2.4 Dosage Calculations

Measuring medications accurately is crucial for safe and effective treatment. Nurses must master various measurement systems, including metric, apothecary, and household, to ensure proper dosing. Converting between systems and interpreting drug labels are essential skills for administering medications correctly.

Calculating drug dosages requires careful attention to detail and mathematical precision. Nurses use formulas to determine the right amount of medication based on prescribed doses, available strengths, and patient factors. Understanding different calculation methods for oral, parenteral, and other routes of administration is vital for patient safety.

Measurement Systems and Calculations

Measurement system conversions for dosing

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• Metric system
  • Based on powers of 10 for easy conversions (move decimal point)
  • Units measure length (meter), volume (liter), and mass (gram)
  • Prefixes indicate magnitude (kilo = 1000, milli = 0.001)
  • Convert by moving decimal point based on prefix (1 kg = 1000 g)
• Apothecary system
  • Used in compounding medications for precise measurements
  • Units include grain (60 mg), dram (3.9 g), and minim (0.062 mL)
  • Conversions involve specific factors (1 fl oz = 29.6 mL)
  • Essential for creating personalized medication formulations
• Household measurements
  • Approximate measurements for liquid medications (teaspoon = 5 mL)
  • Units include drop, tablespoon (15 mL), and cup (240 mL)
  • Useful for patient education and adherence to medication regimens
  • Imprecise compared to metric and apothecary systems
• Conversion between systems
  • Use conversion factors to switch between measurement systems
  • Example: Convert teaspoons to milliliters (2 tsp = 2 × 5 mL = 10 mL)
  • Ensures accurate dosing when different systems are encountered

Critical information from drug labels

• Drug name identifies medication (generic and brand names)
• Dosage form specifies physical properties (tablet, capsule, liquid)
• Strength quantifies active ingredient (mg, mcg, mEq)
• Route of administration indicates method of delivery (oral, IV, IM)
• Expiration date ensures potency and safety (discard if expired)
• Storage requirements maintain stability (room temp, refrigeration)
• Warnings and precautions highlight potential risks and interactions
• Directions for use provide dosing instructions
  • Dose specifies amount of medication per administration
  • Frequency indicates how often to administer (twice daily)
  • Duration of therapy defines length of treatment (7 days)
• Lot number and manufacturer info enable tracking and reporting

Drug dosage calculation methods

• Dosage calculation formula determines amount of medication needed
  • $Desired\ dose = \frac{Ordered\ dose}{Available\ dose} \times Quantity$
  • Aligns prescribed dose with available product strength
• Oral medications involve counting tablets/capsules or measuring liquids
  • Tablets/capsules: Calculate number needed based on strength and dose
  • Liquids: Use formula to determine volume based on concentration
• Parenteral medications require precise calculations for injections and IVs
  • Injections: Calculate volume based on concentration and dose
    • $Volume\ of\ injection = \frac{Ordered\ dose}{Concentration}$
  • IV infusions: Determine flow rate and duration based on dose and concentration
    • $Flow\ rate = \frac{Total\ volume}{Duration}$
    • $Duration = \frac{Total\ volume}{Flow\ rate}$
• Other routes (transdermal, rectal, topical) have specific instructions
  • Follow provided directions for administration
  • Apply dosage calculation formula when appropriate
• Dosage conversions enable switching between units of measurement
  • mg to mL: $Volume\ (mL) = \frac{Dose\ (mg)}{Concentration\ (mg/mL)}$
  • mL to drops: $Drops = Volume\ (mL) \times \frac{Drops}{mL}$ (dropper-dependent)
• Verify calculations to prevent medication errors and ensure patient safety
• Weight-based dosing calculates medication amounts based on patient's body weight

Advanced Calculation Techniques

• Dimensional analysis: A problem-solving method that uses unit conversion factors to calculate dosages
• Ratio and proportion: A technique for solving dosage problems by setting up equivalent ratios
• Rounding rules: Guidelines for determining the appropriate number of decimal places in medication calculations
• Significant figures: The number of digits that carry meaning in a measurement or calculation, ensuring precision in dosage determinations