Module 12 – Drug Calculations

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Introduction

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From your readings in module 2, you would realize the importance of correct drug calculations. In this module you are going to look at drug calculations that you will encounter next year and in your career when you have to revisit these calculations. You dont have to solve these problems (you can if you wish) but to critically study them to understand what they are asking and extract the information needed.

Objectives

On successful completion of this module you should be able to:

  • Read drug calculation problems correctly

  • Recognise the different types of drug calculations

  • Recognise the solutions and units needed in drug calculation problems.

12.1 Why drug calculations

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Case study

Have a look the following video clip (click on the picture).

Here is a transcript of part of this video.

Susan: Gee you dont have to do much calculating here. All the machines do it for you and the doctor tells you what dose to give..Its all on the labels.

Nurse 1: Um...most of the time it is relatively straight forward. But youve gotta know what your doing. Theyre pretty hot on calculating doses cos it can be really bad if theyre wrong. We have to pass a test before we start - and once a year after that.

Susan: Sounds like a waste of time to me. Must get everybody all worked up for nothing. Well at least I can use my calculator.

Susan sees Mary-Anne passing. She looks worried, and is in a hurry.

Susan: Can you excuse me for a minute - theres a friend of mine ...

Nurse 1 nods assent. Susan follows Mary-Anne to a casualty booth. Danny and another two nurses are in there. A doctor has been examining Dannys arm. A nurse takes Danny to the scales.

Susan: Mary-Anne - what happened?

Mary-Anne: (Surprised) Oh Susan. Danny fell of his bike - they think his arms broken.

Nurse 2: How heavy are you Danny?

Danny: About forty Kilos.

Nurse 2: (Weighing) Thats right Danny. Forty Kilos.

Doctor: Right Danny - wed better give you something to ease the pain. How much did we say you weighed?

Dan: 40 kilograms (looking exasperated)

Doctor: Well yes....we need your weight so we can calculate the exact amount of painkiller for you. (to nurses) Give him 15 milligrams IV pethidine.

Nurse 2: Cathy can you check again the standard dose with me? (Nurse 3 (Cathy) looks up the MIMS with Nurse 2 looking on.)

Cathy: point five to 1 milligrams per kilogram dose for IV point five times 40 is 20 milligrams. Its a bit on the low side. Is that OK? (To the doctor)

Doctor: Yep. Thats fine.

Nurse 2: Weve got to give 15 milligrams from the ampule. OK, lets work out how much we have to draw up.

Susan: I thought youd just worked that out.

Nurse 2: We worked out a weight. We need to know the volume to draw from the ampule. We have to calculate the amount.

Susan: Oh.

Nurse 2: Lets see. Theres 50 milligrams in a millilitre (Nurse 2 does the calculation on paper) so for 15 milligrams we divide 15 by 50 then multiply that by one millilitre - equals point three).

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(Cathy and Susan look on and agree. Susan still looks confused but goes along with the other nurses. (The calculation reads 15 mg/50 mg x 1 mL = 0.3) Then Nurse 2 draws up 0.3 mL from the ampule into a syringe and prepares to give it to Dan in a vein....scene fades)

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Exercise
  1. What drug were they using?

  2. Why were they interested in Daniels (the patients) weight?

  3. What was the nurse doing with the book in her hand?

  4. What was the strength of the drug?

  5. How much drug did the doctor order?

  6. Can you explain the calculations performed?

  7. What was the solution?

  8. What were the units?

  9. Do you think this patient would be out of pain?

12.2 Drug calculation questions

Lets have a closer look at some drug questions. One of the most important skills you need is being able to interpret the question correctly. Try this exercise.

When you perform drug calculation problems you will need to interpret what type of question it is. Have a look at the flow chart below. You start by asking yourself what type of drug therapy it is. Look for phrases like volume to deliver, infused over two hours; calculate the rate etc. All these involve infusion therapy. This will be a RATE. You then need to identify the type of infusion - will it be in mLs per hour or drops per minute? The photo to the left will deliver mL per hour. The one on the right will deliver in drops per minute.

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If it is a medication, then it may be tablets or liquid. Both the liquid and the tablets have different strengths and you need to identify the strength.

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For example on the left the label states:

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So the container itself has 10 millilitres of fluid. In each millilitre there is 100 Units of Protaphane.

On the right the box states:

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So the box has 30 tablets. In each tablet there is 40 milligrams of the drug atorvastatin.

In drug calculation written problems you also need to highlight the information given in the problem and determine if any information is irrelevant.

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Example

Calculate the volume of Diamorphine you would need to draw up from stock available in 10mcg per 2 mL of solution, to give the ordered dose of 1.5mcg.

  1. Is it a rate or a medication?

  2. What unit will the answer be in?

  3. Can you identify a dose required? (what you have to give to the patient?)

  4. Can you identify a dose in stock? (what is available?)

  • This is a liquid medication (in millilitres)

  • The answer will be in mL.

  • The patient needs 1.5 micrograms of the drug

  • Available is 10 micrograms of the drug in 2 mLs of solution.

If you are a visual learner, you may like to colour code the important parts of the question.

Calculate the volume of Diamorphine you would need to draw up from stock available in 10mcg per 2 mL of solution, to give the ordered dose of 1.5mcg.

Here I have coloured the word volume (suggesting it is a liquid medication); the phrase available in 10 mcg (dose in stock); ordered dose of 1.5 mcg (dose required); and per 2 mL (the strength)

For your interest the solution to this problem is:

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The last step in this would be to think about the solution. Does this appear to be a reasonable answer? Well 2 mL of the drug delivers 10 mcg so 0.2 mL will deliver 1 mcg (dividing both sides by 10) so yes this does sound reasonable.

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Exercise

In each of the following questions answer the following (You do NOT have to solve the problem)

  1. Is it a rate or a medication?

  2. What unit will the answer be in?

  3. Can you identify the variables? Eg if it is a medication:

  • Can you identify a dose required? (what you have to give to the patient?)

  • Can you identify a dose in stock? (what is available?)

  1. If it is a rate:

  • Can you identify the total volume? (in mLs or drops)

  • Can you identify the time (in hours or minutes)

    1. Calculate the volume per hour (mL/h) that you need to deliver if a patient is prescribed an infusion. The volume of the infusion is 500 mL over an 8 hour period.

    2. You have to run an infusion of 500 ml Dextrose/Saline over the next 3 hrs. How many ml per hour will you be infusing at. How many drops per minutes will the infusion be set to if you are using a giving set of 20 drops/ml.

    3. Your patient is prescribed Lasix 80 mg IV stat this morning. The available dose is 10 mg/mL. How many mL will you draw up?

    4. The doctor orders a volume of 250mL to be infused at 30 mL/hr. You start this infusion at 12 noon. At what time will this infusion be complete?

    5. You patient had Hartmans 1000mL started at 12 noon in the theatre recovery. This was to run for 6 hours. When he returned to the ward at 14:30 there was 650mL remaining in the bag. Is the infusion running on time? Calculate the new rate (20 drops/mL) so that your patient receives his IV on time.

    6. Calculate the flow rate (in drops per minute) which is necessary to deliver 2 litres in 16 hours with a drop factor of 20 drops/mL.

    7. Your Patient is prescribed Heparin of 23000 units over 24 hrs. You hold a stock of Heparin which comes in 5000 units/mL and 1000 units/mL. What volume of Heparin will you draw up?

    8. In order to improve the renal perfusion the doctors have prescribed an infusion of Dopamine to be titrated at 3 mcg/kg/min. Your patient's weight this morning is 85 kgs. How many micrograms will your patient receive each minute?

      8b. The ward stock of Dopamine is 200 mgs in 10 mL. To restrict fluid overload you are asked to mix 300 mgs in 100 mL of 5% Dextrose. How many mL will your patient be receiving each hour?

    9. Your patient is written up for IV Metronidazole 500 mg every 8 hrs. This comes pre-packed in 100mL bag (Metronidazole 500mg/100 mL). The MIMS suggest that this drug should be administered at a rate of 5 mL/min. Over what period of time will you administer the Metronidazole?

    10. The doctors have ordered an infusion of 1500mL of 5% Dextrose to be given at the rate of 50 drops per minute. Assuming a drop factor of 20 drops per mL, how many hours will it take for this infusion to complete?

Solutions can be found here

Conclusion

  • Can you now:

  • Read drug calculation problems correctly

  • Recognise the different types of drug calculations

  • Recognise the solutions and units needed in drug calculation problems?

If you can and you have also successfully completed the CMA test in this course, you are now ready to do the course NUR1090. Good luck!graphics10