When should a serum prolactin be drawn, including timing, fasting, rest, and avoidance of nipple stimulation?

Timing of Prolactin Blood Draw

A single prolactin measurement can be drawn at any time of day without special timing requirements, fasting, or rest periods, though patients should avoid nipple stimulation before the test and serial measurements may be needed for modestly elevated levels to exclude stress-related elevation. 1, 2

Timing and Collection Requirements

• No specific time of day is required for prolactin measurement—a single blood sample collected at any time is sufficient for initial assessment 1, 2
• Fasting is not necessary for prolactin testing 1
• No special rest period is required before blood draw 1
• Results are typically available within hours to one day, allowing for timely clinical decision-making 1

Important Pre-Test Considerations

• Avoid nipple stimulation before testing, as mechanical or tactile breast stimulation can acutely elevate oxytocin and potentially affect prolactin levels 3, 4
• Stress can elevate prolactin up to five times the upper limit of normal, so if initial levels are modestly elevated, repeat measurement on a different day is recommended to exclude stress-induced elevation 1, 5, 2
• Review all medications before testing, as dopamine antagonists are among the most common causes of hyperprolactinemia 1, 5

When to Repeat Testing

• For modestly elevated levels, consider serial measurements on different days to exclude transient stress-related elevation 1, 2
• If a large pituitary mass is found on imaging but prolactin seems paradoxically normal or only mildly elevated, request manual serial dilutions of the serum sample to rule out the "high-dose hook effect," which occurs in approximately 5% of macroprolactinomas 1, 5

Common Pitfalls to Avoid

• Do not assume specialized timing is needed—unlike some other hormones with circadian variation, prolactin can be measured at any time of day for clinical purposes 1, 2
• Do not delay testing waiting for specific conditions—the convenience of any-time sampling eliminates barriers to timely diagnosis 1
• Do not ignore macroprolactinemia—assess for this in cases of mild or incidental elevation, as it accounts for 10-40% of hyperprolactinemia cases and represents biologically inactive prolactin complexes 1, 5