 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.

  4. 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

  5. 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

  6. 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

  7. 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)

  8. 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)

  9. 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.

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

  11. 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.

  12. 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

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

  14. Cardura Calculations 2 tablets = X

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

  16. Norvasc Calculations

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

  18. 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

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

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

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

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

  23. 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

  24. 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

  25. 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

  26. 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

  27. 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

  28. 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

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

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

  31. Figure 7-7 Tuberculin syringe.

  32. Figure 7-8 100-unit insulin syringe.

  33. Figure 7-9 50-Unit insulin syringe.

  34. Figure 7-10 30-unit insulin syringe.

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

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

  37. 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.

  38. 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.

  39. 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.

  40. 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.

  41. 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.

  42. 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

  43. 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).

  44. 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.

  45. 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.

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

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

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

  49. 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.

  50. 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.

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