Need help with a math question....grains?

I'm not a new nurse, just starting a new job. We have a dosage cal test tomorrow and they gave us some practice Q&As. This one I can't see how they got to this answer. (I've been a nurse 3 years and never have I seen a doc write an order in grains....ugh)

Ordered atropine gr 1/300 IM
On hand atropine gr 1/150 IM per cc

The answer sheet says its .5cc

How?!?!?!?!

I thought it would be 2cc.
There's something missing: quantity of drug to be administered.The strength is 1gr/300 ml or 1gr/150 ml but what is the quantity of gr required for the dose?

The metric system uses decimals for units less than 1. For example, you would administer 0.2 mg atropine, not 2/10 mg.The apothecary system, however, is diffferent. In the apothecary system, units less than 1 are expressed as fractions, and the unit of measure is placed in front of the dose rather than behind. A major issue with the aapothecary system is that it is not as precise; depending upon the source, gr I can either be 60 or 65 mg. In the problem that you have presented they are using 60.In your problem you have an order to administer 1/300 of gr I (60 mg) or 0.2 mg. your available concentration is 1/150 of gr I (60 mg) or 0.4 mg in 1 mL.While you will occasionally find a problem on a pharmacology quiz using the apothecary system, in actual practice it is rare.

Let's hear it for metrics! But I do sincerely thank the Sisters of Mercy in Catholic grade school for my understanding of fractions.It is a good thing that the apothecary system is rarely used because they also used Roman numerals. Once in a very blue moon I will see Tylenol gr X or ASA gr X. And by converting grains to milligrams, we're talking Tylenol/ASA 650 mg. I also remember something ordered as gr 7 1/2 (aminophylline?).And do some of us still remember minims and gtts (drops)?>>>>>>>>>>>>>>>>>>>>>To answer OP - the answer is correct. You would be giving 0.5 cc. You'd be giving half of the way the med is provided accdg to the math explanation by chare. I follow her arithmetic OK but it's not always easy to see.

Your ordered dose is half as strong per cc as your available dose; this is a bit confusing since 300 is twice 150, but remember that the larger the denominator number is, the smaller it's value. So to get a 1/300 dose you would need half a 1/150 dose, which would be 0.5 cc in this case.In general though, the correct answer is to make the Doctor re-write it. Apothecary ordering is extremely dangerous and they are the type of Doctor that orders 'eye of newt' they need to accept standardized, safe, units of measurement for ordering.

1/300= .0033333 .0033333*60=0.199998 or 0.2 mg/ ml (or c.c.) ordered1/150=.0066666 .0066666*60=0.399996 or 0.4 mg/ml (or c.c.) on hand0.2/0.4= 0.5 c.c.

Ah yes grains. I remember the rare problem with these in school. I have never seen an order with these. The EMR wouldn't accept it. I think continuing to teach it continues to reinforce that it's acceptable, and I agree it's dangerous.

I'm just finding it amazing that we all do our math different ways, but we all come up with the same answer!

Thanks! I got it now.

Quote from MunoRNYour ordered dose is half as strong per cc as your available dose; this is a bit confusing since 300 is twice 150, but remember that the larger the denominator number is, the smaller it's value. So to get a 1/300 dose you would need half a 1/150 dose, which would be 0.5 cc in this case.In general though, the correct answer is to make the Doctor re-write it. Apothecary ordering is extremely dangerous and they are the type of Doctor that orders 'eye of newt' they need to accept standardized, safe, units of measurement for ordering.

Quantity to be administered is the same as Ordered dose..right?