Serial Dilution of Prolactin Level
Serial dilution of a prolactin level is a laboratory technique where the patient's serum sample is diluted (typically 1:10 or 1:100) before measuring prolactin, used specifically to detect the "high-dose hook effect" in patients with large pituitary tumors who have paradoxically normal or only mildly elevated prolactin levels. 1
What is the Hook Effect?
The hook effect is a laboratory artifact that occurs in two-site immunoradiometric assays when extremely high concentrations of prolactin saturate the signaling antibody, making it less available for binding to the coupling antibody, resulting in falsely low measurements. 1 This phenomenon occurs in approximately 5% of patients with macroprolactinomas. 1, 2
Clinical Scenario Requiring Serial Dilution
You should request serial dilution when there is a discrepancy between imaging and biochemistry: 1
- A large pituitary adenoma (typically >2-3 cm) is visible on MRI
- But prolactin levels are normal or only mildly elevated (<100-200 ng/mL)
- This mismatch suggests possible hook effect rather than a non-functioning adenoma
How Serial Dilution Works
When the serum sample is diluted (commonly 1:10 or 1:100), the excess prolactin is reduced to concentrations that fall within the assay's linear range, revealing the true prolactin level. 3 In documented cases, undiluted samples showed prolactin levels of 66.6-147.7 µg/L, but after dilution, the true levels were revealed to be 2,097-12,722 µg/L. 4
Practical Example
A 65-year-old man with a 10-cm pituitary mass had an initial prolactin of 164.5 ng/mL (seemingly inconsistent with tumor size). Serial dilution testing revealed the true prolactin level was 26,000 ng/mL, confirming a giant prolactinoma rather than a non-functioning adenoma. 3
Clinical Importance
This distinction is critical because it completely changes management: 1, 3
- Prolactinomas respond to medical therapy with dopamine agonists (cabergoline), avoiding surgery in most cases
- Non-functioning adenomas typically require surgical resection
- Missing the hook effect leads to unnecessary surgery in patients who would respond to medical therapy
Patient Characteristics at Higher Risk
Patients with hook effect tend to be: 4, 5
- Younger (mean age 38 years vs. 45 years for typical macroprolactinomas)
- More frequently male (75-100% in hook effect cases vs. 10% in typical macroprolactinomas)
- Very large tumors (mean 51 mm vs. 20-27 mm for typical macroadenomas)
- Grade III-IV adenomas according to Hardy classification
When to Request Serial Dilution
Contact your clinical biochemist to request manual dilution when: 1, 2
- Large pituitary lesion (>2 cm) on MRI
- Prolactin level is normal (<20 ng/mL) or mildly elevated (<100 ng/mL)
- The degree of elevation doesn't match the tumor size
- Patient has symptoms of mass effect (headaches, visual changes) but "low" prolactin
Important Caveat
Some modern prolactin assays have built-in safeguards such as large linear ranges or automatic dilution steps to prevent the hook effect. 1 However, the potential for this artifact remains, so clinical vigilance is essential when imaging and biochemistry don't align. 1