APPM 1313 Students,

The following is a partial transcript of an e-mail I sent to one of your
classmates to explain orders that give a dosage range and, possibly, ask about
the safety of the order. Let me know if
you have questions or need further information.

Chuck Oates

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…

On Worksheet 32, problems 1 - 3, they're basically asking you to solve
the problem twice, once for the LOW limit of the recommended dosage range (25
mg/kg/day) and once for the HIGH limit of this range (50 mg/kg/day).

[In Problem 1] for a 40 lb child the LOW limit of the dosage range is

40 lb x 1 kg / 2.2
lb x 25 mg / kg /
day x 1 day / 4 doses

= ( 40 x 25 ) / ( 2.2 x 4 )
mg / dose

~= 113.7 mg / dose

~= 114 mg / dose (3 SDs since there'll be another calculation to get
this to mL, eventually)

For a 40 lb child, the HIGH limit of the recommended dosage range is

40 lb x 1 kg / 2.2
lb x 50 mg / kg /
day x 1 day / 4 doses

= ( 40 x 50 ) / ( 2.2 x 4
) mg / dose

~= 227.3 mg / dose

~= 227 mg / dose (3 SDs since
there'll be another calculation to get this to mL, eventually)

In real life, and on the test, also, you would have an order for, say,
500 mg / dose of this medication. After you've practiced a while, you'll recognize
that for a 40 lb child, that's quite high. On a test you would be given
the order and asked "Is it safe?" You would then perform
the above calculations and compare the ordered dose of 500 mg / dose to the
high limit of the recommended dosage that for this child is 227 mg /
dose. Since 500 mg/dose is over twice the high recommended limit for a 40
lb patient, you would probably consult the pharmacist or the physician to
be sure that this is really the dose that's desired. The physician might,
for some reason, intend to give this much; however, another very real
possibility is that somewhere along the line the dose for the whole day became
confused with a single dose. On a test you would label such a dosage
"unsafe."

Similarly, for an order of, say, 50 mg / dose, which is less than
half the recommended low limit for a 40 lb child, you would probably need to
consult the physician to make sure that such a low dosage was what was
intended. (Prepare to get an ear full when you contact a physician about
something like this. Be sure to have your ducks in a row before you
call.) On a test you would also label such a dosage “unsafe,” since it is
likely that there’s not enough of the medication being given to achieve the
desired result.

Be sure that you understand these calculations, because they are the
ones you’ll most likely actually need to do in practice. They’re the answer to your own question, “Is
that dosage REALLY correct for my patient?”

…

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[Here’s some information on Problem 8 from that same series of
worksheets. Aren’t you glad we can now
use calculators?! --CLO]

On WS 33, Problem 8, we'll do a calculation to get the number of
milligrams of medication for a 30 lb. child and then use the concentration
information from the label ( 125 mg / mL ) to convert
this number of milligrams to milliliters. For a 30 lb. child, we'd
calculate

30 lb x 1 kg / 2.2
lb x 25 mg / kg /
day x 1 mL / 125 mg x
1 day / 4 doses

= (30 x 25) / (2.2 x 125 x 4) mL / dose

= 0.681 mL / dose

~= 0.68 mL / dose (round to in
0.01 mL increments [2 SDs], since 1 mL tuberculin syringe is marked in 0.01 mL
increments)

…

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[Post Script]

I hope this helps. Please check my arithmetic, and let me know as
soon as possible if you find errors.

Let me know if there are problems that give you difficulty.

Don’t forget that my Tuesday and Tuesday-Thursday sections have Test 4
scheduled for this Tuesday, 7‑Nov-2006.
The Monday evening section’s Test 4 will be given in class

See you Monday or Tuesday.

--CLOates