Endocrine Abstracts

Background: There is no universal reference range for thyroid function tests (TFTs), and laboratories adopt different ranges based on their assay method. However, discrepancies between assay methods used across different health boards can lead to variability in TFT interpretation, potentially impacting clinical decision-making. This is not only affecting patient care, but also incurs an additional cost.

Aim: To evaluate the frequency and nature of discrepancies in TFT results between two different inter-laboratory assay methods, and whether these differences have led to a change in clinical management.

Method: A retrospective review of thyroid function data from a large university health board database was conducted. TFTs originally analysed using a particular assay method and subsequently re-evaluated using a different method at an adjacent university health board were identified. Discrepancies in results and any changes in clinical management were recorded. A chi-squared test was used to assess statistical significance.

Results: 64 TFT results were referred for re-evaluation. 34% showed differing outcomes between the two assay methods. 100% of these cases resulted in a change to the clinical management plan. 14 patients were hyperthyroid and 8 were hypothyroid under the initial assay method, but all were reclassified as euthyroid using the different assay method at the adjacent university hospital, avoiding unnecessary medication changes. The association between discrepant results and altered management was statistically significant with a p-value <0.001. Analysis of the re-evaluated values showed that discrepant cases are usually due to a result that is only marginally outside of the reference range. For TSH, values up to 0.37 below or 1.63 above the reference range were likely to be reclassified as normal when re-evaluated. For FT4, all raised results below 21 pmol/l were reclassified as normal.

Conclusion: Assay variability has a significant impact on the interpretation of TFTs and consequent clinical management in paediatric patients. Re-evaluation delays patient care and has cost implications. Borderline results are particularly susceptible; hence we recommend standardisation of TFT reference ranges irrespective of assay method.