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 13-Nov-2006.

 

See you Monday or Tuesday.   

 

--CLOates