Fishing for answers?

Problem:

Doctor's order:	0.25 mg of digoxin p.o.
On hand:	0.5 mg tablets

          

Your exams may use varied terms to designate the 2 components necessary to calculate the correct dosage to administer.  The key is to clarify the 2 parts of the equation (what to give and what is available).

Process:
 

Set up your "skeleton". (until you become comfortable with the process)	_____	=	_____ x
	x	=

Fill in the 2 units of measurement you are dealing with to make the equation EQUAL.		=
Add the information from the problem.	0.25 mg x tab	=	0.5 mg 1 tab
Cross multiply.  Always place x on the left hand side of the equation.	0.5 x	=	0.25

Place a division sign.
Divide 0.25 by 0.5:  Move the decimal 1 space to the right.
Answer = 0.5 tablet

Tips:

• Make sure both sides of the equation are equal.  As necessary, change mg to mcg, grams to milligrams, etc.  It is easier to calculate if the conversion is changed to the measurement one has "on hand".  LABEL all terms you are dealing with so that it will be apparent if you are trying to calculate something that is not EQUAL.
• X can be placed in any of 4 places on the "skeleton" as shown below.

• Cross multiply. 
• Whatever is cross-multiplied by X will always be placed on the left side of the equation.  This automatically sets up your division schematic.
• Draw your division sign and divide.  This way you will not have to worry what gets divided by what.  It is already set up for you.

• An important principle in setting up your problem is to identify what is critical information for calculation and what is extraneous to calculating the problem.

John has an order "Oxacillin 550 mg IVPB q 6°".  The nurse has a one gram vial with  the following information on the vial: Mix 5.7 ml of sterile water to yield 250 mg/1.5 ml.  How many ml will the nurse withdraw from the reconstituted vial?

• The dosage (550 mg)
• The end concentration (250 mg/1.5 ml)

• Mixing instructions (Adding the 5.7 ml to the vial tells you that this is the volume necessary to add to the powder to yield a specific concentration.)  Can you figure how much volume the powder has in the vial?  (0.3 ml) 
• q 6° (Since you are calculating a single dose, this information is not necessary to calculate.)
• 1 gram vial (This is not important because the end concentration is given to you in this case.)

5550 mg x ml	=	250 mg 1.5 ml
250 x	=	825
x	=	3.3

The nurse will withdraw 3.3 ml of medication from the vial after reconstitution.

Tip:

• Remember, the problem may contain information necessary for preparing the medication and should contain information necessary to solve the problem.
• This may require Critical Thinking to differentiate which is which.