Dosages Based on Body Weight Amount of medication per Weight of the patient Unit of time Learning Outcome: 17.2 Calculate safe dosages based on body weight. Medication orders for pediatric and geriatric orders are frequently based on weight. mg/kg/h

Dosages Based on Body Weight (cont.) Rule 17-2 Calculating dosage based on body weight: Step A Convert: patient’s weight to kilograms Round to nearest tenth Learning Outcome: 17.2 Calculate safe dosages based on body weight. Based on facility policy, weight may be rounded to the whole number, tenths, or hundredths.

Dosages Based on Body Weight (cont.) Rule 17-2 (cont.) Step B Calculate: the desired dose D by multiplying dose ordered by the weight in kilograms such as Learning Outcome: 17.2 Calculate safe dosages based on body weight.

Dosages Based on Body Weight (cont.) Rule 17-2 (cont.) Step C Think!...Is It Reasonable? Confirm whether or not the desired dose is safe by checking the label, package insert, or product literature. Learning Outcome: 17.2 Calculate safe dosages based on body weight. If the dose is unsafe, contact the AP.

Dosages Based on Body Weight (cont.) Rule 17-2 (cont.) Perform dosage calculation. Calculate the amount to administer, using proportion method, dimensional analysis, or the formula method. Learning Outcome: 17.2 Calculate safe dosages based on body weight.

Dosages Based on Body Weight (cont.) Calculate the amount to administer to a 3 yr old weighing 34 lb. Ordered: hysocyamine sulfate 5 mcg/kg subcut 1 h pre-anesthesia On hand: hysocyamine sulfate 0.5 mg/mL Example Learning Outcome: 17.2 Calculate safe dosages based on body weight.

Dosages Based on Body Weight (cont.) Step A Convert 34 lb to kg = 15.5 kg Step B Calculate desired dose: 5 mcg / kg x 15.5 kg = 77.5 mcg 77.5 mcg = 78 mcg = D Example Learning Outcome: 17.2 Calculate safe dosages based on body weight. 1 kg : 2.2 lb = x : 34 lb cancel units - lb 2.2 X x -= 1 kg x 34 2.2 x / 2.2 = 34 kg / 2.2 x = 15.45 = 15.5 kg

Dosages Based on Body Weight (cont.) Step C Confirm desired dose is safe. Based on PDR : 5 mg / kg is the recommended dose for a child over 2 years old. Example (cont.) Learning Outcome: 17.2 Calculate safe dosages based on body weight. Back to Order

Dosages Based on Body Weight (cont.) Find The amount to administer: Step A first factor 1 mg/1000 mcg Step B Calculate amount to administer. A mL = 1 mg /1000 mcg x 1 mL / 0.5 mg x 78 mcg/ 1 Amount to administer = 0.156 mL = 0.16 mL Step C 0.78 is 0.16 of 5, so 0.16 mL is reasonable. Example (cont.) Learning Outcome: 17.2 Calculate safe dosages based on body weight. Dimensional analysis is used here. First factor is the conversion factor for mcg to mg. Second factor is dose on hand. Third factor is desired dose over 1. Can also perform using proportion method and formula method. Back to Order

Dosages Based on Body Weight (cont.) Pediatric injection volume is limited based on the size and age of the child. The length and gauge of the needle also varies with the age and size of the patient and the location of the injection. Learning Outcome: 17.2 Calculate safe dosages based on body weight. Smaller muscles need smaller needles. With geriatric patients, the depth of the injection may vary due to decreased muscle size. Refer to Table 17-5 for Pediatric Injections.

Ensuring Safe Dosages Drug orders may be written in several ways. Check to see if dose ordered is a standard recommended dose: not less than the minimum; not greater than the maximum. Learning Outcome: 17.2 Calculate safe dosages based on body weight.

Ensuring Safe Dosages (cont.) Rule 17-3 Ensuring Safe Dosages When working with special populations, always check the package insert, drug label, or product literature to ensure the safety of the dose to be administered. Learning Outcome: 17.2 Calculate safe dosages based on body weight.

Ensuring Safe Dosages (cont.) Determine if dose is safe for a child weighing 38 lb. If safe, calculate the amount to administer. Ordered: Pediazole suspension 5 mL qid On hand: Pediazole susp (combination drug) Erythromycin 200 mg / 5 mL Sulfisoxazole 200 mg / 5 mL Example Learning Outcome: 17.2 Calculate safe dosages based on body weight.

Ensuring Safe Dosages (cont.) Step A Convert 38 lb to kg = 17.3 kg Step B Calculate recommended dose. Package insert : recommend dose for erythromycin ethylsuccinate = 50 mg/kg/d 50 mg / 1 kg x 17.3 kg = 865 mg Example Learning Outcome: 17.2 Calculate safe dosages based on body weight. 1 kg : 2.2 lb = x : 38 lb cancel units - lb 2.2x =38 x 1 kg x = 38 kg/ 2.2 x = 17.27 kg = 17.3 kg

Ensuring Safe Dosages (cont.) recommended dose for sulfisoxazole acetyl = 150 mg/kg/d 150 mg/1kg x 17.3 kg = 2595 mg Example Learning Outcome: 17.2 Calculate safe dosages based on body weight.

Ensuring Safe Dosages (cont.) Step C Think!...Is It Reasonable? Order is for 5 mL 4 times a day. Daily dose of erythromycin = 800 mg/day Falls under maximum dose of 865 mg/day Daily dose of sulfisoxazole = 2400 mg/day falls under maximum dose of 2595 mg/day Example Learning Outcome: 17.2 Calculate safe dosages based on body weight. Since dose is safe, you may administer medication. Order is already written as a volume of 5 mL, so no further calculation is needed.

A baby who weighs 8 lb 6 oz does not weigh 8.6 lb. Error Alert ! Convert ounces carefully. The weight of babies may be measured in pounds and ounces or grams. 16 oz = 1 lb, an ounce is not a tenth of a pound. A baby who weighs 8 lb 6 oz does not weigh 8.6 lb. Learning Outcome: 17.2 Calculate safe dosages based on body weight. Convert 6 ounces to pounds. Therefore 8 lb 6 oz = 8.375 lb = 8.4 lb

Practice Determine whether the following order is safe. If safe, calculate the amount to administer. For a child weighing 14.5 kg Ordered: Amoxil 175 mg PO q8h On hand: 200 mg/5 mL Usual dose 20-40 mg/kg day Learning Outcome: 17.2 Calculate safe dosages based on body weight. Step A No conversion is needed. Step B Calculate. 20 mg/1 kg x 14.5 kg = 290 mg [cancel units – kg] 40mg/1 kg x 14.5 kg = 580 mg 75 mg x 3 doses/day = 525 mg Step C Compare to recommended dosage. Ordered dose falls within recommended range. Step B H – 200 mg Q – 5 mL D – 175 mg 175mg/200 mg x 5 mL = A [cancel units – mg] 0.875 x 5 mL = 4.357 = 4.4 mL = A Step C Desired dose is slightly over ¾ ordered dose, and 4.5 mL is just over  of 5 mL, so it is reasonable. Answer: Dosage falls within range. Amount to administer is: 4.375 mL (round to 4.4 mL)

Apply Your Knowledge True or False A baby who weighs 6 lb 8 oz weighs 6.8 lb. A baby who weighs 9.5 lb weighs 9 lb 8 oz. Answer: False Learning Outcome: 17.2 Calculate safe dosages based on body weight. 8 oz x 1 lb/16 oz = 0.5 So weight is 6.5 lb Answer: True