Siteimprove cannot track screen reader usage of visitors using assistive technologies on your site. Currently, there are no reliable means of determining whether a user with disabilities is visiting your site. This is because information is not available as the standard information that is used in tracking users, such as user-agent strings.
Instead, web browsers present a structured "accessibility tree" to the disabled users' operating system, which is then read by assistive technologies such as screen readers. This means that a website's tracking code can never detect what devices are used to interpret an operating system's accessibility tree, because this happens as a "post-processing" step, after the browser has downloaded the content from the Internet.