Module 8: Dosage Calculations by Weight and BSA
Part 1: Dosage Calculations by Weight
Sometimes the dosage of a drug that a patient is supposed to receive depends upon how much they weigh. This is particularly the case with drugs that are given to children or infants, since their size will vary greatly from the size of an average adult, and often even varies a lot among children of the same age. This may also happen when we are dosing a patient with particularly sensitive drugs, so, for example, a patient in intensive care may be much more likely to be dosed by body weight. If we think about this, it makes sense that we would want to give people dosages of a drug that depend on how much you weigh: for example, a 350 lb man probably needs a different amount of a pain reliever than a 50 lb child (or even a 100 lb adult woman). For this reason, we need to know how to read drug orders that are given based on body weight. These drug orders will look just like any other drug order, for example:
Order: ampicillin suspension 200mg/20kg p.o. q.6.h.
This looks like an ordinary order for a medication other than the 200mg/20kg. This means we are to give a 200mg dosage per 20kg of the patient’s body weight. We will use the information on the label or given in the order and the weight of the patient to find either a recommended dosage, which is a single dosage amount that is the recommended amount for that patient, or to find a safe dosage range, which is a set of two numbers: a minimum safe dose and a maximum safe dose. Then we will want to compare the order that has been written by the doctor to the recommended dosage or safe range:
· When a label gives us the information needed to calculate a safe range, then the ordered dosage must fall within the range (i.e. between the minimum and the maximum) in order to be considered safe. Any dosage amount given in an order which falls outside of the range, even by a small amount, should be questioned.
· When a label gives us the information needed to calculate a recommended dose, then the ordered dosage must be equal to (or fairly close to) the recommended dose in order to be considered safe. If the ordered dose is not exactly the same as the recommended dose, what do we do? If we are outside of the range, then we will consider the drug unsafe.
Drugs can also be given based on a person’s (usually a child or infant) age. The label shown for KEFZOL® demonstrates an example of this:
Calculation of Dosage Based on Body Weight:
Example 1: A 66-pound child is to receive 25 mg/kg/day of tetracycline tid. Tetracycline is available in 100 mg and 250 mg capsules. How many capsules should the child receive for the first dose?
Solution: Since the child’s weight is given in pounds, not kilograms, we’ll have to include the conversion from pounds to kilograms in the dimensional analysis. Starting with the physician’s order, we will include the 25mg/1kg but the “per day” part of the order will be handled later. It’s a good idea, though, to make a note to yourself that the order is “per day” and the order is tid.
The per day dose is 3 tablets and the medication is to be administered tid – or three times / day. So 1 tablet will be administered for one dose.
Example 2: The physician ordered a drug (0.55 mg/kg po). The patient weighs 32 kilograms. The drug is supplied at the strength of 30 mg/mL. Calculate the number of milliliters you would administer. Round down to the nearest hundredth of a milliliter.
Solution:
0.55mg1kg×32kg1×1mL30mg=0.586=0.59mL
We round down to be sure not to overdose.
Example 3: A physician orders acetaminophen 125mg q4h for an infant with a high fever. The safe dosage range for acetaminophen is 10-15 mg/kg q4h. The infant weighs 22 ½ pounds. Is this a safe dose for the infant?
Solution: According to the label a range of dosage for an infant weighing 22 ½ lbs would be:
On the low side: 10mg1kg×1kg2.2lbs×22.5lbs=102.2mg q4h
On the high side: 15mg1kg×1kg2.2lbs×22.5lbs=153.4mg q4h
The safe dosage range is 102 mg q4h to 153 mg q4h. The doctor’s order is for 125 mg q4h, so the other is safe, because it is within the safe range.
Part 2: Calculating Dosage by Body Surface Area (BSA)
Sometimes using a person's weight to determine how much of a drug they receive isn't the best method. For example, someone who is 4 ft 10 in tall and someone who is 6 ft 4in tall might both be 150 lbs, but they may need more or less of a drug, depending on what portion of their weight is fat, or muscle, or retained fluid, etc. Some ways of dealing with this might include determining a person's dosage not on their actual weight, but on their "ideal" body weight - the weight that would be healthy for someone of their height and build; a hospital should have a chart that would allow us to calculate what this is. Another way to address this issue is to determine how much drug a person should get by their body surface area.
What is Body Surface Area and How Do We Calculate It?
The body surface area is precisely what it sounds like: the surface area of a person's body. You have probably calculated the surface areas of more regular shapes, like cubes and balls and pyramids in a previous math class. A person's surface area would be the area that the skin would take up if we could lay it out flat. Obviously this would be difficult to measure, as people are made up of a lot of irregular shapes, so to determine a person's body surface area, or BSA, we use formulas or charts based on their height and weight.
Note that BSA is always in square meters (m2), no matter which formula we use. The BSA of a person will always be below 3 m2. The BSA of a child will generally be below 1.5 m2, and the BSA of an adult will usually be above 1.5 m2. We will always round BSA to the nearest hundredth of a square meter.
Calculating Dosage by Body Surface Area (BSA) Formulas:
We need to know the patients height and weight to calculate the BSA. (You do not need to memorize these formulas.)
Metric:
Household:
Example 1: The physician orders Deltasone (prednisone) 60 mg/m2 po daily as part of the treatment protocol for a patient with leukemia.
a) How many milligrams of the steroid drug would you administer if the patient is 5 feet 6 inches tall and weighs 140 pounds?
Solution: First, note the order is 60mg/m2 the first time we’ve seen this abbreviation on an order. An order with these specification dictates that we use BSA to calculate dosage. To find the BSA, we need to use the Household formula. Also note that the number found before taking the square root should be rounded to at least 4 decimal places in order for our final answer to be rounded to two decimal places with accuracy.
BSA=66in(140lbs)3131=2.9511=1.72m2
Find the amount of drug to administer to this patient by using the BSA so that the m2 in the order are ‘cancelled’ by the m2 in the BSA.
103.2 mg
b) The drug is supplied in 50 mg per tablet. How many tablets will you administer?
Solution: Divide the found dosage by the amount per tablet:
103.2mg×1tab50mg=2.064=2tabs
Example 2: The physician ordered Retrovir (zidovudine) po, an antiviral drug used for the treatment of CMV retinitis. The order is 160 mg/m2 every 8 hours. The patient weighs 60 kg and is 140 cm in height. How many milligrams of this drug will you give the patient per day?
Solution: Find the BSA first now using the metric formula:
BSA=140cm(60kg)3600=2.3333=1.53m2
Find the amount of drug to administer to this patient by using the BSA multiplied by the order:
160mg1m2×1.53m21=244.8mg q8h
But the question asked for the mg for one day!!! There are three 8-hour periods in one 24-hour day. So the per-day dosage would be .
Example 3: The usual adult dose for Demerol® is 50 mg per m2 IM. A child is 58 cm tall and weighs 23 kg. What is the correct dosage of Demerol for this child?
Solution: Find the BSA first:
BSA=58cm(23kg)3600=0.3706=0.61m2
Find the amount of drug to administer to this patient by using the BSA multiplied by the order:
50mg1m2×0.61m21=30.5mg
Example 4: A physician orders methotrexate 30 mg IM for an 8-year-old girl with cancer. The manufacturer’s recommended dose is 30 mg/m2. The girl is 44 inches tall and weighs 55 pounds. Is 30 mg IM a safe dose for this girl?
Solution: Find the BSA first:
BSA=44in(55lbs)3131=0.7729=0.88m2
Find the amount of drug to administer to this patient by using the BSA multiplied by the order:
30mg1m2×0.88m21=26.4mg
Administering 30mg as ordered could potentially overdose the patient. 30mg is NOT a safe dosage based on the patient’s BSA based upon the manufacturer’s recommendation.