A nurse is reviewing routine laboratory values for several clients who are taking lithium carbonate. Which of the following clients should the nurse assess further for findings indicating lithium toxicity?

Choice A reason: A BUN of 18 mg/dL is within normal range (7–20 mg/dL) and does not indicate lithium toxicity. Lithium is renally excreted, and normal renal function, as reflected by BUN, suggests adequate clearance. Toxicity arises from sodium imbalances or dehydration, not directly from normal BUN levels, making this unremarkable. Choice B reason: A fasting blood glucose of 80 mg/dL is normal (70–100 mg/dL) and unrelated to lithium toxicity. Lithium affects sodium and water balance, not glucose metabolism. Toxicity involves neurological symptoms from elevated serum lithium due to impaired renal clearance, not glycemic changes, so this value requires no further assessment. Choice C reason: A potassium level of 3.6 mEq/L is within normal range (3.5–5.0 mEq/L) and does not indicate lithium toxicity. Lithium primarily affects sodium reabsorption in renal tubules, not potassium. Toxicity symptoms like tremors or confusion stem from sodium imbalances or high lithium levels, not normal potassium levels. Choice D reason: A sodium level of 128 mEq/L (normal 135–145 mEq/L) indicates hyponatremia, increasing lithium toxicity risk. Lithium is reabsorbed in renal tubules like sodium; low sodium reduces lithium excretion, elevating serum levels, causing neurological symptoms like tremors or seizures. This requires immediate assessment to prevent toxicity.