Which laboratory result should a nurse report immediately for a client taking lithium?

When a client is taking lithium, it is critical to monitor their electrolyte levels, particularly sodium. A sodium level of 130 mEq/L indicates hyponatremia, which can significantly affect lithium levels in the body. Lithium's therapeutic range is closely tied to sodium levels, as low sodium can lead to an increase in lithium reabsorption by the kidneys, potentially causing lithium toxicity. Symptoms of lithium toxicity include tremors, confusion, and ataxia, which can become serious if not addressed promptly. Therefore, the immediate reporting of a sodium level of 130 mEq/L is necessary to ensure the safety of the patient and to facilitate timely intervention.