DOSAGE CALCULATIONS MADE
EASY
SOLVING PROBLEMS USING
DIMENSIONAL ANALYSIS
8TH EDITION
• AUTHOR(S)GLORIA PEARL
CRAIG
TEST BANK
,The healthcare provider orders cephalexin 500 mg by mouth
every 6 hours. The pharmacy dispenses cephalexin 250 mg/5
mL. How many mL will the nurse administer per dose?
A. 5 mL
B. 10 mL
C. 15 mL
D. 20 mL
Correct Answer:
B. 10 mL
Rationale:
Step 1:
Identify the desired dose, the available concentration, and the
units needed for the answer.
• Desired: 500 mg
• Available: 250 mg / 5 mL
• Needed: mL
Step 2:
Set up the dimensional analysis equation to cancel out the
unwanted units (mg) and arrive at the desired units (mL).
5 mL / 250 mg x 500 mg = ? mL
,Step 3:
Perform the calculation.
• Cancel out the common unit (mg) in the numerator and
denominator.
• (5 mL x 500) / 250 = = 10 mL
Medication Safety Note:
Always double-check your calculation by estimating. If 5 mL
contains 250 mg, then 10 mL contains 500 mg. This is a simple
but crucial verification step to prevent a ten-fold dosing error.
Question 2
A nurse is preparing to administer 0.9% normal saline at 125
mL/hr. The IV tubing has a drop factor of 15 gtt/mL. What is the
infusion rate in drops per minute? Round to the nearest whole
number.
A. 21 gtt/min
B. 31 gtt/min
C. 41 gtt/min
D. 51 gtt/min
Correct Answer:
B. 31 gtt/min
Rationale:
, Step 1:
Identify the known values and the desired unit for the answer.
• Rate: 125 mL/hr
• Drop factor: 15 gtt/mL
• Desired unit: gtt/min
Step 2:
Set up the dimensional analysis equation to convert hours to
minutes and mL to drops.
(125 mL / 1 hr) x (1 hr / 60 min) x (15 gtt / 1 mL) = ? gtt/min
Step 3:
Cancel out the common units (hr and mL) and solve.
(125 x 1 x 15) / (1 x 60 x 1) = = 31.25 gtt/min
Round to the nearest whole number: 31 gtt/min.
Medication Safety Note:
This is a critical calculation for IV infusions. An error in drops per
minute can lead to significant fluid overload or under-hydration.
Always use the correct drop factor for the specific tubing being
used.
Question 3
EASY
SOLVING PROBLEMS USING
DIMENSIONAL ANALYSIS
8TH EDITION
• AUTHOR(S)GLORIA PEARL
CRAIG
TEST BANK
,The healthcare provider orders cephalexin 500 mg by mouth
every 6 hours. The pharmacy dispenses cephalexin 250 mg/5
mL. How many mL will the nurse administer per dose?
A. 5 mL
B. 10 mL
C. 15 mL
D. 20 mL
Correct Answer:
B. 10 mL
Rationale:
Step 1:
Identify the desired dose, the available concentration, and the
units needed for the answer.
• Desired: 500 mg
• Available: 250 mg / 5 mL
• Needed: mL
Step 2:
Set up the dimensional analysis equation to cancel out the
unwanted units (mg) and arrive at the desired units (mL).
5 mL / 250 mg x 500 mg = ? mL
,Step 3:
Perform the calculation.
• Cancel out the common unit (mg) in the numerator and
denominator.
• (5 mL x 500) / 250 = = 10 mL
Medication Safety Note:
Always double-check your calculation by estimating. If 5 mL
contains 250 mg, then 10 mL contains 500 mg. This is a simple
but crucial verification step to prevent a ten-fold dosing error.
Question 2
A nurse is preparing to administer 0.9% normal saline at 125
mL/hr. The IV tubing has a drop factor of 15 gtt/mL. What is the
infusion rate in drops per minute? Round to the nearest whole
number.
A. 21 gtt/min
B. 31 gtt/min
C. 41 gtt/min
D. 51 gtt/min
Correct Answer:
B. 31 gtt/min
Rationale:
, Step 1:
Identify the known values and the desired unit for the answer.
• Rate: 125 mL/hr
• Drop factor: 15 gtt/mL
• Desired unit: gtt/min
Step 2:
Set up the dimensional analysis equation to convert hours to
minutes and mL to drops.
(125 mL / 1 hr) x (1 hr / 60 min) x (15 gtt / 1 mL) = ? gtt/min
Step 3:
Cancel out the common units (hr and mL) and solve.
(125 x 1 x 15) / (1 x 60 x 1) = = 31.25 gtt/min
Round to the nearest whole number: 31 gtt/min.
Medication Safety Note:
This is a critical calculation for IV infusions. An error in drops per
minute can lead to significant fluid overload or under-hydration.
Always use the correct drop factor for the specific tubing being
used.
Question 3
