# Pharmacy Rx

Ratio & Proportion Pediatric Dosages

The Ratio & Proportion Method is so easy to calculate dosages, all you have to do is cross multiply and divide. Let's start off with some examples!

Ex 1) Order: Give "Drug A" IV piggyback for a child weighing 66 Ib, at a dosage of 50 mg/kg/day in divided doses q6h. How many mg would each dose contain?

Step 1: Convert the divided doses q6h

24 / 6 = 4x a day

Step 2: Convert Ib to Kg

66 Ib / 2.2 = 30 Kg

Step 3: Figure the amount of mg given a day for a 30 Kg individual by using the Ratio & Proportion

If      50 mg = 1 kg

Then    X mg  = 30 kg    ----<   50 mg x 30 kg / 1 kg = 1500 mg/day

Step 4: Figure the amount of mg given per dose for a 30 Kg individual by dividing

1500mg / 4 (q6h) = 375 mg/dose

How many mg would each dose contain? 375 mg  Answer

Ex 2) Order: Cefadyl 5 mg qid IM for a child weighing 44 Ib. The PDR recommends 1-3 mg/kg/day. Is the ordered dose safe?

Step 1: Decipher qid

4x a day

Step 2: Convert Ib to Kg

44 Ib / 2.2 = 20 Kg

Step 3: Calculate doctor's total mg given a day from the order

5 mg x 4 = 20 mg/day

Step 4: Calculate 20 Kg into the PDR recommended dose range by using the Ratio & Proportion

If    1 mg = 1 kg

Then  x mg = 20 kg  ---<  1 mg x 20 kg / 1 kg = 20 mg/day

If     3 mg = 1 kg

Then   x mg = 20 kg  ---< 3 mg x 20 kg / 1 kg = 60 mg/day

Step 5: Recommeded dose range for a 20 kg individual is

20 mg - 60 mg per day

Step 6: Is the order safe?

Well the doctor's total order dose is 20 mg/day (from Step 3) so YES it is safe because 20 mg/day falls within the recommended dose range of 20 mg - 60 mg per day (from Step 5).

Ex 3) Order: Tetracycline Elixir qid for a child weighing 50 Ib at a dosage of 8 mg/kg/day in equally divided doses. How many mg would one dose contain? Tetracycline is supplied at 50 mg per 7 ml. How many ml should be administered?

Step 1: Decipher qid

4x a day

Step 2: Convert Ib to Kg

50 Ib / 2.2 = 22.7 kg

Step 3: Figure the amount of mg given a day for a 22.7 Kg individual by using the Ratio & Proportion

If         8 mg = 1 kg

Then    x mg = 22.7 kg  ---<  8mg x 22.7 kg / 1 kg = 181.6 mg/day

Step 4: Figure the amount of mg given per dose for a 22.7 Kg individual by dividing

181.6 mg / 4 (qid) = 45.4 mg/dose

How many mg would one dose contain? 45.4 mg  Answer

Step 5: Figure the amount of ml to be administered by using the Ratio & Proportion

If         50 mg = 7 ml

Then    45.4 mg = x ml    ---<  45.4 mg x 7 ml / 50 mg = 6.4 ml

1.) Order: Ampicillin IVPB for a child weighing 55 Ib at a dosage range of 20 mg/kg/day in 4 equally divided doses. How many mg would be given per dose? The Ampicillin is supplied as 250 mg in 1.5 ml after reconstitution, give ________ ml.

2.) Order: Garamycin 35 mg IM q6h for a child weighing 42 Ib. The recomended dose is 6-7.5 mg/kg/day given at 6 hour intervals. Is the order within the safe range?