Zinc transporter 8 autoantibody testing requires age-related cut-offs

Sian Louise Grace, Angela Cooper, Angus G Jones, Timothy James McDonald, Sian Louise Grace, Angela Cooper, Angus G Jones, Timothy James McDonald

Abstract

Introduction: Zinc transporter 8 autoantibodies (ZnT8A) are biomarkers of beta cell autoimmunity in type 1 diabetes that have become more widely available to clinicians in recent years. Robust control population-defined thresholds are essential to ensure high clinical specificity in islet autoantibody testing. We aimed to determine the optimal cut-offs for ZnT8A testing.

Research design and methods: 97.5th and 99th centile cut-offs were determined using residual clinical sera from 1559 controls aged between 0 and 83 years with no history of diabetes and a hemoglobin A1c level of less than 6.0% (<42 mmol/mol). ZnT8A were measured by ELISA (RSR, Cardiff, UK) on a Dynex DS2 ELISA robot (Dynex, Preston, UK). We assessed the impact of age-related cut-offs in comparison with the manufacturer's recommended threshold in a mixed cohort of young-onset (<age 30) diabetes (UNITED study (Using pharmacogeNetics to Improve Treatment in Early-onset Diabetes), n=145).

Results: Using the manufacturer's limit of detection, 6 WHO U/mL, 16.2% of people in the control cohort had detectable levels of ZnT8A and those who had detectable ZnT8A were much more likely to be younger (p<0.0001). The 97.5th and 99th centile thresholds were substantially higher in younger participants: 18 and 127 WHO U/mL (tested under 30 years) in comparison with 9 and 21 WHO U/mL (tested 30 years and over). In the UNITED cohort some of those found to be ZnT8A-positive by the manufacturer's threshold but negative using the appropriate 99% centile cut-off (127 WHO U/mL) displayed characteristics suggestive of type 2 diabetes.

Conclusions: Age-related thresholds are needed for ZnT8A testing. In those aged <30 years, use of manufacturers' recommended cut-offs may result in low test specificity and potentially high rates of false positive test results in patients who do not have autoimmune diabetes.

Trial registration: ClinicalTrials.gov NCT01238380.

Keywords: autoantibodies; autoimmunity; biomarkers; diabetes mellitus; type 1.

Conflict of interest statement

Competing interests: None declared.

© Author(s) (or their employer(s)) 2021. Re-use permitted under CC BY-NC. No commercial re-use. See rights and permissions. Published by BMJ.

Figures

Figure 1
Figure 1
Line graph of the 5-year rolling average of the percentage of those with detectable ZnT8A. The rolling average shows a drop to consistently

Figure 2

Plot of ZnT8A titers in…

Figure 2

Plot of ZnT8A titers in healthy controls (n=1559). Dotted lines indicate the 97.5th…

Figure 2
Plot of ZnT8A titers in healthy controls (n=1559). Dotted lines indicate the 97.5th and 99th percentiles for
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Figure 2
Figure 2
Plot of ZnT8A titers in healthy controls (n=1559). Dotted lines indicate the 97.5th and 99th percentiles for

References

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