PriSUD-Nordic -- Diagnosing and treating substance use disorders in the prison population: Protocol for a mixed methods study by Bukten, A, Lokdam, NT, Skjærvø, I, Ugelvik, T, Skurtveit, S, Gabrhelík, R, Skardhamar, T, Lund, IO, Havnes, IA, Rognli, EB, Chang, Z, Fazel, S, Friestad, C, Hesse, M, Lothe, J, Ploeg, G, Dirkzwager, AJE, Clausen, T, Tjagvad, C, Stavseth, MR