A 76-year-old woman comes to the outpatient clinic for a routine follow-up. Two weeks ago,… — USMLE Step 3

A $76$ -year-old woman comes to the outpatient clinic for a routine follow-up. Two weeks ago, she was diagnosed with major depressive disorder, and she is agreeable to starting pharmacotherapy today. A prescription for sertraline is initiated. Her medical history includes essential hypertension, hyperlipidemia, and osteoarthritis. Her daily medications include hydrochlorothiazide, lisinopril, atorvastatin, and as-needed acetaminophen. Her blood pressure is $135/82\text{ mmHg}$ and her heart rate is $74\text{/min}$ . Physical examination is unremarkable. In addition to assessing her clinical response to the new medication, which of the following is the most appropriate monitoring plan for this patient?

Measure serum sodium concentration in $2$ to $4$ weeksAnswer
B
Measure serum potassium concentration in $1$ week
C
Obtain an electrocardiogram in $4$ weeks
D
Measure serum aminotransferase levels in $4$ weeks
E
Measure serum thyroid-stimulating hormone in $6$ weeks

Answer

Measure serum sodium concentration in 2 to 4 weeks.

The correct action is to measure the serum sodium concentration. Elderly patients are at an increased risk of developing the syndrome of inappropriate antidiuretic hormone secretion (SIADH) when treated with selective serotonin reuptake inhibitors (SSRIs) such as sertraline. This risk is significantly magnified when the patient is concurrently taking a thiazide diuretic (e.g., hydrochlorothiazide), which independently depletes sodium. Current clinical guidelines recommend checking a basic metabolic panel for serum sodium levels

2

4

weeks after initiating an SSRI in older adults, particularly those with interacting medications or baseline risk factors.

Step-by-Step Solution

Analyze the patient's demographic profile and current medication regimen.

The patient is a

76

-year-old woman taking a thiazide diuretic (hydrochlorothiazide) who is starting a selective serotonin reuptake inhibitor (SSRI).

Age and polypharmacy significantly increase the risk of specific adverse drug events.

Identify the primary pharmacological risks associated with the new medication (sertraline).

SSRIs are strongly associated with the development of the syndrome of inappropriate antidiuretic hormone secretion (SIADH).

SIADH leads to euvolemic hyponatremia, which can cause severe neurologic complications in the elderly.

Determine compounding synergistic risk factors.

The risk of SSRI-induced hyponatremia is magnified in advanced age and by the concurrent use of thiazide diuretics, which independently cause sodium depletion.

Recognizing overlapping medication toxicities dictates the required surveillance.

Select the appropriate guideline-concordant monitoring strategy.

Measure a basic metabolic panel (specifically serum sodium)

2

4

weeks after initiation of the SSRI.

Timely laboratory evaluation detects asymptomatic hyponatremia, allowing for early intervention before clinical deterioration occurs.

Key Concept

Elderly patients initiating SSRI therapy, particularly those concurrently using thiazide diuretics, are at high risk for developing hyponatremia secondary to SIADH and require serum sodium monitoring within 2 to 4 weeks.

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