Basic formula to solve for the unknown drug strength (tablets, capsules, milliliters)
D
A X Q = answer
Symbol Translation:
D = desired strength prescribed by doctor
A = available strength
Q = quantity or amount of drug that contains the available strength (if it's a capsule, tablet, or caplet then the quantity would always be 1, and if it's in ml then the quantity will vary depending on the drug).
This formula is very similar like the ratio and proportion method only difference is that symbols are placed differently.
1.) The physician orders ampicillin 0.5 g po qid. The drug is supplied in 250mg- capsules. How many capsules will the nurse administer in one dose?
Step 1: Set up formula
D
A X Q = ?
Step 2: Plug in
0.5 g
250 mg X 1 capsule = 0.002 (convert answer to mg because drug is available in mg) 0.002 x 1000 = 2 capsules
Notice we did not calculate qid (4x a day), that is because the question only asked amount of capsules for one dose.
Calculate using the ratio & proportion method for the above question
If 250 mg = 1 capsule
Then 0.5 g = X capsule ----< cross multply---< 0.5 g x 1 cap / 250 mg = 0.002 caps
****Remember convert 0.002 into mg (0.002 x 1000 = 2 capsules)
2.) The physician orders aspirin gr xv po qid. Aspirin tablets gr v are available. How many tablets total will be administer?
Step 1: Set up formula
D
A X Q = ?
Step 2: Plug in
15 gr
5 gr X 1 tablet = 3 tablets
Step 3: Calculate total quantity
*qid (4x a day) was recommended
*We know that 3 tablets will be taken in a single dose
*3 tablets X 4x a day = 12 tablets total will be taken in 1 day
Calculate using the ratio & proportion method for the above question
If 5 gr = 1 tablet
Then 15 gr = x tablet ---<cross multiply---< 15 gr x 1 tab / 5 gr = 3 tablets (for 1 dose)
*** 3 tabs x 4x a day = 12 tablets total
3.) The physician orders Prozac 40mg po daily in a.m. Prozac is available in 20mg/5ml liquid. How many milliliters would be administer per dose?
Step 1: Set up formula
D
A X Q = ?
Step 2: Plug in
40 mg
20 mg X 5 ml = 10 ml
Calculate using the ratio & proportion method for the above question
If 20 mg = 5 ml
Then 40 mg = x ml ---<cross multiply---< 40 mg x 5 ml / 20 mg = 10 ml
