What is the preferred assay, Roche (Electrochemiluminescence Immunoassay) or Abbott (Chemiluminescent Immunoassay), for measuring free Thyroxine (fT4) and free Triiodothyronine (fT3) levels?

Comparison of Roche vs Abbott Assays for Measuring Free Thyroid Hormones

The Roche Electrochemiluminescence Immunoassay (ECLIA) is preferred over the Abbott Chemiluminescent Immunoassay for measuring free T4 and free T3 levels, particularly in patients with suspected thyroid autoantibodies, as it demonstrates less interference and more accurate results in these cases.

Assay Performance Characteristics

Interference Issues

• The Roche ECLIA has been specifically shown to be more accurate in patients with triiodothyronine-binding autoantibodies, providing correct values that align with clinical thyroid status while RIA methods produced spuriously high results 1
• Abbott Alinity measures markedly lower fT4 and fT3 concentrations compared to other methods, particularly at high fT4 concentrations, which can affect clinical decision-making in hyperthyroid patients 2
• Antiruthenium antibodies can cause interference in Roche thyroid assays, including TSH, fT4, and fT3, though Roche has introduced a new generation fT4 assay to address this issue 3

Clinical Implications of Assay Differences

• Large non-linear method differences exist between platforms, especially at high fT4 concentrations 2
• These differences can significantly impact treatment decisions for hyperthyroid patients:
  • Patients appear much less hyperthyroid when measured with Abbott Alinity compared to Roche Cobas
  • Antithyroid drug dosages based on fT4 levels need to be adjusted according to the method used 2

Recommendations for Clinical Practice

When to Consider Alternative Testing Methods

• When thyroid hormone levels are elevated and TSH is not appropriately suppressed, consider:
  • Confirmatory testing with equilibrium dialysis LC-MS/MS, which is not susceptible to interference from autoantibodies 4
  • Testing for thyroid hormone autoantibodies (THAAs) if immunoassay interference is suspected 4

Documentation Best Practices

• fT4 and fT3 results from different methods should be clearly distinguished (e.g., separate lines) in medical records to avoid confusion 2
• When switching between assay platforms, clinicians should be aware of method-specific reference ranges

Special Considerations

Monitoring Thyroid Function

• For routine monitoring of thyroid function, both TSH and free T4 should be ordered when:
  • Patients are symptomatic and hypothyroidism is suspected
  • Patients are on immune checkpoint inhibitor therapy 5
  • Following up on previously abnormal results

Situations Requiring Particular Attention

• In patients with severe hyperthyroidism, the method differences between assays are most pronounced 2
• In patients with known or suspected thyroid autoantibodies, Roche ECLIA provides more reliable results 1
• When monitoring patients on levothyroxine replacement therapy, consistent use of the same assay platform is recommended to avoid misinterpretation of changing values

Pitfalls and Caveats

• Be aware that immunoassays for fT4 and fT3 can be affected by:
  • Endogenous autoantibodies to thyroid hormones
  • Heterophile antibodies
  • Anti-reagent antibodies (such as antiruthenium antibodies in Roche assays) 3
• In cases of discordant results (e.g., elevated fT4 with non-suppressed TSH), consider assay interference and use alternative methods such as equilibrium dialysis LC-MS/MS 4
• The relationship between Abbott Alinity and other methods is non-linear, particularly at high fT4 concentrations, which can lead to significant clinical misinterpretation 2