Since the fall 2015 it is mandatory to sign up for all written exams at LTH. This includes both regular exams and re-exams.
You sign up at the student portal, www.student.lu.se.
In some cases, e.g., when you wish to improve your grade, it will not be possible to sign up through the portal. In order to still take the exam you need to sign up directly to the department. You do this by sending an email to Erik Göthe during the regular time period for exam sign-up.
More information about anonymous exams can be found at http://www.student.lth.se/studieinformation/anonyma-tentor/.