NAPLEX Question of the Week: Calculations

A common calculation within the hospital is the subject of this week's question.

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Mr. PE is a 48 year old, 72 kg male who was recently diagnosed with COVID-19. He was mostly asymptomatic when he developed sudden onset shortness of breath, when he presented to the emergency department. A CT scan showed acute bilateral pulmonary emboli. His vitals on presentation were: Blood pressure 118/74 mmHg, respiratory rate, 16 breaths per minute, and heart rate 110 BPM. The providers elected to start him on a therapeutic 18 unit/kg/hr intravenous heparin drip. Your hospital stocks heparin as a 25,000 unit in 500 mL of 1/2 normal saline premix bag. At what rate in mL/min should he be started on?

 

Rationale:

Calculations are a very important component of the NAPLEX. In fact, one of the sample questions in the candidate's guide involves a calculation with a constructed response example i.e. "fill in the blank". Therefore students must prepare not only to select an appropriate answer from a calculation, but also perform the calculation themselves and input the answer into the exam.

For our question here, this is how we would work the problem:

18 unit/kg/hr * 72 kg = 1296 units/hr

1296 units/hr / 60 minutes/hr = 21.6 units/minute

Heparin 25,000 unit in 500 mL of 1/2 NS.  = 50 units/mL

21.6 units/minute / 50 units/mL = 0.432 mL/min

 

Start with calculating the number of units per hour needed based on his actual body weight. Then convert the units per hour to units per minute by dividing 1 hour into 60 minutes. Divide this by the per mL concentration of your drug. Lastly, check for errors by rationalizing. If we multiplied by 60 minutes per hour instead of dividing, we would get 1555 mL/minute. Does that make sense as a real life application? Definitely not so always good for a common sense last evaluation to ensure the math makes sense.

Devoting study time to working a lot of calculation question will definitely pay
"dividends" on your exam (I know I inserted a dad joke).

See everyone next week!

CB

Christopher M. Bland

Clinical Associate Professor, University of Georgia College of Pharmacy