# Question about a question on dosage calculation test

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• Specializes in Student but interested in neuroscience..

My friend had taken the dosage calculation test this morning and after she took it, she called me hysterical because there was a question on there she wasn't sure about. The doctor ordered 1 grain/200; availible is 30 mg,how much should you give the patient? I already took my test and got 100% on it, but being a prenursing student, I didn't know what this meant exactly. She said that there was no unit on the 200. Was this a typo on the test or can you tell me how to figure this out. Thanx alot!

1,381 Posts

Specializes in Critical Care.

Grain is part of the outdated and soon to be (if not already) banned by JCAHO apothecary system.

One grain is equal to anywhere from 60-65 mg depending on your source. Now you see why it's being banned.

The way the problem was written, I have no idea what the 200 is in reference to: 1/200 of a grain (~0.3mg)? What it's diluted in? Dunno.

168 Posts

Specializes in Orthopedic, Corrections. Has 5 years experience.

I had a similar question on my pharm exam I took yesterday

Doctor's order: atropine gr 1/150 IM now Available: atropine 0.4 mg/mL How many mL will you administer Correct answer: 1mL

I took 60/150 and got 0.4 then devided the 0.4/0.4=1mL

so for 1gr/200 (60/200=0.3) then devide ordered by have (0.3/30=0.1) if that is per 1ml then 0.1x1=0.1mL

So I think the answer would be 0.1 mL

4 Articles; 14,603 Posts

Specializes in med/surg, telemetry, IV therapy, mgmt. Has 40 years experience.
my friend had taken the dosage calculation test this morning and after she took it, she called me hysterical because there was a question on there she wasn't sure about. the doctor ordered 1 grain/200; availible is 30 mg,how much should you give the patient? i already took my test and got 100% on it, but being a prenursing student, i didn't know what this meant exactly. she said that there was no unit on the 200. was this a typo on the test or can you tell me how to figure this out. thanx alot!

there was a unit on the 200. it was a fraction (1/200). a unit label is never afixed to the denominator of a fraction; always to the numerator. in this case, the "1". thus, "1 grain/200". to do this by dimensional analysis the fraction needs to be flipped in order to cancel out both the "mg" and "grain" labels. i think that typing "1 grain/200" may have been a hint for the students doing the calculation that way.

200/1 grain
(dose on hand)
x 30 mg tablet/1
(dose to give)
x 1 grain/60 mg
(conversion factor)
=
100 tablets
(amount to give)

approaching this another way. . .1/200th grain is
0.005 grain
.

30 mg coverted to grains using 60mg/grain as a conversion is 30mg/60mg or
0.5 grains
. then, doing dose desired (0.5 grains) divided by dose on hand (0.005 grains), you get the same 100 tablets.

i would so not give this drug and be on the phone calling this doctor if this were a real life situation.

1,381 Posts

Specializes in Critical Care.
there was a unit on the 200. it was a fraction (1/200). a unit label is never afixed to the denominator of a fraction; always to the numerator. in this case, the "1". thus, "1 grain/200". to do this by dimensional analysis the fraction needs to be flipped in order to cancel out both the "mg" and "grain" labels. i think that typing "1 grain/200" may have been a hint for the students doing the calculation that way.

200/1 grain
(dose on hand)
x 30 mg tablet/1
(dose to give)
x 1 grain/60 mg
(conversion factor)
=
100 tablets
(amount to give)

approaching this another way. . .1/200th grain is
0.005 grain
.

30 mg coverted to grains using 60mg/grain as a conversion is 30mg/60mg or
0.5 grains
. then, doing dose desired (0.5 grains) divided by dose on hand (0.005 grains), you get the same 100 tablets.

i would so not give this drug and be on the phone calling this doctor if this were a real life situation.

you had the dose ordered and the dose on hand backwards.

if the doc ordered 1gr/200, if you divide 60 by 200 to convert to milligrams, you see he ordered 0.3 mg. if the dose on hand is 30 mg, you're looking at the opposite problem: instead of 100 tablets you're needing 1/100th of a tablet- another impossibility.

either way, the problem doesn't add up.

4 Articles; 14,603 Posts

Specializes in med/surg, telemetry, IV therapy, mgmt. Has 40 years experience.

So, what does "The doctor ordered 1 grain/200" mean? OP, you want to clarify that, please.

4 Posts

How I did it:

(1/200)gr * 60mg/1gr*tablet/30mg = 0.01 tablet

Can't give 0.01 of a tablet

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