What is "Washout" in Intraoperative PTH Monitoring
"Washout" refers to the clearance of PTH from the circulation after removal of hyperfunctioning parathyroid tissue, measured by the rapid decline in PTH levels due to its short half-life of only several minutes. 1
Physiologic Basis of Washout
The concept of washout is based on the biological half-life of PTH:
- PTH has a half-life of only several minutes, which allows for rapid clearance from the bloodstream once the source of excess PTH production is removed 1
- After successful resection of a parathyroid adenoma, PTH concentration drops instantly because the hyperfunctioning tissue is no longer secreting hormone 1
- This rapid decline provides surgeons with real-time feedback about whether the diseased parathyroid tissue has been adequately removed 1
Clinical Application During Surgery
The standard criterion for successful washout is a ≥50% decline in PTH compared to the preoperative level, which defines treatment success during parathyroidectomy 1, 2
Timing of Washout Assessment
- PTH measurements are typically obtained at 5 and 10 minutes after adenoma removal to assess for adequate washout 3
- With third-generation PTH assays, the washout drop can be observed after 10 minutes, whereas second-generation assays require an additional 30 minutes to demonstrate the same decline 1, 2
- The faster washout detection with third-generation assays occurs because they measure primarily intact PTH rather than C-terminal fragments, which have longer half-lives 1
Assay-Specific Considerations
The type of PTH assay used significantly impacts washout assessment:
- Second-generation assays recognize C-terminal fragments that have a longer half-life, making the washout appear slower 1
- Third-generation assays show a more rapid decrease in PTH concentrations because they don't measure these longer-lived fragments 1, 2
- The difference is more pronounced in patients with secondary hyperparathyroidism (SHPT) compared to primary hyperparathyroidism (PHPT) 1
Clinical Pitfalls
A common pitfall is removing the wrong gland and misinterpreting inadequate washout: In one reported case, PTH levels remained high after excision of a preoperatively localized lesion, which turned out to be a normal parathyroid gland rather than the adenoma—continued exploration revealed a contralateral adenoma, and subsequent PTH measurement showed proper washout 3
Residual PTH secretion from suppressed normal glands can complicate interpretation: Normal-sized parathyroid glands associated with an adenoma are not completely suppressed and continue to secrete PTH, particularly amino-terminal PTH fragments, which can affect the washout pattern 4