[grannypatches]

Help! I have a drug dosage calculations test Wed. If I don't pass this test then I can't complete the last semester of the lpn program. I am a december candidate for graduation. The practice problems we were given to work on has confused me. THere are two questions that no matter what formula I use I don't get the right answer.

#1
Amoxicillin 50mg/kg po per day
Available: Amoxicillin 125g/2ml
Child weighs: 22 lbs.
Give:_______


#2
Order: Amoxicillin 75mg PO every 8 hrs.
Available: Amoxicillin 125mg/5ml
child weight: 70lb
pediatric dose parameter: 20-40 mg/lg in three divided doses

a) How many mg of Amoxicillin would the child receive per day?
b) Is the dose within safe parameters?
c) How many ml of Amoxicillin would the child receive per dose?

Answers given:
#1 8ml

#2 225mg/day
yes
3 ml

The formulas I am using are not giving me the correct answers. There is something I am doing wrong. Can someone plz help me? It would be deeply appreciated.

[Sweetooth EMT-P, RN]

hi,

first you need to convert lbs to kg:

weight in pounds divided by 2.2 = kilogram weight.


multiply kg weight time dose per kilogram and that will give you your mg dose.


when you figure out the total daily mg dose divide that by the number of times per a day the child will receive a dose and that will give you the amount per dose.


* obviously when you find the mg dose you need to figure out how many mLs to give. Figure out how many mg are in 1 ml and then divide ordered dose by whatever is in 1 mL.

** Do not round until you get your final figure


Good luck

Sweetooth

[traumaRUs]

I always use ratio and per cent because I have to keep things simple. So...for the first problem:

Amoxicillin 50mg/kg po per day
Available: Amoxicillin 125g/2ml
Child weighs: 22 lbs.
Give:_______



So, first I would get the child's weight from pounds to kilos:

22 pounds divided by 2.2 = 10 kg

Then, you know the dosage is 50mg per kg per day, so you multiply the 10 kg (child's weight) x 50mg = 500mg per day is the total dosage that the child receives per day. However, my next question is how often is this ordered: 3-4 times per day. The 500mg is what is to be given for the entire day, not in one dose! So..I would ask how often is this to be given???

However, just for the sake of figuring this problem: you have 125mg/2ml (or cc). You need to give 500mg, so you set it up as a ratio:


125mg 500mg
______ : : ______

2ml X

So, you multiply 500 x 2 = 1000
Then, you divide the 1000 by 125 = 8cc which is the dosage that you give.

[grannypatches]

That is what was puzzling me. It did not say how many times a day. It seemed to be in one dose and that seemed to me to be too much. Thanks for all the posts. It has got me started. Have a nice weekend everyone.

[Daytonite]

I like to do these problems by dimensional analysis (factor label method) when they involve applying conversion factors. I still use the formula "dose desired divided by dose on hand" as a guide to what I am doing. However, I manipulate all the terms of the resulting problem to that I am going to end up with the correct label on the final answer.

#1
Amoxicillin 50mg/kg po per day
Available: Amoxicillin 125mg/2ml
Child weighs: 22 lbs.
Give:_______
Answer: 8 mL

I want "mL" as the label on the final answer and I will make sure I set the problem up so that "mL" is going to end up being a label on the number in the final answer. The dose desired is 50 mg/kg and the patient weighs 22 pounds. Already you are going to have to apply a conversion factor to change the "kg" to "pounds".

The dose on hand is 125 mg/2 mL.

If you plug these into the formula "dose desired divided by dose on hand" you end up with a fraction that has a fraction in it's numerator and a fraction in it's denominator (a complex fraction).

50 mg/1 kg (numerator)/125 mg/2 mL (denominator)

The fraction in the denominator is cleared out by multiplying both the numerator and denominator of this complex fraction by another complex fraction that is actually a reciprocal of the fraction in the denominator. This is a mathematical manipulation.

50 mg/1 kg (numerator)/125 mg/2 mL (denominator) X 2 mL/125 mg/2 mL/125 mg = 50 mg/1 kg x 2 mL/125 mg (NOTE: the complex fraction in green actually represents the number 1. The number, 1, can be written as 1/1, 2/2, 10/10, or in this case 2 mL/125 mg/2 mL/125 mg. This is math trickery done to simplify that horribly huge fraction.)

To that equation you now apply your conversion factor as well as the patient's weight and it now will look like this:

50 mg/1 kg (dose desired) x 2 mL/125 mg (dose on hand) x 1 kg/2.2 lb (conversion factor to convert kg to lb) x 22 lb/1 (the weight of the patient) = 2200 mL/275 (what you are left with after cancelling out the duplicated labels in the numerators and denominators and performing the actual math--the mL is left standing alone, but it's what you want in the final answer) which simplifies to 8 mL, your answer, when you divide 275 into the 2200 mL.

#2
Order: Amoxicillin 75mg PO every 8 hrs.
Available: Amoxicillin 125mg/5ml
child weight: 70lb
pediatric dose parameter: 20-40 mg/kg in three divided doses

a) How many mg of Amoxicillin would the child receive per day?
b) Is the dose within safe parameters?
c) How many ml of Amoxicillin would the child receive per dose?

Answers: 225mg/day, yes, and 3 ml

Let me tackle "C" first:

C. The dose desired is 75 mg and for purposes of performing the math, I'm going to re-write this as 75 mg/1 which is still the same number. It's going to make it easier for me to do the math manipulation later.

The dose on hand is 125 mg/5 mL

If you plug these into the formula "dose desired divided by dose on hand" you end up with a fraction that, again, has a fraction in it's numerator and a fraction in it's denominator (a complex fraction).

75 mg/1 (numerator)/125 mg/5 mL (denominator)

The fraction in the denominator is cleared out by multiplying both the numerator and denominator of this complex fraction by another complex fraction that is actually a reciprocal of the fraction in the denominator, like this:

75 mg/1 (numerator)/125 mg/5 mL (denominator) X 5 mL/125 mg/5 mL/125 mg = 75 mg/1 x 5 mL/125 mg

Do the math and clear out the duplicated "mg" labels in the numerator and denominator and you are left with 3 mL. This is the answer to "C", the amount of Amoxicillin the child receives for one dose.

A. Asks how many mg of Amoxicillin would the child receive per day. If the child is getting a dose every 8 hours, and there are 24 hours in a day, then

24 hours divided by 8 hours = 3, or 3 doses per day.

3 doses x 3 mL/dose (determined in part C above) = 9 mL will be given per day.

9 mL (total amount to be given per day) x 125 mg/5 mL (dose on hand) = 225 mg per day

B. Is the dose within safe parameters? The problem tells you that the safe dose is 20-40 mg/kg per day. Your patient weighs 70 pounds. To determine the safe dosage parameters, you have to calculate the 20mg and 40mg daily dose for a 70 pound individual. Here are the calculations, by dimensional analysis for each:

20mg/kg/day for a 70 pound child:
20 mg/1 kg (low safe dose per day) x 1 kg/2.2 lb (conversion factor) x 70 lb/1 (weight of patient) = 636.36 mg

40mg/kg/day for a 70 pound child:
40 mg/1 kg (high safe dose per day) x 1 kg/2.2 lb (conversion factor) x 70 lb/1 (weight of patient) = 1272.73 mg

The pediatric dose parameter converted to mg would be 636 mg - 1273 mg in three divided doses. (I rounded off the numbers.) Since you know from part A of the problem that the patient is only going to get 225 mg total per day, the dose that was ordered is below these parameters and therefore safe. So, the answer is yes, the dose is within safe parameters.

Hope that helps you and didn't confuse you. You might want to print this out and rewrite the fractions in a vertical position. I am not able to do that with this software.

[grannypatches]

Thank you so much for your help. I understood it completely. This is going to help me so much. I cannot thank you enough. Have a very nice weekend.