Endocrine Abstracts

Thyroglobulin (Tg) is a tumoral marker of differentiated thyroid cancer (DTC). Iterative Tg assays are then mandatory for their follow-up. The maximum imprecision of any Tg measurement should be <5%. However, it is unlikely that current Tg assays can maintain such tight precision over the 6–12 months time period used for monitoring patients with DTC. This precision can be overcome by measuring archived frozen samples from the patient in the same run as the current specimen. This aims to minimise inter-assay variability and to avoid inappropriate therapeutic action.

We retrospectively investigated the systematic re-assay of archived serum samples from the last 10 year: 2992 out of a total of 16689 sera. From the re-assayed sera, we analysed only those <2.5 ng/ml (n=2063). The same kit was always used: thyroglobulin IRMA, CIS bio international. Intra-assay CV were 6.4 and 3.5% at 11.6 and 37.1 ng/ml, respectively. Inter-assay CV was 4.6 (1.1–10.2)% and 4.5 (0.8–8.6)% for the last 2 year for the two controls of the kit (about 11 and 36 ng/ml).

One thousand seven hundred seventy four results were ≤1 ng/ml. The median (2.5th-97.5th percentile) coefficient of variation between the two assays was 0.0 (0.0–2.2)%. When reassayed, 1748 results were ≤1 ng/ml. The 26 ‘discordant’ results ranged from 1.01 to 1.52 ng/ml. When the recommended threshold (1.5 ng/ml) for analytical variability was considered there was 1 ‘discordant’ result (1.52 ng/ml).

Two hundred eighty nine results were >1 and <2.5 ng/ml The coefficient of variation between the two assays was 8.7 (0.4–36.7)%. When reassayed, 64 results were “discordant”: 23 were ≤1 ng/ml and 41 were ≥2.5 ng/ml (range (2.5–4.1) ng/ml). When analytical variability was considered, this amounted to 3 ‘discordant’ results (4.02, 4.07, 4.1 ng/ml).

Although some physicians do not draw clinical conclusion from Tg values <1–2 ng/ml, the assay precision is excellent throughout this measuring range questioning the opportunity of systematic retesting prior archived samples at each individual run.

Declaration of interest: The authors declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research project.

Funding: This research did not receive any specific grant from any funding agency in the public, commercial or not-for-profit sector.