Dosage Calculation Review

RNSG 1341

Some Basic Rules

• Zeros lead, they don’t follow.– Ex. 0.01 or 0.1 not 1.00 (could be mistaken for 100)

• ‘ac’ means ‘before meals’• ‘pc’ means ‘after meals’• ‘hs’ means ‘hour of sleep’ = bedtime• If a med was to be given pc and hs, that would

be 4 times

Rounding Rules

• 1. Be as precise as possible/practical in the calculation process. If calculating using multiple formulas (i.e., ratio::proportional), round to the nearest hundredth (2 decimal places) with each step. For example, if converting pounds to kilograms, keep at least 2 decimals until you reach your solution.

• If calculating using dimensional analysis, do not clear your calculator when multiplying or dividing until you reach your solution.

• 2. Round at the end of the calculation process. If the final answer is:

• a. Greater than 1mL, round to the tenth decimal place. – Ex: 2.44 = 2.4 – 2.45 = 2.5 – 2.49 = 2.5

• b. Less than 1mL, round to the hundredths place. – Ex: 0.082 = 0.08 – 0.084 = 0.08 – 0.085 = 0.09

• c. Zeroes “always Lead, never Trail”. (Ex: “0.5”, is correct, NOT “.5” and 1 is correct, NOT”1.0” for medication purposes.)

• 3. Calculating Intravenous infusion rates:

– a. For an infusion pump, (calculated to milliliters per hour, mL/hr), round to the nearest tenth if necessary.

• Ex: preliminary answer = 32.8 mL/hr, then show answer as 33 mL/hr.

– b. For gravity drip (calculated in drops per minute, gtt/min), use the above rounding rules, and then round to the whole number. The human eye cannot calculate a fraction of a whole drop.

• Ex: If your preliminary answer is 20.2, show 20 gtt/min. • If your preliminary answer is 20.45, show 20 gtt/min • If your preliminary answer is 20.5, show 21 gtt/min.

• A.D.N./Faculty/Forms and Resources/Dosage Calc/Rounding Rules • Rev. 11-27-06, 7/11

• Questions?

Dimensional Analysis• What does that mean?– Using equivalents to change your unit of

measurement– Multiplying anything by one doesn’t change the

value• 1000 mg = 1 g *1 cup = 240 mL• 1000 mL = 1 L * 2.2 lb = 1 kg• 1000 mcg = 1 mg *1 kg = 1000 g• 1 gr = 60 mg *1 tsp = 5 mL

• The physician orders 10 mL of Milk of Magnesia. How many teaspoons is this equal to?

• The physician orders 10 mL of Milk of Magnesia. How many teaspoons is this equal to?

Hint: use 1 tsp = 5 mL10mL x 1 tsp = X 1 5 mL

X = 2 tsp

• The physician orders Digoxin 0.5 mg po qd. The medication available is Digoxin 0.125 mg/5mL. How many mL per dose will you administer to the patient?

• The physician orders Septra 0.25g po tid. Available is Septra 200 mg/5mL. How many mL will you administer to the patient per dose?

• Order: morphine gr 1/4 po q6h prn for pain.• Available: morphine 15 mg per scored tablet.• How many tablet(s) per dose will the nurse

administer?

• Order: Keflex 425 mg po bid.• Available: Keflex 125 mg/5mL.• How many mL will the nurse administer per

dose?

• The physician orders Biaxin (clarithromycin) 7.5 mg / kg PO q12h. If the drug strength is 250 mg / 5 mL, how many mL will you administer to a client who weighs 70 kg per dose?

• A drug is ordered 6 mg/kg po b.i.d. How many tablets will a client receive if he weighs 148 lbs and the strength of the tablet is 200 mg/tab?

• Keftab (cephalexin) 50 mg/kg PO in two equally divided doses is prescribed for a client who weighs 88 lb. If each tablet contains 500 mg, how many tablets of this medication will the client receive per dose?

• A patient is scheduled to receive 0.015 g of a drug by mouth every morning. The drug is available as 7.5 mg tablets. How many tablets would you administer?

• Sustacal 240 mL infuse over 2 hours via a feeding tube stat. Calculate the flow rate.

• 850 mL IV fluid in 8 hours. Drop factor of IV tubing is 10 drops / mL. –Calculate drip rate for gravity flow.–Calculate rate with IV pump.–What if the tubing where microdrip?

• The physician orders 500 mL of 0.45%NaCl (type of IV solution) to be infused in 4 hours. You will be using an infusion pump. Calculate the flow rate (to the nearest tenth) for this IV remembering you will be using an infusion pump.

• A patient has an order for aminophylline IVPB 250 mg in 250 mL D5W to infuse at 0.5 mg/kg/h. The patient weighs 80 kg. What is the flow rate in mL/h?

• A patient has an IV infusing at 25 gtt/min. How many mL/h is the patient receiving if the drop factor is 15 gtt/mL?

• Order: Ticar (ticarcillin disodium) 1 g IM q6h• The package insert states to reconstitute each

1 g vial with 2 mL of sterile water for injection. Each 2.5 mL = 1 g. How many milliliters will you administer?

• The recommended dose of Cleocin (clindamycin) pediatric oral solution is 8 to 25 mg/kg/day in three to four equally divided doses. A child weighs 60 pounds.– A. Calculate the minimum safe daily dose in mg/d.– B. Calculate the maximum safe daily dose in

mg/d.