Endocrine Abstracts

Background/aims: Thyroglobulin (Tg) is used for monitoring patients who have undergone total thyroidectomy (TT) and radioiodine (RAI) ablation therapy for thyroid cancer. The current method is the Siemens Immulite assay with limit of quantification of 2 ng/mL following in-house evaluation. Recent guidelines suggest the use of high sensitivity Tg (hs-Tg) as an alternative to TSH stimulated Tg levels. The aim is to evaluate the hs-Tg Beckman Access II assay with a stated functional sensitivity of 0.1 ng/ml.

Method: Bias was assessed relative to the Siemens assay using Deming regression and Bland-Altman analysis on 45 patient samples. Bias relative to the method mean was calculated using nine NEQAS samples. Intra-assay and inter-assay imprecision were calculated from three replicates of four patient samples for hs-Tg over one and five days respectively. Clinical utility of the assay was assessed by measurement of 126 patients with an Immulite result <2 ng/ml after TT and RAI.

Results: Bland Altman analysis showed a 50% negative proportional bias and Deming regression showed a slope of 0.604. Intra-assay imprecision (%CV) was 10.2% at 0.13 ng/ml, 2.0% at 0.77 ng/ml, 2.4% at 1.13 ng/ml and 1.7% at 2.62 ng/ml. Total imprecision was 11.3% at 0.13 ng/ml, 3.6% at 0.77 ng/ml, 11.3% at 1.13 ng/ml and 7.3% at 2.62 ng/ml. Time post RAI ranged from one month to twenty years, median four years. A Tg≥0.1 ng/mL was obtained in 37/126 (29%) patients. Of these patients, 25 were <5 years post RAI and 12 were >5 years.

Conclusion: A period of paired analysis would be required due to the significant negative bias observed. The assay has acceptable imprecision and EQA performance. A subset of patients with detectable hs-Tg were identified; further investigation is required to determine the clinical significance.