Gluten index is a measure of gluten strength: the share of wet gluten, on a 0 to 100 scale, that stays on a sieve after centrifuging.
In trade practice
A high gluten index means strong, elastic gluten for bread; a low one means weak, extensible gluten for biscuits and wafers. It is measured with wet gluten under ISO 21415-2. Two flours with the same protein can have very different indices.
In a specification
State a minimum, and for biscuit flour a maximum, with ISO 21415-2 named.