Systems of Measurement and Drug Calculation By: Bryan Mae Degorio

Objectives:

Classify unit of systems and their conversion units.

Describe the role of the nurse in safe drug calculations.

Perform accurate drug calculation with specific consideration to pediatric client and elderly.

Calculate infusion rate for intravenous fluid.

Roles of the Nurse in Drug Calculations:

Drugs are formulated into medicines in such a way that most adult doses are easily calculated and predictable

Before calculation, it is sensible to estimate the dose you are likely to require so that you know whether your calculated answer seems reasonable.

Dose volumes of oral liquid medicines are typically: 5-20 mls for adults and 5 mls or less for children

Crushing tablet should be avoided wherever and whenever possible.

Always check children's and babies’ weight carefully and make sure that their weights are recorded in kilogram.

If a calculation using weight or surface area gives an answer equivalent to or greater than the normal adult dose, reconfirm.

If you are in any doubt about a calculation, STOP and contact the ward pharmacist or on-call pharmacist

Systems of Measurements:

Metric System- is a decimal system

that is logically organized to allow easy conversion through multiplication and division.

Household measurements- is a form of

measurement that uses household utensils.

Apothecary- is a system of units

use chiefly in compounding and dispensing liquids.

Conversions of Units…Weight Volume

1 kg 1000 grams 1 L 1000 ml

1 gram 1000 mg 1 ml 1 gram weight1 mg 1000 microgram 1000 ml 1000 g or 1 kg

1 microgram 1000 nanogram

Note:- it is essential to use the SAME UNITS in drug calculation. You must

convert the drugs doses in the calculation in the same units.Mass:

mcg → mg → g → kg    ( ÷ by 1,000 )mcg ← mg ← g ← kg    ( x by 1,000 )lb → kg    ( ÷ by 2.2 )lb ← kg    ( x by 2.2 )

Volume: mcL → mL → L → kL    ( ÷ by 1,000 )mcL ← mL ← L ← kL    ( x by 1,000 )

Time: min → hr    ( ÷ by 60 )min ← hr    ( x by 60 )

Example: Convert 5,000 mcg to mg. mcg → mg → g → kg ( ÷ by 1,000 )5,000 mcg ÷ 1,000 = 5 mg

Example: Convert 44 lb to kg. lb → kg ( ÷ by 2.2 )44 lb ÷ 2.2 = 20 kg

Example: Convert 0.003 L to mcL. mcL ← mL ← L ← kL ( x by 1,000 )0.003 L x 1,000 = 3 mL3 mL x 1,000 = 3,000 mcL

Example: Convert 5 hours to minutes. min ← hr ( x by 60 )5 hr x 60 = 300 min

Dosage Calculations…General FormulaTablets Dose/Stock Dose= # of tablesFluids Dose Prescribes/Stock dose X Stock volumes

1. Metroprolol (Lopressor), 25 mg PO, is ordered. Metropolol is available as 50 mg tablets. How many tablets would the nurse administer?

2. Potassium chloride is available as 10 mg per tablet. Potassium Chloride (K-Dur), 40 mg, is ordered. How many tablets would the nurse administer?

Exercises:a. 200 mg trimethropin required. Stock dose= 100 mgb. 100 mg chlorpromazine required. Stock = 25 mg tabletsc. 10 mg diazepam elixir required. Stock dose = 5 mg/5mld. 1. 2 augmentin required. Stock dose = 600 mg tabletse. 240 mg paracetamol elixir required. Stock dose = 120 mg/5 mlf. 50 mg morphine elixir required. Stock dose= 10 mg/5mlg. 40 mg pethidine required. Stock ampule= 50 mg/mlh. 6 mg morphine is required. Stock ampule= 10 mg/mli. Heparin 2000 units required. Stock ampule= 5000 units/ml

IV Fluids Calculations…gtts: dropsDrop Factor: Number of drops

per volume of IV fluid. Varies depending on the tubing used. Usually measured in gtts/mL. Common drop factors:a. 10 gtts/ml – blood setb. 15 gtts/ml- macrosetc. 60 gtts/ml- microset

Flow Rate: Measure of the flow of liquid from an IV.

a. gtts/minute (how many drops are released

every minute) b. mL/hour (how many mL

flow through each hour).

Weight Range

Required Daily Fluid

0-10 kg 100 mL per kg

10-20 kg1,000 mL + 50 mL per each kg above 10 kg

20-70 kg1,500 mL + 20 mL per each kg above 20 kg

Over 70 kg 2,500 mL (adult requirement)

Calculating Fluid Requirements:

Percentage ConcentrationConcentration in % X Volume (ml) = dosage amount in grams

100IV Drop Rate

Volume (ml) X Drop factors = gtts/min Time (minutes)

Examples:1. Calculate the amount of dextrose in 1000 mL D5W. 2. Calculate the amount of sodium chloride in 2000 mL NS.

Recall NS is 0.9% NaCl (sodium chloride)3. Infuse 250 mL over the next 120 minutes by infusion pump.4. Ordered 1000 mL D5W IV to infuse in 10 hours by infusion pump.5. Calculate the IV flow rate for 1200 mL of NS to be infused in 6 hours. The

infusion set is calibrated for a drop factor of 15 gtts/mL. 6. Calculate the IV flow rate for 200 mL of 0.9% NaCl IV over 120 minutes. Infusion

set has drop factor of 20 gtts/mL.

Pediatric Calculations…Dosage based on weight

Often drugs are prescribed based on an adults or Childs' weight. However, you must pay close attention to whether the dosage has been prescribed per Kg or Lbs.Example:132 Lb woman is prescribed Acyclovir 5mg/kgFirst convert Lbs. to Kg(132 ÷ 2.2) = 60kgThe prescription calls for 5mg per kg

60 x 5mg = 300mg

Young's RuleYoungs Rule uses age.(which makes it easier to remember, the word young refers to age)Here is the formula:Adult Dose X (Age ÷ (Age+12)) = Child's Dose

Example: 11 year old girl / 70 Lbs

500mg X (11 ÷ (11+12)) = Child's Dose

500mg X (11 ÷ 23) = Child's Dose

500mg X .48 = Child's Dose

Child's Dose = 240mg

Clark's RuleClark's Rule uses Weight in Lbs, NEVER in

Kg.

Here is the formula:

Adult Dose X (Weight ÷ 150) = Childs Dose

Example11 year old girl / 70 Lbs

500mg X (70 ÷ 150) = Child's Dose

500mg X ( 0.47 )= Child's Dose

500mg X 0.47 = 235mg

Child's Dose = 235Mg

Fried's rule Is a method of estimating

the dose of medication for a child by dividing the child's age in months by 150 and multiplying the result by the adult dose.

Pediatric dose = child's age in months 150 x Adult Dose