What is the proper procedure for sending a sample for inactivated Parathyroid Hormone (PTH) testing?

Proper Sample Collection and Handling for PTH Testing

Collect blood in EDTA tubes (not serum) and process within 24 hours, storing plasma at 4°C until analysis within 72 hours. This recommendation is based on the IFCC working group guidelines, which specifically state that PTH is most stable in EDTA plasma compared to serum 1.

Sample Collection Specifications

Tube Type

• Use EDTA (ethylenediaminetetraacetic acid) anticoagulant tubes exclusively 1, 2
• Do not use serum tubes, as PTH degrades more rapidly in serum compared to EDTA plasma 1, 3
• Avoid interchangeable use of serum and plasma, as they yield different PTH concentrations 1

Timing Considerations

• Optimal collection time: between 10:00 AM and 4:00 PM (10:00-16:00) to minimize circadian rhythm effects 3
• PTH exhibits a circadian rhythm with nocturnal peak and mid-morning nadir 2, 3
• Avoid sampling during winter months if possible, as PTH concentrations are higher in winter than summer in the northern hemisphere 3

Sampling Site

• Use peripheral venous blood rather than central venous access 2, 3
• Central venous PTH concentrations are significantly higher than peripheral venous concentrations 3
• Document sampling site when interpreting results, as location affects measured values 3

Sample Processing and Storage

Immediate Handling (Room Temperature)

• PTH remains stable in EDTA whole blood for at least 24 hours at room temperature 3, 4
• EDTA plasma is stable for at least 48 hours at room temperature 3
• In contrast, clotted blood shows losses after only 3 hours, and serum after 2 hours at room temperature 3

Optimal Storage Protocol

• Separate plasma within 24 hours of venepuncture 3
• Store separated EDTA plasma at 4°C (refrigerated) 1, 2, 3
• Analyze within 72 hours of venepuncture when stored at 4°C 3, 4
• PTH is stable in EDTA plasma at 4°C for at least 72 hours, compared to only 24 hours in serum 3

Frozen Storage Considerations

• Frozen storage data are limited and contradictory 3
• If freezing is necessary, serum may be more stable than EDTA plasma after 5 days at -20°C 5
• However, for routine clinical practice, avoid freezing when possible and use refrigerated storage instead 3

Critical Pitfalls to Avoid

Biotin Interference

• Screen patients for biotin supplement use (vitamin B7, B8, or H) 1, 2
• Biotin can cause either underestimation or overestimation of PTH depending on the specific assay design 1, 2
• Consider discontinuing biotin supplements 2-3 days before testing if interference is suspected

Assay-Specific Considerations

• Confirm which PTH assay generation your laboratory uses (second-generation vs. third-generation) 1, 2
• Second-generation assays measure both full-length PTH and inactive fragments, potentially overestimating biologically active PTH 1, 6
• Third-generation assays are more specific for full-length 1-84 PTH only 1, 6
• Results can vary up to 47% between different assay generations 6, 2

Sequential Monitoring

• Always use the same assay in the same laboratory for serial measurements 6
• PTH assays lack standardization against internationally recognized reference materials, causing interlaboratory variability 6, 2

Summary Protocol

Step-by-step collection and handling:

1. Collect blood between 10:00-16:00 from peripheral vein into EDTA tube 3
2. Keep whole blood at room temperature if processing within 24 hours 3, 4
3. Separate plasma within 24 hours of collection 3
4. Store separated EDTA plasma at 4°C 1, 2, 3
5. Analyze within 72 hours of venepuncture 3, 4
6. Document any biotin supplement use by patient 1, 2