# Drug Calculation Help!!

2 Posts

Specializes in Research Nurse.

I have recently sat an exam and failed on one question but I do not understand how.

Here is  the question:

A child has been prescribed 0.5 g vancomycin in 100 mL sodium chloride 0.9%. This must be infused at a rate not exceeding 10 mg/min. What is the rate in mL/hr?

The Formulation I used is dose prescribed in mg (500) divided by maximum infusion rate (10mg/min). Can anyone tell me where I have gone wrong please?

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3,902 Posts

Specializes in Geriatrics, Dialysis.

This question requires a two part calculation to get to the right answer.  You first need to determine the length of time in minutes to achieve the ordered dose without exceeding the maximum mg/min which appears to be the step you completed.  However there is another step to determine the answer the question is actually asking for.  Once you had the max mg/min calculated  you then need to do the conversion to get the ml/hr final answer the question is asking

4,299 Posts

@London1984, you have a mismatch in units.  Look at what you are being asked to solve.  And, In addition to providing your answer as @sleepwalker requested, show us your work; this will let us see where exactly  you might be having difficulty and better assist you.

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2 Articles; 5,581 Posts

If you learn and use dimensional analysis you will never get these wrong ?

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Specializes in Programming / Strategist for allnurses.

This may help...

41 Posts

Specializes in Student Nurse.
London1984 said:

I have recently sat an exam and failed on one question but I do not understand how.

Here is  the question:

A child has been prescribed 0.5 g vancomycin in 100 mL sodium chloride 0.9%. This must be infused at a rate not exceeding 10 mg/min. What is the rate in mL/hr?

The Formulation I used is dose prescribed in mg (500) divided by maximum infusion rate (10mg/min). Can anyone tell me where I have gone wrong please?

I'm just venturing a guess here based solely on the units being used. You have a max of 10mg/min = 60 mg/hr max. So how much of the solution must not be exceeded in order to keep below this rate? Well, there's 500mg in the 100mL bag, and, at most, 60 mg must come out of the bag in an hour's time. And so what percentage of the bag must be used? The law of entropy would suggest an equal amount of medicine in each mL (because of diffusion). So, if 60 mg came out of a 500 mg bag, that's no more than 12% of the bag being used in one hour. And 12% of a 100mL bag is 12mL. So, I'm thinking the rate should be no more than 12mL/hr. But I haven't even started my program yet, so I could be way off. This just seems like the intuitive answer. What is the correct answer?

4,299 Posts

1 hour ago, bsd058 said:

Sorry﻿. 600. ﻿I’ll see if I﻿ can edit﻿ it﻿.

[...]

120 mL/hour is correct.

8 hours ago, bsd058 said:

... This﻿ just seems like the intuitive﻿ answer. What is﻿ the ﻿correct﻿ ﻿answer﻿﻿﻿?

Yes, much of what you are going to be asked to solve is intuitive.  You need to be able to look at the problem and determine what you are being asked to solve, and what of the information that you have been provided do you need to do so as you are often provided more information than you will need.

8 hours ago, bsd058 said:

... But﻿ I haven’t even started my progra﻿m﻿ yet, so I could be way off. ...

Not at all.  But this does emphasis the need to pay attention to your calculations.  While much of this is basic math, you do need to develop a process that works for you, which you apparently have done; although this is not how I would have worked this, it does work for you.

There are 5 or 6 formulae that will work for most, if not all problems that you will asked to solve.  However, I haven't encountered a problem that couldn't be solved using dimensional analysis (DA).  If you're not familiar with DA and are interested in reviewing it, you might find the attachments in first 2 posts in the thread @Joe V referenced here.

2,957 Posts

Specializes in oncology.

you can not  get a number without knowing the drip factor, a micro drip (ususally or peds) or macro (adults).

With a drug limited to such a narrow range not to exceed 10mg/minute it would be imperative to use a pump. You do not need to know the drip factor.

London1984 said:

A child has been prescribed 0.5 g vancomycin in 100 mL sodium chloride 0.9%. This must be infused at a rate not exceeding 10 mg/min. What is the rate in mL/hr?

429 Posts

Specializes in Occupational Health.

1,344 Posts

Specializes in NICU.

you can not  get a number without knowing the drip factor, a micro drip (ususally or peds) or macro (adults).

4,299 Posts

59 minutes ago, bsd058 said:

I’m just venturing a guess here based solely on the units being used. You have a max of 10mg/min = 60 mg/hr max﻿﻿﻿. ...

How many mg/hour?

41 Posts

Specializes in Student Nurse.
chare said:

How many mg/hour?

Sorry. 600. I'll see if I can edit it.

You have a max of 10mg/min = 600mg/hr max. So how much of the sol﻿ution mu﻿st﻿ not be exceeded in order to keep below this rate? Well, there's 500mg in the 100mL bag, and, at most, 600mg must co﻿me out of these bags in an hour's time. And so what percentage of the bag must be used? The law of entropy would suggest an equal amount of medicine in each mL (because of diffusion). So, if 600 mg came out of these 500 mg bags, that's n﻿﻿o more than 120% of a bag being used in one hour. And 120% of a 100mL bag ﻿is 120mL. So, NOW I'm thinking the rate should be no more than 120mL/hr. What a difference a few hours of sleep makes. LOL