Question

1) If the patient had been given one dose of procainamide resulting in a peak concentration of 22 μg/mL, what would the theoretical blood level have been 24 hours later if no further drug was administered (half-life is 8 hours) and no other factors influenced the results?

2) In a salicylate poisoning, what would be the expected results of arterial blood gas analysis?

3) . Explain the mechanism of action of methotrexate and purpose of leucovorin administration to patients receiving high dose methotrexate.

4) . What is the specimen of choice for measurement of cyclosporine and why?

Answer 1

1) Half life is 8 hours.After each half life, the drug concentration is half of the original concentration.

So in 24 hours, 3 half lives have passed.So in 3 half lives, drug concentration becomes 1/8 of the initial concentration (1/2 * 1/2 * 1/2).

So blood level = 1/8 * 22 = 2.75 microgram/ml

2) pH is acidic (< 7.35)

So there is metabolic acidosis .

PCO2 decreases to compensate (< 35 mm of Hg).

3) Methotrexate is inhibitor of dihydrofolate reductase.

So dihydrofolate is not converted to tetrahydrofolate(THF).THF is the active form and acts as coenzyme in many biochemical reactions.

So high dose methotrexate can inhibit the metabolic reactions in the normal cells.Hece the active form ,folinic acid is administered to prevent the side effect.

4) The specimen of choice is whole blood which is anti coagulated with EDTA.Whole blood is used instead of plasma because cyclosporine binds to red blood cells.