Chapter 10 Dosage Calculations

Objectives Define key terms. Use dimensional analysis to calculate dosages for administering drugs accurately. Use the formula method to calculate dosages accurately. Use fractions to solve dosage calculations accurately.

Objectives Use ratios and proportions to solve dosage calculation problems by cross-multiplying to calculate or check a dosage given. Correctly solve word problems. Accurately calculate dosages ordered by weight. Use a child’s weight to calculate the desired dosage correctly.

Objectives Use a body surface area nomogram to calculate pediatric dosages. Discuss how pediatric patients differ from adults. Correctly reconstitute powdered medication and calculate desired dosage. Use dimensional analysis to calculate IV drip rates accurately. Calculate fluid intake accurately.

Methods for Calculating Drug Dosages Dimensional analysis Formula Fractions Ratio and proportions Word problems

Dimensional Analysis Write the unit of the dose ordered. Write the units that are on the label and the unit you plan to give the patient. Fill in the numbers, after you set up the problem. Calculate. Check your work for reasonableness.

Formula Method D/H × Q = Answer D is the dimension ordered. H is what you have on hand in that dimension. Q is the quantity of the unit you are giving. The answer is in the same unit as Q.

Fraction Method Dosage on hand (H)/Dosage unit (Q) = Desired dose(D) /Dose given OR Ordered dosage unit (D)/label dosage unit (H) = Dose given/Quantity on hand (Q)

Critical Thinking Does it make any difference if you align the units on one side and desired units on the other? Explain.

Using Several Methods Healthcare professionals usually find a method they prefer to use and use it for all calculations. Preference may depend on how your brain may have first learned how to calculate. You can use a different method to check your work.

Critical Thinking Can you use the fraction method to check the accuracy of other methods?

Ratio and Proportions Set the equation up with label on one side and desired on the other. e.g., 250 mg : 1 mL :: 500 mg : ? Multiply the means (middle section) and the extremes (far ends) and solve for ? e.g., 250 × ? = 1 × 500 ? = 2 mL

Word Problems Dimensional analysis If the prescriber orders ___ mg, multiply the order by the amount of tablets, caplets, milliliters, etc. on the label and divide by the number of milligrams on the label.

Formula Method If the prescriber orders _________ mg, divide that number by the milligrams on the label and multiply by the unit per milligram.

Fraction Method If the prescriber orders ______ mg, place it over the milligrams on the label. Place the quantity of units on the label and write it as the denominator of a fraction on the other side of the equals sign. Then find what numerator makes the two fractions equal.

Ratio and Proportions Express the label quantities as a ratio of mg : units. On the other side of the two colons, place the prescriber’s order in milligrams. Solve for the units that you should give by multiplying the extreme (far right and left) amounts together and the mean (inside right and left) amounts together—solving for what you need to give.

Special Circumstances

Weight-Based Calculations in Adults and Children Find weight in kg. Multiply by ordered mg/kg/day. Adjust if dosage is required: bid 2 × per day tid 3 × per day qid 4 × per day

Critical Thinking You may find it bothersome that dosages sometimes come out to be odd numbers relative to the usual dosages—for example, when the dose is 337.5 mg/dose. If the medication comes in 200 mg tablets, what would you do?

Calculations Using Body Surface Area Find child’s height and weight on BSA chart (Figure 10.4). Put a ruler or piece of paper on those two values. Record the point at which they intersect. Place that number over the average 1.7 and multiply by the adult dose to get the child dose.

Reconstituting Powders Reconstitute as specified on the label and then use the reconstituted ratio for the calculations.

Calculations of IV Dosages Electronic regulator pumps Total mL ordered/Total time ordered in hours = mL/hour (rounded to whole number)

Critical Thinking In the above case, where the flow rate has not been consistent, you see that the IV insertion site is swollen. Would you increase the flow rate to make up the difference? What would you do?

Manual IV Sets Total volume (V) to be infused (mL) divided by total time in minutes × Drop factor (D) in gtts/mL = Rate of flow (R) in gtts/minute

Critical Thinking Because the actual order is the same, the real factor in these calculations is the drip rate. Which would drip faster at the same hourly rate—a macrodrip or a microdrip?

Fluid Balance Add up the total fluids consumed in oz. Multiply this number by 30 to get mL. (1 oz = 30 mL) Compare this total with the fluid balance order. If greater than order, patient has exceeded it. If less than order, patient has not.

Critical Thinking What instructions do you need to give this child’s parents? (page 196)

Hydration Patients may have restricted fluids if they are storing fluids inappropriately, such as with congestive heart failure. Patients who are dehydrated may have, instead, a plan to increase fluids to exceed a certain threshold. This patient may be required to drink more than 1,500 mL per day, for example.

Critical Thinking Coffee, caffeinated sodas, and beer have a diuretic effect on the kidneys, meaning that they increase urination. Although consumption of these fluids does count as hydration, what would be better choices for dehydrated patients?

Summary What new piece of information in this chapter were you most interested to learn? What questions do you still have about the information in this chapter? Return to Objectives to determine extent of learning.