Abstract :
[en] Background
Islet cells antibodies (ICA), tested by indirect immunofluorescence (IIF) on monkey pancreas, is a marker for Type 1 diabetes. However, the sensitivity of this method is weak, its results might be inconsistent with other assays (anti-GAD or anti-IA2 antibodies) and its recommended screening dilution varies according to references, leading to false negative results.
Objective
We sought to test different screening dilutions for ICA IIF testing, and to compare it with anti-GAD65kd method.
Methods
A total of 188 samples were prospectively collected during 15 months. All sera were tested for ICA positivity by IIF (Menarini, ICAb Primate Pancreas) at 1:5 dilution according to the manufacturer recommendations. In addition, the samples were also tested at 1:20, 1:50 and 1:100 dilutions to verify prozone (Hook) effect. Anti-GAD65kd was measured by ELISA (anti-GAD, IgG, Euroimmun) to confirm the results.
Results
Amongst the 188 samples, 33 (17.5%) samples were positive with both ICA and anti-GAD tests. However, only 16 of them (48,5%) were positive at the 1:5 screening dilution recommended by the manufacturer. The other 17 (51,5%) sera showed negative ICA at the 1:5 screening dilution, while being positive with 1:100 (or sometimes at 1:20 or 1:50). This is due to a prozone effect at low dilution which is avoided at higher dilutions, and was confirmed by anti-GAD positivity. One hundred fifty-five samples (80%) were negative with both anti-GAD and ICA at 1:5 dilution. Finally, four patients had positive anti-GAD antibodies but no positive ICA at any dilution, which is in accordance with previously published data.
Conclusion
We showed that the 1:5 screening dilution recommended by the manufacturer leads to high false negative rate. Prozone effect is a well-known drawback of assays using antibodies, however it is still underestimated. Hence, we recommend to use a 1:100 screening dilution for ICA IIF testing.
