Formulas for Drug calculations
There are many occasions when it is necessary to convert one unit of measurement to another when undertaking drug calculations.
The metric system is based on a number of basic measures or units e.g.
Large and small quantities of these units often have a prefix to make writing quantities more compact for example 0.000001g may be written as 1 mcg or 1µg.
Some common units of measurement are:
Prefix symbol multiplication factor
Converting a number from one metric unit to another involves moving the decimal place to the left or the right.
To work out how many decimal places to move:
- Write the metric scale
- Identify the two units in the problem
- Count the number of units from the unit given to the unit desired
- Move the decimal point the number of places from the given unit to the desired unit
Example 1:
Convert 0.1 grams to milligrams.
The decimal place is moved 3 places to the right.
Mg - - kg hg dag g dg cg mg - - µg
Example 2:
Convert 375 millilitres to litres.
The decimal place is moved 3 places to the left.
Ml - - kl hl dal l dl cl ml - - µl
375 ml = 0.375 l
Intravenous fluid
Intravenous fluid must be given at a specific rate, neither too fast nor too slow. The specific rate may be measured as ml/hour, L/hour or drops/min. To control or adjust the flow rate only drops per minute are used.
The burette contains a needle or plastic dropper which gives the number of drops per ml (the drop factor). A number of different drop factors are available (determined by the length and diameter of the needle).
Common drop factors are:
10 drops/ml (blood set), 15 drops / ml (regular set), 60 drops / ml (microdrop).
To measure the rate we must know:
(a) the number of drops (b) time in minutes.
The formula for working out flow rates is:
|
volume (ml) X drop factor (drops/ml)
-----------------------------------------
time (min)
|
= drops / minute
|
Example:
1500 ml IV Saline is ordered over 12 hours. Using a drop factor of 15 drops / ml, how many drops per minute need to be delivered?
|
1500 (ml) X 15 (drops/ml)
--------------------------------------------
12 x 60 (gives us total minutes)
|
=
|
Tablet Administration
This formula is used to calculate the number of tablets to be administered when given the required dose.
|
Required Dose
-----------------
Stock Dose
|
= Number of tablets to be given
|
N.B. Units for required dose and stock dose must be the same.
Example 1:
A client is ordered 150mg of aspirin. 300 mg aspirin tablets are available. How many tablets would you give?
|
150 mg
-------------
300 mg
|
= 1/2 tablet
|
Example 2:
How many tablets containing 62.5 mcg will be required to give a dose of 0.125 mg?
STEP 1 : CONVERT TO SAME UNITS
Convert 0.125 mg to mcg.
The decimal place is moved 3 places to the right.
Mg - - kg hg dag g dg cg mg > > µg
0.125 mg = (0.125 * 1000 mcg)
= 125 mcg
STEP 2: CALCULATE
|
125 cmg
--------------
62.5 mcg
|
= 2 tablets
|
OR
Convert 62.5 mcg to mg.
The decimal place is moved 3 places to the left.
Mg - - kg hg dag g dg cg mg < < µg
62.5 mcg = (62.5 * 0.001 mg)
= 0.0625 mg
STEP 2: CALCULATE
|
0.125 mg
---------------
0.0625 mg
|
= 2 tablets
|
Medication Administration
This formula is used to calculate the amount of medication in solution for oral, intramuscular, intravenous or subcutaneous injection to be administered when given a dosage, stock dose and volume:
|
Required Dose
-------------
Stock Dose
|
x
|
Stock Volume
-------------
1
|
= Volume to be given
|
N.B. Units for required dose and stock dose must be the same.
Example1:
A client is ordered 15mg of stemetil. You have 2ml of solution on hand which contains 25 mg Stemetil.
What volume of solution would you give?
|
15 mg
---------
25 mg
|
x
|
2 mg
---------
1 ml
|
=
|
30
----------
25
|
= 1.2 ml
|
Example 2:
A dose of 75 mg of pethidine has been ordered. It is available in ampoules containing 100 mg in 2 ml.
What volume must be administered?
|
75 mg
-------------
100 mg
|
x
|
2 ml
------------
1 ml
|
=
|
150
--------------
100
|
= 1.5 ml
|
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