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Dosage Calculations

Dosage Calculations. The elements of the prescription can be curtailed down to several elements Dose is the amount of drug taken at one time Q is a variable that represents the quantity or units that consists of a dose of a medication. i.e. 2 tablet give a 650 mg dose if each dose is 325 mg.

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Dosage Calculations

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  1. Dosage Calculations • The elements of the prescription can be curtailed down to several elements • Dose is the amount of drug taken at one time • Q is a variable that represents the quantity or units that consists of a dose of a medication. i.e. 2 tablet give a 650 mg dose if each dose is 325 mg. • F is the frequency of administration per day i.e. BID • N is the number of days, or the “day supply” • T is the total quantity to dispense • Q*F*N=T

  2. Example One • Renagel ® Sig: iii TID AC for 30 days with 11 refills. • Q= 3, F=TID, N= 30 days therefore T=3*3*30= 270 tablets • Renagel® is the brand name of sevelamer chloride which a drug given to patient with end stage renal disease to lower their phosphate levels in the blood.

  3. Example Two • i.e. Timoptic® sig: instill 1 gtt OU BID. Dispense 5 ml ophthalmic container. • What is the day supply? • T= 5ml* 20 gtt/1 ml= 100 gtt • Remember Q is the quantity per dose which should take into account the route of administer OU is both eyes so Q=2 gtt • Q= 2, F= BID, T=100 what is N? • Answer: 2*2*N=100 or N= 25 days

  4. Example three • Amoxicillin 250 mg/5 ml suspension • Sig: ii tsp tid for 14 days • What is the total quantity to dispense? • Q= ii tsp • Tsp=teaspoon so ii tsp is 2 teaspoons • 2 teaspoons * (5 ml /one teaspoon)= 10 ml • F=tid or three times a day, N=14 days • Q*F*N= 10*3*14 • 420 ml

  5. Example Four • Phoslo® Sig: 1,334 mg tid ac with 6 refills • What is the total quantity needed to fill this Rx for one month? For the full 7 months? • Q= 2 (1,334 mg * (1 tab/667 mg)= 2) • F= tid is 3 times a day • N=30 days • T= 2*3*30=180 tablets • T for 7 months = 180 * 7= 1,260 tablets

  6. Insulin Example • Novolin N sig: inject 10 units bid and 20 units qhs dispense one vial (10 ml) • Commercially Novolin N is available as a 100 unit/ml, 10 ml vial size. Novolin N is human insulin neutral protaminesuspension where is a type of long acting insulin. • When the prescription is written with composition instructions (i.e. 2 parts- bid dosing and qhs dosing) calculate quantity per day (q/d). • What is the days supply of this Rx?

  7. Insulin Example • Quantity per day is 10 units bid which is 20 units • 20 units qhs is 20 units • Adding both is 40 units/days • (q/d)*Days supply= total quantity • Total quantity is 100 units/ml*10 ml= 1,000 units • Day supply = total quantity/(q/d) or 1,000/40 or 25 days

  8. Pediatric Dosing • Pediatric dosing includes dosing for neonates, infants and children up to about 12 years of age • Neonates and infants metabolize drugs differently that adults often slower in neonates. Kidney function and renally cleared drugs are removed more slowly • Drugs applied topically on the skin are absorbed much better on neonates and infants than adults because of increased surface area and thinner stratum corneum • Caution must always be exercised in these circumstances

  9. Drugs that dangerous in children

  10. Pediatric Dosing Formulas • Fried’s Rule: PD (pediatric dose)=Age (months)/150X adult’s dose • Young’s Rule : PD= Age (years)/(age (years)+12)Xadult dose • Clark’s rule: PD= weight (lbs)/150Xadult dose

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