title: Is nephrolithiasis an unrecognized extra-articular manifestation in ankylosing spondylitis? A prospective population-based Swedish national cohort study with matched general population comparator subjects.
abstract: BACKGROUND
  Ankylosing spondylitis (AS) is associated with several extra-articular manifestations. Nephrolithiasis (NL) has not been recognized as one of those, however, several factors known to increase the risk of NL are at play in AS patients. The objective was to estimate rates and predictors of NL in Swedish patients with AS compared to the general population.
  
  
  RESULTS
  We performed a prospective population-based nationwide cohort study based on linkage of data from Swedish registries. 8,572 AS patients were followed for 49,258 person-years (py) and 39,639 matched general population comparators were followed for 223,985 py. Patients were followed prospectively together with comparator subjects from January 2001 through December 2009. The first occurrence of NL during follow-up was the primary outcome. Hazard Ratios (HR) were used to compare these rates adjusting for comorbidities and treatment, and to assess predictors for NL. Mean age at study entry was 46 years (inter quartile range 36-56 years), 65% were males. Based on 250 vs. 466 NL events, the adjusted HR of NL in AS patients was 2.1 (95%CI 1.8 to 2.4). Predictors of NL within the AS group included prior diagnosis of inflammatory bowel disease (IBD) (HR 2.3; 95%CI 1.7 to 3.3), prior diagnosis of NL (HR 16.4; 95%CI 11.5 to 23.4), and patients receiving anti-TNF treatment (HR 1.6; 95%CI 1.2 to 2.1). Male sex was a risk factor for NL both in AS patients and in the general population.
  
  
  CONCLUSIONS
  The risk for residual confounding and inability to study the chemical nature of NL were considered the main limitations of the study.
  
  
  CONCLUSIONS
  Patients with AS are at increased risk of NL, which may be considered a novel extra-articular manifestation. Previous history of NL, IBD, AS disease severity and male sex were identified as predictors of NL in AS.
authors.affiliation: Department of Urology, Skåne University Hospital, Malmö, Sweden.
authors.forename: Ane Krag
authors.initials: AK
authors.lastname: Jakobsen
authors.affiliation: Department of Rheumatology and Inflammation Research, Sahlgrenska Academy at University of Gothenburg, Gothenburg, Sweden; Department of Rheumatology, Lund University, Malmö, Sweden.
authors.forename: Lennart T H
authors.initials: LT
authors.lastname: Jacobsson
authors.affiliation: Department of Urology, Skåne University Hospital, Malmö, Sweden.
authors.forename: Oliver
authors.initials: O
authors.lastname: Patschan
authors.affiliation: Clinical Epidemiology Unit and Rheumatology Unit, Department of Medicine (Solna), Karolinska Institutet, Stockholm, Sweden.
authors.forename: Johan
authors.initials: J
authors.lastname: Askling
authors.affiliation: Department of Rheumatology and Inflammation Research, Sahlgrenska Academy at University of Gothenburg, Gothenburg, Sweden; The Parker Institute and Department of Rheumatology; Frederiksberg and Bispebjerg, Denmark.
authors.forename: Lars Erik
authors.initials: LE
authors.lastname: Kristensen
mesh_terms: D000328:Adult; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D053040:Nephrolithiasis / Q000150:complications*; D011446:Prospective Studies; D013167:Spondylitis, Ankylosing / Q000150:complications*; D013548:Sweden