## Saturday, January 2, 2016

### Dosage Calculation - Dosage By Weight Questions

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Dosage By Weight Questions
Given the weight of a patient and a dosage specified in terms of weight, calculate the necessary dosage. These problems are a type of pediatric dosage calculations.
Formula:

$\overline{){\mathbf{Weight in Kg}}x{\mathbf{Dosage Per Kg}}={\mathbf{Y\left(Required Dosage\right)}}}$

Example: A doctor orders 200 mg of Rocephin to be taken by a 15.4 lb infant every 8 hours. The medication label shows that 75-150 mg/kg per day is the appropriate dosage range. Is this doctor's order within the desired range?
${\mathbf{Weight in Kg}}x{\mathbf{Dosage Per Kg}}={\mathbf{Y\left(Required Dosage\right)}}$
Convert 15.4 lb to kg.
lb → kg    ( ÷ by 2.2 )
15.4 lb ÷ 2.2 = 7 kg

7 kg x 75 mg/kg = 525 mg (Minimum Desired Dosage)
7 kg x 150 mg/kg = 1,050 mg (Maximum Desired Dosage)
24 hours in one day and the medication is ordered every 8 hours.

24 hrs / 8 hrs = 3 times per day doctor ordered medication
200 x 3 = 600 mg ordered per day
600 mg is within the desired range of 525-1,050 mg

$\overline{){\mathbf{\text{Yes}}}}{\mathbf{\text{doctor has ordered a dosage within the desired range.}}}$

Example: Solumedrol 1.5 mg/kg is ordered for a child weighing 74.8 lb. Solumedrol is available as 125 mg / 2mL. How many mL must the nurse administer?
${\mathbf{Weight in Kg}}x{\mathbf{Dosage Per Kg}}={\mathbf{Y\left(Required Dosage\right)}}$
Convert 74.8 lb to kg.
lb → kg    ( ÷ by 2.2 )
74.8 lb ÷ 2.2 = 34 kg

${\mathbf{34 kg}}x{\mathbf{1.5 mg/kg}}={\mathbf{51 mg}}$
This is now an ordinary Mass/Liquid For Liquid Question. 51 mg is ordered and the medication is available as 125 mg / 2 mL.
$\frac{{\mathbf{Ordered}}}{{\mathbf{Have}}}x{\mathbf{Volume Per Have}}={\mathbf{Y\left(Liquid required\right)}}$
$\frac{{\mathbf{51 mg}}}{{\mathbf{125 mg}}}x{\mathbf{2 mL}}=\overline{){\mathbf{0.82 mL}}}$
Alternatively, 125 mg = 2mL
Therefore 51 mg = 2/125 x 51 = 0.82 mL

Question (6): (From Ngee Ann Polytechnics Dosage Calculation Worksheet)
ORDER: Drug G 5 mg per kg body weight P.O.
AVAILABLE: Drug G 250 mg per 5 mL
BODY WEIGHT: 500 kg

${\mathbf{\text{GIVE:}}}\overline{)\phantom{\rule{30pt}{12pt}}}{\mathbf{\text{mL}}}$
${\mathbf{Weight in Kg}}x{\mathbf{Dosage Per Kg}}={\mathbf{Y\left(Required Dosage\right)}}$
${\mathbf{50 kg}}x{\mathbf{5 mg/Kg}}={\mathbf{250 mg}}$
$\frac{{\mathbf{Ordered}}}{{\mathbf{Have}}}x{\mathbf{Volume Per Have}}={\mathbf{Y\left(Liquid required\right)}}$
$\frac{{\mathbf{250 mg}}}{{\mathbf{250 mg}}}x{\mathbf{5 mL}}=\overline{){\mathbf{5 mL}}}$
Alternatively, 250 mg = 5 mL
Therefore 250 mg = 1/250 x 250 = 5 mL

Question (10): (From Ngee Ann Polytechnics Dosage Calculation Worksheet)
ORDER: Drug N 15 mg per kg body weight P.O.
AVAILABLE: Drug N 60 mg per mL
BODY WEIGHT: 100 kg

${\mathbf{\text{GIVE:}}}\overline{)\phantom{\rule{30pt}{12pt}}}{\mathbf{\text{mL}}}$
${\mathbf{Weight in Kg}}x{\mathbf{Dosage Per Kg}}={\mathbf{Y\left(Required Dosage\right)}}$
${\mathbf{100 kg}}x{\mathbf{15 mg/Kg}}={\mathbf{1,500 mg}}$
$\frac{{\mathbf{Ordered}}}{{\mathbf{Have}}}x{\mathbf{Volume Per Have}}={\mathbf{Y\left(Liquid required\right)}}$
$\frac{{\mathbf{1,500 mg}}}{{\mathbf{60 mg}}}x{\mathbf{1 mL}}=\overline{){\mathbf{25 mL}}}$
Alternatively, 60 mg = 1 mL
Therefore 1,500 mg = 1/60 x 1,500 = 25 mL

Continue (Go to IV Terms & Abbreviations)

Reference