CASE ONE: TP, a 40-year-old woman withchronic myelogenous leukemia,is admitted to the hospital for anallogeneic bone marrow transplant.Since her diagnosis 6months earlier, TP has beenreceiving imatinib mesylate therapy,which has produced ahematologic but not a cytogeneticremission. Due to the lack ofhematologic remission, one of TP's brothers, who is a 6/6 humanleukocyte antigen match, offered to donate his bone marrow.
After the bone marrow infusion, the physician writes anorder for TP to receive 15 mg/m2 of methotrexate to be giventhe day after the procedure. The pharmacist who receives theorder verifies that the dose is appropriate for prevention ofgraft-versus-host disease. She then must calculate the dosebased on TP's body surface area (BSA).
If TP is 5 ft tall and weighs 160 lb, how many milligrams ofmethotrexate should she receive?
CASE TWO: When the pharmacist deliversTP's methotrexate to hernurse, hospital policy dictatesthat the pharmacist and nursereview the order again, determineif the dose per BSA isappropriate, and compare calculationsof the dose. Whilethey are completing this task,the nurse notices in her nursing drug guide that sodium bicarbonateis often added to the patient's intravenous fluids whenmethotrexate is given. The guide explains that sodium bicarbonatewill alkalinize the urine and increase the solubility ofmethotrexate, thereby providing renal protection from theprecipitation of methotrexate within the renal tubules. Sheasks the pharmacist if they need to call the physician to addan order for sodium bicarbonate.
How should the pharmacist reply?
CASE THREE: Two students from Mortar& Pestle University are compoundinga solution of potassiumchloride (KCl) in the teachinglaboratory. The studentswere asked to compound 250mL of a 5% solution of KCl.
The students calculate thenumber of milligrams of KClnecessary for the final product.Since a 5% solution is equivalentto 5 g of KCl per 100 mL or 50 mg/mL, the students knowthey will need 12,500 mg of KCl. When the students look atthe stock bottle, the concentration is written in milliequivalents,rather than milligrams.
Convinced that an error has been made on the stock bottle'slabel, the students inform the professor of the discrepancy.The professor informs the students that the label is correctand that the students will need to calculate the numberof milliequivalents needed for the solution.
How many milliequivalents will the students need for 250mL 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 convertedto 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 squareroot 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. Thisis 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 isfound 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, 12,500mg divided by 74.5; therefore the students will need 167.8 mEq.