Case STUDIES

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/m2 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.

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/m2. TP's dose is much lower at 15 mg/m2.

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.