Author: Lauren S. Schlesselman, PharmD

**CASE ONE:** TP, a 40-year-old woman with
chronic myelogenous leukemia,
is admitted to the hospital for an
allogeneic bone marrow transplant.
Since her diagnosis 6
months earlier, TP has been
receiving imatinib mesylate therapy,
which has produced a
hematologic but not a cytogenetic
remission. Due to the lack of
hematologic remission, one of TP's brothers, who is a 6/6 human
leukocyte antigen match, offered to donate his bone marrow.

After the bone marrow infusion, the physician writes an
order for TP to receive 15 mg/m^{2 }of methotrexate to be given
the day after the procedure. The pharmacist who receives the
order verifies that the dose is appropriate for prevention of
graft-versus-host disease. She then must calculate the dose
based on TP's body surface area (BSA).

If TP is 5 ft tall and weighs 160 lb, how many milligrams of methotrexate should she receive?

**CASE TWO: **When the pharmacist delivers
TP's methotrexate to her
nurse, hospital policy dictates
that the pharmacist and nurse
review the order again, determine
if the dose per BSA is
appropriate, and compare calculations
of the dose. While
they are completing this task,
the nurse notices in her nursing drug guide that sodium bicarbonate
is often added to the patient's intravenous fluids when
methotrexate is given. The guide explains that sodium bicarbonate
will alkalinize the urine and increase the solubility of
methotrexate, thereby providing renal protection from the
precipitation of methotrexate within the renal tubules. She
asks the pharmacist if they need to call the physician to add
an order for sodium bicarbonate.

How should the pharmacist reply?

**CASE THREE:** Two students from Mortar
& Pestle University are compounding
a solution of potassium
chloride (KCl) in the teaching
laboratory. The students
were asked to compound 250
mL of a 5% solution of KCl.

The students calculate the number of milligrams of KCl necessary for the final product. Since a 5% solution is equivalent to 5 g of KCl per 100 mL or 50 mg/mL, the students know they will need 12,500 mg of KCl. When the students look at the stock bottle, the concentration is written in milliequivalents, rather than milligrams.

Convinced that an error has been made on the stock bottle's label, the students inform the professor of the discrepancy. The professor informs the students that the label is correct and that the students will need to calculate the number of milliequivalents needed for the solution.

How many milliequivalents will the students need for 250 mL of the 5% solution?

*Dr. Schlesselman is an assistant clinical professor at the University of Connecticut School of Pharmacy.*

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**CASE ONE: **Body surface area (BSA) is equal to the square root of height x weight/3600. The height and
weight should be converted
to centimeters and kilograms, respectively. Five feet is equivalent to 152.4 cm, while 160 lb is equivalent to
72.6 kg. TP's BSA is the square
root of 3.073, which is 1.75 m2. Since the physician prescribed 15 mg/m2, TP's dose equals 15 x 1.75, or 26.3 mg.

**CASE TWO: **The addition of sodium bicarbonate to intravenous fluids for renal protection is only necessary
at higher doses of methotrexate. This
is typically reserved for doses of methotrexate in excess of 1 g/m^{2}. TP's dose is much lower at 15
mg/m^{2}.

**CASE THREE: **To determine the number of milliequivalents, the students will need to know the formula weight
of KCl. The formula weight of KCl is
found by adding the atomic weights of potassium and chlorine, 39.1 + 35.4 = 74.5.
Since the students will need 12,500 mg of KCl, the milliequivalents necessary is equal to milligrams divided by
formula weight—in this case, 12,500
mg divided by 74.5; therefore the students will need 167.8 mEq.