Med Calc: Dosage Calculations

Unit 4 Dosage Calculations Welcome to Unit 4. It’s time for us to learn about the details of how we will use the principles gleaned from the first 3 units and now calculate correct dosages and solutions for our clients. As we mentioned earlier, we will continue to use the principles of Basic Math, Ratio and Proportions, Conversions and now learn about using the Formula. We will continue the principles of dimensional analysis and ratio and proportion to calculate amounts and doses to be given to a variety of clients. We are going to encourage you to continue to pay attention to numbers and units and treat them the same in our mathematical processes. We will also continue the use of the ? as our unknown to avoid confusion with a multiplication sign (x). Ok, let’s get rolling ;-)

WHERE DO I START TO DO DRUG CALCULATIONS? When we are working with clients, they frequently need medications as part of their treatment. Nurses provide many non-drug services and therapies but medications can be the most frightening to the new nurse. It IS a huge responsibility to safely administer medications! The health care team is responsible for all aspects of a client’s care but doctors, nurses, nurse practitioners, dentists and physician assistants all work together to complete the cycles of medication therapies. It requires accurate assessment, diagnosing, prescribing for the identified problems and then the accurate dosing and administration of prescribed medications and the watchful eyes to determine if the drugs are safe and therapeutic. Therapeutic is that the drugs are acting in a way to improve function or alleviate the conditions for which those drugs have been prescribed. Dosage calculations can be accomplished in multiple ways. Your text (Section 4 and Chapter 11 illustrate use of ratio and proportion to determine correct dosages of medications. In Chapter 12, they discuss “a formula method” for another means of calculating dosages for the clients. We have thought long and hard about ways to simplify and make the dosage calculations safer and as fool proof as possible for our future nurses. The basic formula identified in your text (page 158) is one means of calculating desired volumes (liquids, capsules or tablets). You may learn this formula and use it for most of the calculations you will be required to do. However, it assumes that the desired volume will be the only unknown you will need to determine. It will require significant thought and determination to undo the formula and

determine the desired dose of the drug if the physician orders the dosage in cc or teaspoons. This formula also requires that you recognize and complete conversions prior to use of the formula. This may be cumbersome for some and introduces new opportunities to make errors. The formula from Chapter 12 in your text identifies the same 4 critical pieces of data that we will use. D = the Desired dose as prescribed in a measure of weight (mg, g, gr, etc) H = the strength of the drug on Hand in a measure of weight (mg, g, gr, etc) Q = stands for the quantity of the drug on hand and available for use. This is a volume measure (mL, tabs, caps, tsp, etc) X = the unknown… but be cautious. This refers to an unknown VOLUME that should be given to administer the desired dose. The formula in the book is represented by:

We will teach you a slightly different means of using a standard, easy-to-remember formula in the format of a ratio and proportion that allows you to consistently identify the critical pieces of information needed to compute the correct dosages, take short cuts for conversions and uses the same ratio and proportion strategies that you have already mastered. So what are the 4 elements that we need to complete our drug calculations? As you can see from our previous discussions we need to know: Desired Dose = (always measures of weight; ordered by the doctor or others). Have Dose = (always measures of weight; what you have available to give). Desired Amount = (always measures of volume; the amount you will give). Have Amount = (always measures of volume; the amount you have available). Please note that we will always have 3 of these 4 elements available and 1 unknown. Generally, the unknown will be the Desired Amount but regardless of what is unknown you will use the same processes to determine the unknown element… ie ? So what is the magic formula…taaaa…. daaaaa…. Ladies and Gentlemen…introducing:

Ok, Ok, so I took a little theatrical liberty with the introduction of the formula…but note that it’s not all that different from the text except that you can use exactly the same processes as we did with ratio and proportions and conversions to determine any of the 4 elements that may be unknown. Let’s analyze the formula a bit. Note that you can readily divide the formula into 4 quadrants. On the upper 2 quadrants you will find values that are “DESIRED”. Those elements reflect what the doctor/other prescriber has ordered. On the bottom 2 quadrants you find values that are “HAVE”. Those elements are what you have available and sometimes it helps to remember it’s in the bottom drawers of your medication cart…those on the bottom of the formula. On the left 2 quadrants (upper and lower), you will find values that are “DOSEs”. Dose elements are always measures of WEIGHT for the drug. These are the measures in the middle of your conversion chart such as g, gr, mcg, mg, etc. And last but not least….On the right 2 quadrants (upper and lower), you will find values that are “AMOUNTs”. Amount elements are always measures of VOLUME of the drug. These include liquid measures but don’t forget that capsules and tablets are also measures of volume.

NOTE: THE DOCTOR ORDERS “PER DOSE” AND THE FORMULA IS “PER DOSE.” Let’s work an example of how to use “The Formula”!!

Dr. Comfort has ordered Zantac 70 mg IM now for Mrs Quirk. When you go to the patient’s supply of medications, you find that the pharmacy supplied Zantac in a 10 mL vial with a strength of 25 mg/mL. How much Zantac will you prepare and give to Mrs Quirk? Let’s first read and identify what we are being asked to determine. Is this a conversion problem or a formula problem? Recall that a conversion problem is asking us to convert between 2 of the same type of measures… volume, weight or length. The formula problem is asking us to translate the doctor’s order into a form that we can administer. With the formula you will find 2 units of volume and 2 units of weight with one of them unknown.

In our sample problem, we can identify a doctor’s order (Zantac 70 mg) in a weight form. We are being asked to determine “how much” we should give. It seems to imply that we are searching for a volume. Let’s look a little further. Zantac is in the drawer in a 10 mL bottle and the concentration of the drug in those 10 mL is 25 mg per mL (also written 25 mg/mL). While some may think we need to calculate the 10 mL as the HAVE AMOUNT, that is the total amount in the vial, not the concentration of the drug (25 mg/mL). Don’t be distracted by extra information. HAVE AMOUNT generally is found by reading the problem and hearing something similar to “the drug is available as 25 mg PER mL.” That per is your clue that that is the unit of the “have amount” rather than being confused by the total volume available in a vial. So let’s set up the formula:

Go back to the original formula and you find the units associated with ? are mL, so the answer is you would prepare and give 2.8 mL of Zantac IM to Mrs Quirk.

TIME FOR SOME PRACTICE PROBLEMS Stop here and work some of the practice problems with the unit and in your textbook. Use the labels on the practice problems as your Have supply and use the formula to determine how much of the drug you will administer. Don’t try to work the “Pediatric” (dose by the client’s weight) problems yet, we’re going to talk about those next. What questions do you have after a little practice? Short cuts Did you begin to discover that you can take a short cut with this formula? Note that unlike the

textbook’s formula, THE Formula allows you to do some reduction of units and numbers before you cross multiply. Let’s take a closer look:

Note that ? is already isolated here and after the reduction steps, we merely have to divide 14 by 5 making the math errors less likely and using smaller more manageable numbers. If you feel confident in the process, look for opportunities to take short cuts. If you get stuck, you can always use the rules of ratio and proportion to figure your way to the right answer. [TOP]