# Drug dosage calculations - question on rounding

Posted

Specializes in Hospice, Palliative Care. Has 3 years experience.

Good day:

From dosagehelp.com practice questions: Phenytoin (Dilantin), 11,000 mg PO, is ordered to be given through a nasogastric tube. Phenytoin is available as 19 g / 12 mL. How much would the nurse administer?

I came up with 6.947368421 mL. Dosagehelp.com expects the answer to be 6.9 mL. How do you know how many significant figures to use based on the type of question?

Thank you.

Specializes in MICU. Has 3 years experience.

Because you can only round up fom 0.5 to 0.9 not 0.0 to 0.4. That is why it is 6.9 and mostly the question would tell you whether to roubd up to one decimal place which is 6.9 and two decimal place is 6.95

Edited by loveofrn

Specializes in Hospice, Palliative Care. Has 3 years experience.

Good day, loveofrn:

Thank you for your input; I do know about rounding up vs. down. I just don't know the rules for how many decimal places to use when doing dosage calculations. Do you know if there are any guidelines based on the type of question or calculation to determine how many significant figures should be in the outcome?

Thank you.

Specializes in MICU. Has 3 years experience.

One decimal place is the general rule but the question might tell you whether 2 or 3 decimal place

Specializes in MICU. Has 3 years experience.

24.35= 24.4

24.54= 24.5

24.56= 24.6

24.03= 24.0

also think about your measuring device - you maybe able to measure 6.9 mL in a cup or syringe.. but you'd never realistically be able to measure 6.947368421 mL :)

We had issues with this as it seemed to vary from instructor to instructor. I always figure I round to whatever is reasonable to actually draw up. What size syringe will you be using? What are the increments on the syringe? So, thinking along that line, you'd use a 10mL syringe which is divided into tenths. It is reasonable that you could draw up 6.9mL of med, plunger being halfway between the 6.8mL and 7.0mL marks. Can you reasonably measure out 6.94mL (or 6.95mL if you round) out on a syringe that is broken into tenths? No.

Anything under a mL can be drawn in a tb syringe, so those can reasonably be drawn out to the hundredths place.

Hope that helps!!

Specializes in Hospice, Palliative Care. Has 3 years experience.

Good day, smf0903, 203bravo, and loveofrn:

I'm studying for a pre-clinical dosage calculation test. I've no experiences with syringes or medical devices (other than from a patient perspective; and, unfortunately I wasn't paying attention all the time for those in terms of noticing the details of the syringe or device). Therefore, I don't know what I could measure with a syringe or other device from a visual / experiential point at this time.

I do understand I need to round, and I do know how to round. I'm just working through how to determine how many decimal places to round.

Thank you.

From dosagehelp.com practice questions: Phenytoin (Dilantin), 11,000 mg PO, is ordered to be given through a nasogastric tube. Phenytoin is available as 19 g / 12 mL. How much would the nurse administer?

I came up with 6.947368421 mL. Dosagehelp.com expects the answer to be 6.9 mL. How do you know how many significant figures to use based on the type of question?

Significant figures are very simple... they communicate the precision to which a value is known. The basic rule is that you cannot have any more significant figures in a product than are present in the least significant of its factors.

In this case, your three factors are 11000, 19, and 12... each of which has two significant figures (the zeroes in 11000 are just place holders).

Two sig figs in the factors, two sig figs in the product... 6.9

If one of the factors had only a single significant figure then you'd only be permitted one sig fig in your answer.

Note... you do not round until the final product...

In our program, we were told that if the number is less than 1 to use 2 decimal places. If the number is greater than 1 then only 1 decimal place is needed. The exception is when calculating weight in kg, in which case we always use 2dp. I'm not sure if that helps...it's just how we were taught.

In our program, we were told that if the number is less than 1 to use 2 decimal places. If the number is greater than 1 then only 1 decimal place is needed. The exception is when calculating weight in kg, in which case we always use 2dp. I'm not sure if that helps...it's just how we were taught.

If that's what they want then do it that way...

but don't use the term "significant figures" because that is *not* how significant figures works.

If that's what they want then do it that way...

but don't use the term "significant figures" because that is *not* how significant figures works.

Where did I use the term significant figures? I was simply offering the original poster an insight into how my program has taught us how to round for nursing. I didn't use the term significant figures deliberately because those aren't the rules we use. Geesh.