A nurse is reviewing the laboratory results of a client who received a dose of sodium polystyrene sulfonate. Which of the following labs should the nurse monitor to evaluate if the medication has been effective?

Sodium polystyrene sulfonate is a cation-exchange resin utilized in the management of hyperkalemia. It works in the large intestine by exchanging sodium ions for potassium ions, allowing the excess potassium to be excreted from the body via the feces to prevent cardiac complications. Rationale: A. While sodium polystyrene sulfonate can lead to a secondary decrease in calcium levels as a side effect, calcium is not the target of the medication. Monitoring calcium is important for safety, but it does not measure the primary effectiveness of the drug. The drug is specifically formulated to treat potassium imbalances that pose immediate risks to the myocardium. B. Potassium is the primary laboratory value to monitor because the therapeutic goal of sodium polystyrene sulfonate is to lower serum potassium levels. Effectiveness is confirmed when the lab results show a return toward the normal range of 3.5 to 5.0 mEq/L. Since high potassium can cause fatal arrhythmias, this lab provides the definitive proof of the medication's clinical success. C. Sodium levels may increase during therapy because the resin releases sodium in exchange for potassium, but this is a side effect rather than a measure of efficacy. Monitoring sodium is necessary to prevent hypernatremia and fluid retention, especially in heart failure patients. However, the nurse prioritizes the potassium level to ensure the life-threatening hyperkalemia has been resolved. D. Magnesium levels are not the primary focus for evaluating the effectiveness of this exchange resin. While electrolyte shifts can occur broadly during diuresis or bowel clear-out, sodium polystyrene sulfonate has a specific affinity for potassium. The nurse should monitor magnesium for general electrolyte balance, but it does not serve as a benchmark for the medication's intended therapeutic action.