Safe Drug Preparation

Two ways to calculate doses• Basic Calculation & Ratio and Proportion

– Basic Calculation 1st

– Desired dose x quantity of on-hand dose on hand dose

In short: D_____ X QOH

Example 1

• The physician orders aspirin gr 10 q4h PRN for fever over 101. On hand aspirin gr 5 tabs.– Step one: check to see if all measures are in

the same system. No conversion is necessary. Both measures are in grains

– Step two: use the formula 10 gr

___ x 1 tab = 10 divided by 5 = 25 gr 2 x 1 = 2 tabs

Example 2

• Ampicillin 0.5g. The unit dose packet reads 250mg/cap– Check to see if all measures are in the same

system 1g = 1000mg• 0.5g = 0.5 x 1000 = 500mg

• Then use the formula D 500mg ____ X Q or

____ X 1 cap = OH 250

• Reduce fraction 500/250 = 50 divided by 25 = 22 X 1 = 2 caps

Example 3

• Order: Versed 3 mg IM preoperatively. On hand are vials labled 5mg per ML– Check to see if all measures are in the same

system– Use formula: 3mg

_____ X 1ML = 3 divided by 5 = 0.6

5mg

0.6 X 1ml = 0.6 ml

Ratio

• Ratio describes a relationship between two numbers.– Example: 1g : 15gr

Proportion

• A proportion consists of two ratios that are equal– Example: 1g : 15gr = 2g : 30gr

Solving problems using ratio and proportion

• Known unit of measure : known equivalent = desired unit of measure : unknown equivalent

• Example: 1000 mg : 1 g = 500 mg : xg

– 1000 x = 500 = 500 divided by 1000 = 0.5X = 0.5g

Example 1 using ratio and proportion

• Ordered: Demerol 60mg IM on call. The narcotics draw contains labled Demerol 100mg/2ml– Verify measures are in same system– Set up problem:

Dose on hand : know quantity = dose desired : unknown quantity

100mg : 2ml = 60mg : X ml

100X = 120 = 120 divided by 100 = 1.2

X = 1.2ml

Pediatric and Infant doses

• Remember that 1lb = 2.2 kg (always divide # of lbs by 2.2)– 33lbs = ? kg– 24lbs = ? kg– 66 lbs = ? kg

• Most orders for peds will read: give xmg/kg/24h

Peds example

• Give Demerol 6mg/kg/24h for pain in divided doses every six hours prn. Demerol is available 50mg/ml. How much Demerol would be appropriae for a 33lb child every 6 hours?

• 1st convert weight = 33 divided by 2.2 = 15kg• Next get your daily dose 6mg x 15kg = 90mg in 24 hours• Next calculate ml needed in 24 hours =

– Dose on hand : know quantity = dose desired : unknown quantity• 50 mg : 1 ml = 90mg : X ml

50X = 90 = 90 divided by 50 = 1.8

X = 1.8ml in 24 hours

Then calculate the ml per dose (every 6 hours)

24h : 1.8ml = 6h : xml

24X = 10.8 = 10.80 divided by 24 = 0.45

You will give 0.45ml every 6 hours