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Ankylosing spondylitis (AS) is a chronic inflammatory disease primarily involving the spine and sacroiliac joint and rarely the kidneys. This study aimed to define the clinical and histological features and biology of renal disease in AS. We reviewed the medical records of 681 patients diagnosed with AS from November 2008 to November 2009. Baseline characteristics and laboratory and urinalysis results were reviewed. We identified patients with proteinuria or hematuria and analyzed their risk factors. After providing informed consent, 6 patients underwent a renal biopsy to determine the cause of proteinuria or hematuria. Of the 681 enrolled patients, 547 were men and 134 were women; 81 % were HLA B27 positive, and 8 % had abnormal urinalysis findings (proteinuria, 5.9 %; hematuria, 2.8 %; both, 0.7 %). Incidences of peripheral arthritis and uveitis were 29 % and 18.6 %, respectively. Immunoglobulin (Ig)A and uric acid levels were significantly different between patients with and without proteinuria. Erythrocyte sedimentation rate (ESR), total cholesterol, creatinine, and C-reactive protein (CRP) levels were not statistically significantly different between the 2 groups nor were there any significant differences in IgA, uric acid, ESR, total cholesterol, creatinine, and CRP levels between patients with and without hematuria. Six patients who had >1 g/day proteinuria underwent a renal biopsy; 2 were diagnosed with IgA nephropathy, 1 with amyloidosis, and 3 with non-specific glomerulonephropathy. In the amyloidosis patient, severe proteinuria was the dominant feature. For patients with renal amyloidosis and other forms of glomerulonephritis who initially had normal creatinine levels, tumor necrosis factor (TNF)-alpha blocker therapy resolved proteinuria, but this was not the case for patients with initial renal insufficiency. Renal involvement is not a rare complication of AS, and prognoses differ depending on kidney pathology. Serum levels of uric acid and IgA may predict renal involvement in AS. In cases where abnormal urine sediment is identified, renal biopsy is required to determine prognosis and decide the treatment protocol. Baseline serum creatinine level is important for predicting treatment response. [ABSTRACT FROM AUTHOR]