What is the appropriate test to evaluate parathyroid hormone (PTH) function?

Parathyroid Hormone Testing: Appropriate Assay Selection

The appropriate test to evaluate parathyroid hormone function is a second-generation intact PTH (iPTH) assay using immunoradiometric assay (IRMA) or immunochemiluminometric assay (ICMA), which remains the standard screening tool for most clinical situations including hyperparathyroidism diagnosis and chronic kidney disease monitoring. 1, 2

Primary Testing Recommendations

Standard Clinical Practice

• Second-generation intact PTH assays are the established standard for routine clinical use, measuring both full-length PTH (1-84) and the 7-84 fragment using a sandwich immunoassay with antibodies directed against the N-terminus (amino acids 12-24 or 26-32) and C-terminus (amino acids 39-84). 1

• These assays provide adequate screening to differentiate high-turnover bone disease (osteitis fibrosa) from low-turnover bone disorders (adynamic bone disease), particularly in chronic kidney disease patients. 1

• PTH testing is indicated when GFR falls below 60 mL/min/1.73 m² (CKD stages 3b-5), for evaluation of hypercalcemia to distinguish primary hyperparathyroidism, and for disorders of calcium metabolism. 2

Third-Generation Assays: Specialized Applications

• Third-generation "whole PTH" or "bio-intact PTH" assays measure only full-length PTH (1-84) using an N-terminal antibody directed against the first four amino acids, eliminating detection of the 7-84 fragment. 1

• While more specific, third-generation assays have not replaced second-generation assays as standard clinical tools due to insufficient accumulated research establishing their predictive power and clinical superiority. 1

• Third-generation assays may be more useful for intraoperative PTH monitoring during parathyroidectomy, showing more rapid PTH decline and potentially more accurate prediction of surgical success. 3

Critical Technical Considerations

Assay Variability and Standardization

• PTH measurements can vary up to 47% between second- and third-generation assays, making it essential to use the same assay type for serial measurements in individual patients. 1, 4

• Different second-generation assays can also vary significantly depending on whether the N-terminal antibody targets amino acids 12-24 versus 26-32. 1

• PTH assays are not yet standardized against an internationally recognized reference material, contributing to interlaboratory variability even within the same assay generation. 3

Pre-analytical Factors Affecting Results

• PTH is more stable in EDTA plasma than in serum and should be stored at 4°C rather than room temperature for optimal stability. 2

• PTH exhibits a circadian rhythm that may affect results depending on sampling time. 2

• Biotin supplements can cause significant interference, leading to either overestimation or underestimation of PTH levels depending on the assay platform used. 2, 4

Clinical Interpretation Framework

Understanding What Each Assay Measures

• Second-generation assays overestimate biologically active PTH by detecting C-terminal fragments (particularly 7-84 PTH) that lack biological activity but have long half-lives, especially in kidney disease. 1

• The 7-84 fragment may have hypocalcemic properties and inhibit osteoclastic bone resorption, representing a distinct biological activity from full-length PTH. 5

• Third-generation assays still detect posttranslationally modified PTH forms (including oxidized PTH and amino-PTH), which are biologically inactive but measured as active hormone. 1, 4

Patient-Specific Factors

• Reference values vary based on vitamin D status, BMI, race, age, and the specific assay used, requiring interpretation in clinical context rather than rigid adherence to laboratory reference ranges. 2

• In chronic kidney disease, attempting to maintain PTH in the "normal" range can cause adynamic bone disease; stage-specific targets should be used instead. 3

Common Pitfalls to Avoid

Assay Selection Errors

• Do not switch between assay generations when monitoring individual patients over time, as this creates artificial changes in PTH trends that do not reflect true physiological changes. 4, 3

• Be aware that guidelines may reference specific assay generations, and absolute cutoff values may not apply when using a different generation assay. 3

Interpretation Mistakes

• In CKD patients, elevated PTH measured by second-generation assays may overestimate true biologically active hormone due to accumulation of 7-84 fragments that are cleared renally. 1, 5

• The ratio of 1-84 PTH to non-(1-84) PTH fragments has been proposed as a bone turnover marker but shows limited diagnostic utility using a cutoff value of 1.0. 6

Laboratory Communication

• Always verify which PTH assay generation your laboratory uses and document this in the medical record for future reference. 3

• When referring patients or reviewing outside records, confirm assay compatibility before comparing PTH values across different laboratories or time periods. 1