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# Dosage Calculation Using Formula Method

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### Dosage Calculation Using Formula Method

1. 1. Calculate with Confidence 5 th edition Gray Morris Mosby items and derived items © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.
2. 2. Dosage Calculation Using the Formula Method Unit Three: Chapter 15 Mosby items and derived items © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.
3. 3. Dosage Calculation Using the Formula Method: Objectives <ul><li>After reviewing this chapter, you should be </li></ul><ul><li>able to: </li></ul><ul><ul><li>Identify the information from a calculation problem to place into the formula given </li></ul></ul><ul><ul><li>Calculate dosages using </li></ul></ul><ul><ul><li>Calculate the number of tablets or capsules to administer </li></ul></ul><ul><ul><li>Calculate the volume to administer for medications in solution </li></ul></ul>
4. 4. Formula for Calculating Dosages <ul><li>Nurse must use formula consistently and in its entirety to avoid errors </li></ul><ul><li>Units of measure must be in same system before solving with the formula </li></ul><ul><li>Memorize the formula </li></ul>
5. 5. Terms in the Formula <ul><li>D = Desired dose </li></ul><ul><ul><li>Ordered in unit of measure desired – mg, units, mEq </li></ul></ul><ul><li>H = Strength available </li></ul><ul><ul><li>Have on hand in unit of measure available – mg, g, units </li></ul></ul><ul><li>Q = Quantity </li></ul><ul><ul><li>Unit of measure that carries what is on hand cited in </li></ul></ul><ul><ul><li>“ H” - tabs, mL, caps </li></ul></ul><ul><li>x = Unknown </li></ul><ul><ul><li>Number of “Q”s needed to give the prescribed dose </li></ul></ul>
6. 6. Steps to Use Formula <ul><li>Memorize or verify formula from resource </li></ul><ul><li>Place info in formula and label terms </li></ul><ul><li>Make sure everything is in same system </li></ul><ul><li>Apply logic test for reasonable answer </li></ul><ul><li>Calculate </li></ul><ul><li>Label answer with correct unit of measure </li></ul><ul><li>Note: Convert apothecary and household to metric equivalents when possible – metric is the principal system used for medications </li></ul>
7. 8. “ Q” Key Points <ul><li>When “Q” is 1 in value, it can be omitted in the equation, but should be included </li></ul><ul><li>CAUTION: “Q” value must be used when it is greater than 1 </li></ul><ul><li>“ Q” and the unknown “ x ” will always have the same unit of measure </li></ul>
8. 10. Convert to Same System First <ul><li>Order: Phenobarbital gr i p.o. at bedtime </li></ul><ul><li>WRONG! </li></ul><ul><li>RIGHT </li></ul><ul><li>Conversion: gr i = 60 mg </li></ul>= x = x
9. 11. Convert Toward No Decimals <ul><li>Order: Augmentin 0.25 g p.o. q8hr </li></ul><ul><li>WRONG! </li></ul><ul><li>RIGHT </li></ul><ul><li>Conversion Factor: 1 g = 1,000 mg </li></ul><ul><li>Therefore, 0.25 g = 250 mg </li></ul><ul><li>X = ½ tab </li></ul>= x = x