How should a serum prolactin sample be collected, including timing, patient preparation, and specimen handling?

Prolactin Sample Collection: Timing and Technique

For initial diagnostic evaluation of hyperprolactinemia, a single blood sample collected at any time of day is sufficient, as timing is not critical for diagnosis. 1

Standard Collection Protocol

Timing Considerations

• No specific time of day is required for the initial prolactin measurement 1
• Morning samples are commonly drawn for convenience, but this is not mandatory 1
• Results are typically available within hours to one day, allowing for timely clinical decision-making 2

Patient Preparation

• Minimize stress during collection, as stress can elevate prolactin levels up to five times the upper limit of normal 3
• Avoid venipuncture-related stress when possible 4, 5
• No fasting or special dietary restrictions are required 1

When Serial Sampling is Indicated

For patients with modestly elevated prolactin levels (typically <2,000 mU/L or <94 ng/mL), serial measurements should be considered to differentiate stress-related elevation from true hyperprolactinemia. 1, 6

Serial Sampling Technique

• Insert an indwelling venous cannula (typically in the antecubital vein) 1, 4, 6
• Draw samples at 20-60 minute intervals after cannulation 1, 4
• Most protocols use 3 samples: baseline (time 0), 30 minutes, and 60 minutes after cannulation 4, 5, 7
• Patient should remain at rest during the entire collection period 4, 5, 6
• Some protocols extend rest period to 120 minutes for maximum stress reduction 6

Clinical Value of Serial Sampling

• 68% of patients with initially elevated prolactin had normal levels on serial sampling, indicating artefactual hyperprolactinemia 5
• 27% of referred patients normalized after resting samples, avoiding unnecessary investigation 6
• Serial sampling is particularly valuable when referral prolactin is <2,000 mU/L (<94 ng/mL), as this threshold has 97% specificity for true hyperprolactinemia 6

Specimen Handling

Collection Tubes

• Use standard serum separator tubes or plain tubes for serum collection 8
• EDTA or heparin tubes can be used for plasma collection 8
• Avoid glass tubes if plastic alternatives are available per OSHA guidelines 8

Processing Requirements

• Centrifuge within 2 hours of blood collection at 1,500-2,000 × g for 10 minutes at 4°C 8
• Use swing-out rotors rather than fixed-angle rotors 8
• Separate serum/plasma from cells immediately after centrifugation 8
• Do not store samples in contact with cells for more than 2 hours 8

Storage

• Aliquot samples to minimize freeze-thaw cycles 8
• Store at -80°C or colder for long-term preservation 8
• Avoid storage at -20°C, which is inadequate for protein stability 8

Critical Pitfalls to Avoid

Stress-Related Elevation

• Single random samples may overdiagnose hyperprolactinemia in up to 68% of cases due to venipuncture stress 5
• Symptoms (galactorrhea, amenorrhea, infertility) do not reliably predict true hyperprolactinemia and can occur with stress-related elevation 5, 6

Laboratory Considerations

• Exclude macroprolactinemia (10-40% of cases) when prolactin is mildly or incidentally elevated 1, 3
• Request serial dilutions if large pituitary lesions show paradoxically normal/low prolactin to detect the "high-dose hook effect" (occurs in ~5% of macroprolactinomas) 1, 2, 3
• Use age-specific and sex-specific reference ranges, as prolactin varies significantly with age and sex 1, 3

Pre-Analytical Variables

• Exclude hypothyroidism before confirming hyperprolactinemia diagnosis 1, 3
• Review all medications, particularly dopamine antagonists, which are among the most common causes of hyperprolactinemia 3
• Document any renal or hepatic impairment, as these conditions affect prolactin clearance 3