Overview
This lecture explains how to calculate weight-based medication dosages, including key formulas, step-by-step examples, and important considerations for both children and adults.
Why Weight-Based Dosages are Used
- Pediatric doses are weight-based due to wide size differences among children.
- Some adult drugs require weight-based dosing due to fat solubility or a narrow therapeutic index (small dose changes have large effects).
Steps for Weight-Based Dosage Calculations
- Convert patient weight from pounds to kilograms (1 kg = 2.2 lbs).
- Multiply weight in kg by dose in mg/kg (or mg/kg/day) to get the total dose.
- If the dose is per day and divided into multiple doses, divide the total daily dose by the number of doses per day.
Converting Pounds to Kilograms
- Divide weight in pounds by 2.2 to get kilograms.
- Example: 150 lbs ÷ 2.2 = 68.2 kg.
Calculating Total and Per Dose Amounts
- For mg/kg dosing, multiply kg by mg/kg to get mg per dose.
- For mg/kg/day dosing with multiple doses, divide total mg/day by number of doses.
- Example: 75 mg/kg/day for 13.6 kg child = 1,020 mg/day; divided by 4 (qid) = 255 mg per dose.
Example Problems
- Lovenox 1.5 mg/kg qd for 176 lbs: 176/2.2=80 kg; 80×1.5=120 mg/day.
- Vancomycin 10 mg/kg q12h for 148 lbs: 148/2.2=67 kg; 67×10=670 mg per dose.
- Cephalexin 75 mg/kg/day qid for 30 lbs: 30/2.2=13.6 kg; 13.6×75=1,020 mg/day; 1020/4=255 mg per dose.
- Amoxicillin 50 mg/kg/day q12h for 21 lbs: 21/2.2=9.5 kg; 9.5×50=477 mg/day; 477/2=239 mg per dose.
- To convert mg to mL: set up proportions, e.g., 239 mg/x mL = 400 mg/5 mL, solve for x.
Key Terms & Definitions
- Weight-based dose — Medication dose calculated using patient’s weight.
- mg/kg — Milligrams of drug per kilogram of body weight.
- Therapeutic index — Ratio of toxic dose to effective dose of a drug.
- q6h/q12h/qid/qd — Every 6 hours; every 12 hours; four times daily; once daily.
- IV — Intravenous (into a vein).
- SQ/SQD — Subcutaneous (under the skin)/Subcutaneous daily.
Action Items / Next Steps
- Practice converting patient weights and solving dosage calculations.
- Memorize the conversion: 1 kg = 2.2 lbs.
- Review how to set up and solve dose-to-volume ratios for liquid medications.