What is the most appropriate initial test for a 23-year-old woman presenting with breast fullness and white discharge from her nipple?

Serum Prolactin Level is the Most Appropriate Initial Test for This Patient

The most appropriate initial test for this 23-year-old woman with breast fullness and white nipple discharge is a serum prolactin level.

Rationale for Serum Prolactin Testing

Nipple discharge can be classified as either physiologic or pathologic. This patient's presentation suggests pathologic discharge for several reasons:

• Unilateral discharge (left nipple only)
• Associated breast fullness
• White discharge in a non-lactating woman

Diagnostic Algorithm for Nipple Discharge

1. First step: Serum prolactin level

   • Most appropriate initial test for young women with nipple discharge
   • Hyperprolactinemia is a common cause of galactorrhea in young women
   • Can identify pituitary adenomas and medication-induced causes

2. Secondary testing based on prolactin results:

   • If elevated: MRI of the brain to evaluate for pituitary microadenoma
   • If normal: Breast ultrasound to evaluate for intraductal lesions

Evidence Supporting Prolactin Testing

The American College of Radiology (ACR) guidelines recognize that while imaging is important for pathologic nipple discharge, the initial laboratory evaluation is crucial in young women 1. For a 23-year-old woman with white discharge and no concerning features like bloody discharge or mass, a serum prolactin level should be the first test to rule out hyperprolactinemia 2.

Galactorrhea (milky nipple discharge) is often associated with elevated prolactin levels, which can be caused by:

• Pituitary microadenomas (most common pathologic cause)
• Medications (though patient denies medication use)
• Hypothyroidism
• Chronic renal failure
• Cirrhosis 3

Why Other Options Are Less Appropriate

• Mammography: Rated only 5/9 (may be appropriate) by ACR for women <30 years with nipple discharge 1. The ACR explicitly recommends ultrasound as the initial imaging study (rated 9/9) for women <30 with nipple discharge, not mammography.

• MRI of the brain: Should only be ordered after confirming hyperprolactinemia with serum prolactin testing, not as an initial test 3, 4.

• Serum estrogen level: Not indicated as an initial test for nipple discharge evaluation.

• Urine β-hCG: While pregnancy should be ruled out in women of reproductive age with breast symptoms, the patient's last menstrual period was 3 weeks ago, making pregnancy less likely as the cause of her symptoms.

Management Considerations

If prolactin levels are elevated:

• Evaluate for medication causes
• Order thyroid-stimulating hormone to rule out hypothyroidism
• Consider renal function testing
• Obtain MRI of the pituitary gland if no other cause is identified 3, 4

If prolactin levels are normal:

• Proceed with breast ultrasound, which is the recommended initial imaging study for women <30 years with pathologic nipple discharge 1, 2

Pitfalls to Avoid

• Dismissing white discharge as physiologic without proper evaluation
• Proceeding directly to imaging without laboratory evaluation in young women
• Failing to consider medication-induced causes of hyperprolactinemia
• Relying solely on imaging when endocrine causes are more likely in this age group

In conclusion, while breast imaging is an important component of the evaluation, serum prolactin level is the most appropriate initial test for this 23-year-old woman with unilateral white nipple discharge and breast fullness.