# A client has been on carbamazepine for 6 weeks. The nurse reviews labs and notes serum sodium 126 mEq/L. What is the priority nursing action?

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> subject: Medical Emergencies  
> category: PA

## Question

A client has been on carbamazepine for 6 weeks. The nurse reviews labs and notes serum sodium 126 mEq/L. What is the priority nursing action?

## Option

1. Hold the medication and instruct the client to increase daily sodium intake while monitoring levels.
2. Initiate an infusion of 0.9% sodium chloride to rapidly increase serum sodium levels.
3. Start a 0.45% sodium chloride infusion to cautiously address the low sodium level.
4. Withhold the next dose and notify the provider — carbamazepine-induced SIADH should be evaluated **✔ Correct answer**

**Correct answer: 4**

## Explanation

Correct (1): Carbamazepine can cause SIADH (syndrome of inappropriate antidiuretic hormone) — water retention and dilutional hyponatremia. At Na 126 with neurologic symptoms, hold the dose and notify the provider for evaluation (further fluid restriction, possible drug change, careful sodium correction). (2) More free water worsens hyponatremia. (3) Hypotonic fluid worsens it further. (4) Abrupt discontinuation requires provider order to avoid seizure rebound.

## In-depth explanation

Memory Tip Carbamazepine = "Crystals lock in water" → SIADH·hyponatremia. CPG Check baseline and periodic sodium during carbamazepine therapy; risk highest in elderly and women.

## Clinical scenario

Scenario 58-year-old male on carbamazepine 400 mg PO BID × 6 weeks reports headache, mild confusion, and decreased urine output. Serum sodium 126 mEq/L (low).

## Key concepts

- **SIADH** — Excess ADH secretion causing water retention, dilutional hyponatremia, and concentrated urine despite low serum osmolality.
- **Dilutional hyponatremia** — Low serum sodium from water excess rather than sodium loss; treatment is fluid restriction.
- **Carbamazepine adverse effects** — SIADH, agranulocytosis, hepatotoxicity, SJS, and CYP3A4 autoinduction — all require monitoring.

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