Use ～すら in written or formal spoken Japanese to spotlight an extreme, minimal case — "not even X," implying that if even X doesn't hold, everything beyond it certainly doesn't. It is a stiffer, more emphatic cousin of ～さえ, common in reports, complaint emails, and press releases, and it very often pairs with a negative predicate. Avoid it in casual conversation, where ～さえ or ～も sounds more natural.