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Easy you say?
Easy you say?
The formula method is likely the easiest way to calculate the proper dose during medication administration. Medication orders do not always match the amount of medication supplied by the pharmacy. This requires the nurse to make a calculation based on the order and what is supplied.
The formula:
Desired/Have x Vehicle (or Volume)
Let's break this down:
Desired is the amount of medication that has been ordered, such as 10 mg morphine, 125 mcg digoxin, or 1 g ceftriaxone
Have is the amount supplied by the pharmacy or manufacturer of the medication, such as 5 mg, 250 mcg, or 1 g
Vehicle or Volume is the type of product or volume of liquid supplied, such as 1 ml, 1 tablet, or 250 ml
An example would be: a nurse wants to give 650 mg of acetaminophen by mouth, supplied by the pharmacy are 325 mg tablets of acetaminophen. What will the nurse administer?
Desired is 650 mg
Have is 325 mg
Vehicle is 1 tablet (tab)
We can type this out in two ways
650 mg/325mg x 1 tab = 2 tab PO (by mouth)
650/325 x 1 tab = 2 tab PO
I may use either method in my examples
notice: make sure units match (mg, mcg, etc) in the desired and have section, then you can drop the unit indicators from the formula (second example)
If you don't drop the unit indicators or names, that is okay- it still works the same (they cancel out as in the first example)
Also make sure to include the route with every answer if your nursing program so requires (both examples)
More Examples
More Examples
Order is for 1 mg lorazepam IV. Supplied is lorazepam 2 mg per ml in a vial. What will the nurse administer?
Desired is 1 mg
Have is 2 mg
Vehicle or volume is 1 ml
*both desired and have are in mg, so safe to move forward with formula
We can write this as 1 mg/2 mg x 1 ml or simply as 1/2 x 1 ml = 0.5 ml IV
Order is for digoxin 125 mcg oral. Supplied is 250 mcg tablets. What will the nurse administer?
Desired is 125 mcg
Have is 250 mcg
Vehicle or volume is 1 tablet
*both desired and have are in mcg, so safe to move forward with formula
Again we can do 125 mcg/250 mcg x 1 tab or... 125/250 x 1 tab = 0.5 tab PO
Order for 500 mg amoxicillin PO. Available is amoxicillin 200 mg/5 ml oral suspension. What will the nurse administer?
Desired is 500 mg
Have is 200 mg
Vehicle or volume is 5 ml
*desired and have match in units
500/200 x 5 ml = 12.5 ml PO
Order for 1,000 mg ceftriaxone by intramuscular injection (IM). Available is ceftriaxone 1 g per vial. Each vial is to be diluted with sterile water prior to administration. What will the nurse administer?
Desired is 1,000 mg
Have is 1 g
Vehicle or volume is 1 vial
*desired and have do not match- must convert desired units to match. Recall that 1,000 mg = 1 g (move decimal point 3 places). Now the desired changes to 1 g
1/1 x 1 vial = 1 vial IM
Ordered is heparin 7,500 units by sub-cutaneous injection (SQ). Available is heparin 5000 units/ml... what will the nurse administer?
Desired is 7,500 units
Have is 5,000 units
Vehicle or volume is 1 ml (if label just states "amount/ml" it means amount/1 ml)
*desired and have units match
7,500/5,000 x 1 ml = 1.5 ml SQ
Now it's your turn for some practice- the last few are tricky
A patient with a headache asks for ibuprofen. Ordered is 500 mg PO. Available is ibuprofen 200 mg tablets. What will the nurse administer?
2.5 tab PO Desired is 500 mg; Have is 200 mg; Vehicle is 1 tab; the units match. 500/200 x 1 tab = 2.5 tab
This same patient can't swallow pills and would like a liquid form. Available is ibuprofen 100 mg/5 ml... what will the nurse administer?
25 ml PO Desired is 500 mg; Have is 100 mg; Vehicle/volume is 5 ml; the units match... 500/100 x 5 ml = 25 ml
Order for lisinopril 15 mg PO daily. Available is lisinopril 10 mg tablets. What will the nurse administer?
1.5 tab PO Desired 15 mg; Have 10 mg; Vehicle 1 tab... 15/10 x 1 tab = 1.5 tab
Ordered warfarin 1.5 mg PO daily. Supplied is warfarin 3 mg tablets. What will the nurse administer?
0.5 tab PO Desired 1.5 mg; Have 3 mg; Vehicle 1 tab... 1.5/3 x 1 tab = 0.5 tab
Patient is in pain and would like hydromorphone. Order is for hydromorphone 2 mg IV every 4 hours for pain. Supplied is hydromorphone 10 mg/ml... what will the nurse administer?
0.2 ml IV Desired 2 mg; Have 10 mg; Vehicle/volume 1 ml... 2/10 x 1 ml = 0.2 ml
A patient has developed a severe cough. Ordered is for codeine 20 mg PO for cough or 30 mg PO for pain, every four hours. Supplied is codeine 25 mg/5 ml oral solution. What will the nurse administer?
4 ml PO Desired 20 mg; Have 25 mg; Vehicle/volume 5 ml... 20/25 x 5 ml = 4 ml
Order for sumatriptan 6 mg SQ for severe migraine. Supplied is 3 mg/0.5 ml autoinjector syringes. What will the nurse administer?
2 autoinjector syringes SQ In this case, the drug is supplied in a prefilled autoinjector syringe, that is the vehicle
Desired 6 mg; Have 3 mg; Vehicle/volume 1 autoinjector... 6/3 x 1 autoinjector = 2 autoinjectors
Ordered is spironolactone 200 mg per day PO in two divided doses. Supplied is spironolactone 50 mg tablets. What will the nurse administer for each dose?
2 tab PO Order is for 200 mg daily in 2 divided doses. You must calculate for each dose. 200/ 2 doses = 100 mg per dose
Desired 100 mg; Have 50 mg; Vehicle/volume 1 tab... 100/50 x 1 = 2 tab
Order for furosemide 20 mg IV once. Supplied is furosemide 40 mg tablet. What will the nurse administer?
Do Not Give Notice that the ordered route is IV and the supplied form is tablets (PO)
Patient has a fever of 100.9 degrees F. Order for acetaminophen 325 mg PO for fever greater than 101 degrees F. Supplied is acetaminophen 325 mg tablets. What will the nurse administer?
Do Not Give Notice the order states that the acetaminophen can only be given for fever of greater than 101 degrees F. The patient has a temp of 100.9 degrees F.
Great work so far! Next we will look at weight-based dose calculations
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