A patient diagnosed with bipolar disorder is prescribed lithium carbonate. Which laboratory result should prompt the nurse to hold the medication and notify the healthcare provider?

A. Potassium 4.2 mEq/L is within the normal range (3.5–5.0 mEq/L). B. Sodium 136 mEq/L is slightly low but does not warrant withholding lithium. However, sodium levels should be monitored regularly as low sodium increases the risk of lithium toxicity. C. A calcium level of 9.0 mg/dL is within the normal range (8.5–10.2 mg/dL). D. An elevated creatinine level (2.1 mg/dL) indicates possible renal dysfunction, which is a concern for lithium use. Lithium is excreted by the kidneys, and impaired renal function increases the risk of lithium toxicity.