Dosage Calculations

Daytonite, BSN, RN · Oct 22, 2006

trilli18 said:
ok very frustrated thought i was getting this dosage calculation stuff and im not. my professor suggested doing it as desired over have. i tried but am still coming up with wrong answer. can some please help me
here's the problem need to find desired dose:
ordered 1 1/2 teaspoon zithromax 200mg/5ml poq6h
on hand 200mg per 5ml
if u could show me how to do this problem by solving it as desired over have i would appreciate it ---thanks for ur help

this is a situation where you are being asked to use a formula. you merely need to plug your known information into the parts of the formula. i think what is throwing you is that in this case you are already given the answer and one of the elements making up the formula is what you have to find. this requires using some algebra and doing some manipulation of the equation in order to find the solution.

the formula you are wanting to use is: desired dose divided by dose on hand equals the dose to give. you are being asked to determine the desired dose, so make that x. the dose on hand is 200mg / 5ml. the dose to give is 1.5tsp (teaspoons).

plugging this information into the equation you get the following:

x / 200mg / 5ml =1.5tsp (note that your first term is a complex fraction with a fraction in the denominator)

Simplify the term on the left and clear the fraction out of the denominator by multiplying both the numerator and denominator by it's reciprocal 5ml / 200mg. you will now have the following equation:

(5ml)(x) /200mg =1.5tsp

Isolate x by multiplying both sides of the equation by the reciprocal fraction, 200mg / 5ml. you will now have the following equation:

x=(1.5tsp /1)(200mg /5ml)

Teaspoons, the label, need to be cleared from the equation. that is done by applying a conversion factor which will leave you with your answer, the desired dose:

x=(1.5tsp /1)(200mg /5ml)(5ml /1tsp)=300mg

Daytonite, BSN, RN · Oct 22, 2006

it occurred to me after i had posted the above, that i could have eliminated a couple of steps by simply altering the formula before plugging the given information into it. duh! :trout:

so, in the formula desired dose divided by dose on hand equals dose to be given (dd/dh = dg), dd becomes x, the answer you are looking for. isolating x gives you a new formula, x = (dg)(dh). plug your given information into the formula and proceed from there. you still have to apply a conversion factor to clear the label for "teaspoons" out of the resulting equation and factor out the "ml" so you can be left with the label "gm" for the final answer.

Nurse Education · Nov 6, 2006

minnielynn said:
I am in a desperate situation. I thought I was getting my calculations correctly and today I got back my math results and i got a 70% I am so disappointed I dont know what to do. On my first exam I received an 80%.
My professor said that if i get above 95% on my final (which is next week) I may be able to average a b+ or a- , but it will all depend on the overall average, if he curves grades or not. I went and bought 2 self help books on calculations .... My question is ..... do you think it is humanly possible to ingest all this info by next week (exactly a week from today)! I am having problems with the drips and the three step conversions. Is there a website that can help me? To top it off my professor sucks, so what I have learned, I have learned on my own. I am so frustrated, so much so, that it is getting harder and harder for me to pick up a book .....sorry for venting

Hello, sorry you're having a rough time. I found that the only way to get through math calculations was to memorize the formulas. I know of a website that has many nursing reference tools. Not sure if it will help, but it's called quick-reference.

trilli18 · Nov 11, 2006

on IV's - got to calculate the original flow rate. then determine if an adjustment is necessary and calculate the adjusted flow rate. Adjustments can't exceed 25%

Ordered: 375mL RL over, 3h (10gtt/ml tubuing)

After 1h, 175ml has infused

please can someone help im going bonker been trying to figure this out for over an hour

Daytonite, BSN, RN · Nov 11, 2006

the calculation, by dimensional analysis (factor labeling), to find the original flow rate would have been:

375 ml/3 hours (amount to infuse)x10 gtts /1 ml (drip factor of tubing being used)x1 hour /60 minutes (conversion factor)=20.833 gtts/min =21 gtts/min(rounded off)

After one hour you have 200ml of fluid left to infuse. (375ml minus 175ml = 200ml). you have 2 hours to infuse the remaining 200ml with 10gtt/ml tubing. to complete the calculation by dimensional analysis (factor labeling), keep in mind that you want an answer in gtts/min, so. . .

200 ml/2 hours (amount left to infuse)x10 gtts/1 ml (drip factor of tubing being used)x1 hour/60 minutes (conversion factor)=16.666 gtts /min =17 gtts/min(rounded off)

Somebody, or somehow, things got screwed up and for the first hour the infusion ran at 29 drops a minute, that's 175 ml an hour with 10 gtt per ml tubing.

WickedRedRN, BSN, RN · Nov 12, 2006

Priddyeyez said:
not completely sure that this is right- I used manuels web.
3000mL D5W over 24 hours.
Drop Factor: 10
1. ____mL/hr answer is- 125
2. ____gtt/min
how do I get the answer to drop factor??
what is D5W?

I am not very good at setting these up online, but I will do my best to explain it. Daytonite will probably chime in here, she is awesome at setting the problems up, helped me a lot by reading her explanations. If you get a chance, read the rest of this post, I am betting you will find she has already answered this question before.

Anyway... here is how I set these up.

Total ml X drop factor ~over~ total hours x 60.

In your problem, 3000 X 10 = 30000 24 X 60 =1440 (the 60 is because there are 60 minutes in one hour) Then 30000/1440= 20.8. Since we cannot account for less than one whole drop, we round. Therefore, 20.8= 21. So youre answer is 21 gtt/min.

Oh, and D5W is Dextrose 5% in Water

Hope this helps!

Daytonite, BSN, RN · Nov 13, 2006

Yes! lorirn2b calculated that correctly. it actually the same as the problem i did just above it. here is what it looks like in dimensional analysis so that the labels get factored out and you are left with gtts/min in the final answer:

3000 ml/24 hours(amount to infuse)x10 gtts/1 ml (drip factor of tubing being used)x1 hour/60 min (conversion factor)=20.8333 gtts/min =21 gtts/min(rounded off)

5% dextrose in water is a standard iv fluid that is used in iv therapy.

trilli18 · Nov 17, 2006

Hi all,

I had a math exam in my math for health science class -which i think i did ok but would like someone to review my answers and if they are wrong can u please correct with the proper way to do them so I will know for next time. Can someone check these and let me know. Thanks for all ur help this site is a blessing in a half.

1. O: streptomycin sulfate 800mg

h: streptomycin injection 1g/2.5ml

this is what i did --- 1g = _1000mg__ = 1000mg,

1g

800mg/1000mg X 2.5ml = 2ml, standard syringe

0.5752. O: Prostigmin 0.4 sc Q6h

H:prostigmin 1:4000 sol

this is what i did ----

ratios 1g/4000ml, 1g=1000mg/1g = 1000mg ~ 0.4mg X 4000ml/1000mg = 1.6 and u would use stan eoiodard syringe

3. o:Epogen 2300U

H:Epogen 4000U/ml

this is what i did 2300u/4000u X 1ml = 0.575ml, umg/se a tuberculin syringe

4. O:Dilaudid 0.8mg sc q4h Prn/Pain

H: 2mg/ml

this is what i did --- 0.8mg/2mgX1ml = 0.4ml, tuberculin syringe

5. O: Tigan 200mg deep IM/TID

H: Tigan 100mg/ml for injection

this is what i did ---- 200mg/100mg X 1ml = 2ml, standard syringe

6. Calculate flow rate to nearest drip factor, this is what i did

1800ml X 10 gtt/Ml

_________________

12h (720min) = 25 gtt/min

7. calculate flow rate

100ml o.45%NS Q8h (infusion pump)

this is what i did---- 1000ml/8hrs = 125ml/h

8. original flow rate and adjustment can is adjustment safe to do

1200ml 8h 10gtt/ml, after 3hrs 500ml has infused

this is what i did ---- 1200ml/8h = 150ml/h

150ml/h X 10gtt/ml / 1h

_________________________

60 = 25 gtt/min

then did this 1200 - 500 =700ml

8hrs - 3hrs = 5 hrs

700ml/5hrs = 140ml/h

140ml/h X 10gtt/mlX1h

_________________

60 = 23gtt/min

and i wrote yes can do adustment safely

9. find when infusion will be complete

o:750 ml RL @ 50ml/h your start time is 1000

this is what i did

750 ml / 50ml/h = 15hrs ~ 1pm, 1300

10. Total volume to administer

O:NS at 125ml/h over 7 hrs 30min.

*** this one is was stuck on for a while

but this is what i put 125ml/h X7h = 875

875/30min = 29ml/min

trilli18 · Nov 18, 2006

working on some hw and am stuck can't figure this out, can some help me please!!!!!

I have to determine if the order is safe. If it is then determine the amt to administer.

the pt is a 3day old newborn who weighs 6lbs 5oz

O: Nebcin 5mg Im q 12h

H:Nebcin mult. dose vial 20mg/2ml

According to the package insert, a premature or full - term neonate up to 1 week of age may be administered up to 4mg/kg/day in 2 equal doses q 12 hrs.

This is what i have done so far : 5oz = 1lb

____ = 0.3125 lbs

6lbs + o.3125 = 6.31 lbs 16oz

6.31lbs /2.2x1 = 2.87kg

5mg/20mgX 2ml = o.5ml which is the amt to administer but now stuck not sure how to go about the rest

If someone can please get back to me tonight thanks so much

Daytonite, BSN, RN · Nov 18, 2006

Quote
i have to determine if the order is safe. if it is, then determine the amount to administer. the pt is a 3-day old newborn who weighs 6lbs 5oz
o: nebcin 5mg im q 12h
h: nebcin multi-dose vial 20mg/2ml
According to the package insert, a premature or full - term neonate up to 1 week of age may be administered up to 4mg/kg/day in 2 equal doses q 12 hrs.

First order of business is to determine the normal safe dose. in order to make any kind of comparison you have to convert the baby's weight to kilograms. to do that, i am first going to converting the baby's weight to kilograms by applying two conversion factors. one to change pounds and ounces to ounces only and then to change the ounces to kilograms.

16 ounces = 1 pound, so6 pounds x16 ounces +5 ounces =101 ounces(baby's total weight in ounces)

Then, change the weight from ounces to kilograms:101 ounces/1 (baby's weight in ounces)x1 pound/16 ounces (conversion factor)x1 kilogram/2.2 pounds (conversion factor)=2.8693 kilograms

You were told the safe dose is 4 mg/1 kg/1 day/12 hours. if you divide 4 mg/1 kg into two doses, you get 2 mg/1 kg per dose as the safe dose. use a ratio to figure the baby's safe dose, since you now know the baby's weight in kilograms:

2 mg nebcin/1 kilogram(safe dose)=xmg nebcin/2.8693 kilogram(safe dose for your baby), cross multiply. you getx=5.7386 mg

This is telling you that the safe dose for a baby that weighs 2.8693 kilograms (or, 6 pounds 5 ounces) is 5.7386 mg. compare this to the dose the doctor ordered which is 5 mg. so, the answer to the first part of the question is: yes, the order is a safe dose.

The second part of the question is to determine the amount (in ml) to administer. this, very simply, is a dose desired divided by dose on hand formula calculation:

5 mg/1(dose desired)x20 mg/2 ml (dose on hand)=1/2 ml or 0.5 ml (after completing the math and canceling out the labels). The amount you will administer is 0.5 ml of nebcin.

Daytonite, BSN, RN · Nov 18, 2006

Quote
Ordered: streptomycin sulfate 800mg
On hand: streptomycin injection 1gram/2.5 mL

800 mg/1 (dose desired) X 2.5 mL/1 gram (dose on hand) X 1 gram/1000 mg (conversion factor) = 2 mL

Quote
Ordered: Prostigmin 0.4 subcutaneously Q6h
On hand: Prostigmin 1:4000 solution

Parameters of problem not clear.

Quote
Ordered: Epogen 2300 units
On hand: Epogen 4000 units/mL

2300 units (dose desired) X 1 mL/4000 units (dose on hand) = 0.575 mL = 0.6 mL (rounded off)

Quote
Ordered: Dilaudid 0.8 mg subcutaneously q4h prn pain
On hand: Dilaudid 2 mg/mL

0.8 mg/1 (dose desire) X 1 mL/2 mg (dose on hand) = 0.4 mL

Quote
Ordered: Tigan 200 mg deep IM TID
On hand: Tigan 100 mg/1 mL for injection

200 mg/1 (dose desired) X 1 mL/100 mg (dose on hand) = 2 mL

Quote
Calculate the drip rate for 1800 mL to be infused over 12 hours using IV tubing that delivers the fluid at 10 gtts/min.

1800 mL/12 hours (dose desired) X 10 gtt/1 mL (drip factor) = 25 gtts/hour

Quote
Calculate flow rate for 1000 mL of 45% NS to be given q8h by infusion pump

This is a simple ratio that is reduced to lowest terms: 1000 mL/8 hours = 125 mL/1 hour

Quote
You are to give 1200 mL of an IV fluid over 8 hours with 10 gtt/mL IV tubing. After 3 hours, 500 mL has infused. What was the flow rate during those 3 hours that the 500 mL infused and what adjustment to the flow rate do you need to make to infuse the remainder of the IV fluid safely?

500 mL/3 hours = 166.6667 mL/hour

1200 mL (total amount to give) minus 500 mL (already given) = 700 mL (remaining to be infused)

700 mL/5 hours (remaining amount and time to be infused) X 10 gtts/1 mL X 1 hour/60 minute (conversion factor) = 23 gtts/minute, and yes, you can do the adjustment safely.

Quote
Total volume to administer
O:NS at 125ml/h over 7 hrs 30min.

Have no idea what problem #10 is asking! Sorry!

Daytonite, BSN, RN · Nov 22, 2006

trilli18 said:
Hi All,
I have been on some math problems for a couple of days now and am unable to figure out how to solve this problem, If any of u know can u please respond and show me how its done.
I need to determine if the dosage ordered is safe.
* The patient is a 92yr old female, 5ft 6in. tall, 130lb, and CLcr of 61 ml/min. Patient is in ideal wgt range.
Order: Amikacin 375mg IM Q12H
According to the Package insert, pt. with normal renal function may be administered 7.5mg/kg q12h or 5mg/kg q8h. This patient has normal renal function.
Thanks for your help with this I have a bunch of these problems to do, but have been unsuccessful in figuring out how to answer it. I think If I see one done I will be able to accomplish the rest on my own. Thanks again.

You asked how to do the same kind of problem in post #78 on this thread. A step-by-step explanation of how to approach and answer this kind of problem was given to you in post #79. Please review that post. This is the very same type of problem. The only thing that is different is the numbers.