What does an intact Parathyroid Hormone (PTH) lab test show?

Intact PTH Lab Test: Clinical Significance and Interpretation

The intact Parathyroid Hormone (PTH) lab test measures the biologically active form of PTH and is the primary method for evaluating parathyroid function, bone metabolism disorders, and mineral homeostasis, particularly in patients with chronic kidney disease (CKD). 1

What the Test Measures

• Intact PTH assay: Second-generation immunoassay that uses a sandwich technique with two antibodies - one directed against the C-terminal part and another against the N-terminal part of the PTH molecule 1
• Measures both the full-length PTH (1-84) and some large C-terminal fragments (particularly 7-84 PTH) 1
• Provides a more accurate assessment of parathyroid function than older first-generation assays that only measured C-terminal fragments 1, 2

Clinical Significance

Primary Uses

• Evaluation of calcium-phosphorus homeostasis disorders
• Assessment of bone metabolism in CKD patients
• Diagnosis and monitoring of:
  • Primary hyperparathyroidism
  • Secondary hyperparathyroidism (especially in CKD)
  • Hypoparathyroidism
  • Vitamin D deficiency
  • Hypercalcemia of various etiologies

Interpretation in CKD

In CKD patients, intact PTH is particularly valuable for:

• Distinguishing between high-turnover bone disease (osteitis fibrosa) and low-turnover bone disorders (adynamic bone disorder) 1
• Guiding treatment decisions for secondary hyperparathyroidism 1
• Monitoring response to therapy with vitamin D analogs, phosphate binders, and calcimimetics 1

Target Ranges

PTH target ranges vary by CKD stage:

• Normal range (non-CKD): Typically 10-65 pg/mL (1.1-7.15 pmol/L)
• CKD patients: Higher target ranges are recommended to account for PTH resistance and the presence of inactive PTH fragments 1, 2

Limitations and Considerations

1. Assay variability: Different intact PTH assays can vary up to 47% in their results 1, 3

   • Sequential measurements should use the same assay for proper trend analysis 2

2. Detection of inactive fragments: Intact PTH assays measure both active PTH and some inactive fragments 1

   • Third-generation ("whole" or "bio-intact") PTH assays are more specific for the biologically active form 1, 3

3. Post-translational modifications: Oxidized and phosphorylated forms of PTH may be detected but are biologically inactive 1

4. Interpretation context: PTH results should be interpreted alongside:

   • Serum calcium
   • Serum phosphorus
   • Alkaline phosphatase (bone turnover marker)
   • Vitamin D status 1, 2

Clinical Applications

1. CKD Management:

   • Guides treatment of secondary hyperparathyroidism
   • Helps prevent both high-turnover and low-turnover bone disease
   • Elevated levels (>600 pg/mL) may indicate need for vitamin D therapy 1
   • Very low levels (<65 pg/mL) suggest adynamic bone disease 1

2. Cardiovascular Risk Assessment:

   • PTH levels >70 pg/mL are independently associated with increased cardiovascular events in CKD stages 3-4 4
   • Both high and low PTH levels correlate with increased mortality in dialysis patients 1

3. Surgical Applications:

   • Rapid PTH measurements during parathyroid surgery can assess surgical success 2
   • Helps differentiate parathyroid adenoma from hyperplasia 5

Emerging Considerations

• Newer third-generation assays that specifically measure only the full-length PTH (1-84) may provide more accurate assessment of true parathyroid activity 1, 3
• The ratio of whole PTH to intact PTH may provide additional diagnostic information in certain clinical scenarios 3

Common Pitfalls

• Interpreting PTH in isolation without considering calcium, phosphorus, and vitamin D status
• Comparing results from different assay generations or manufacturers
• Targeting normal PTH ranges in CKD patients (can lead to adynamic bone disease)
• Failing to recognize that intact PTH assays measure both active and inactive fragments

By understanding what the intact PTH lab test shows and its limitations, clinicians can better diagnose and manage disorders of mineral metabolism, particularly in patients with CKD.