Serum Uric Acid Shows Inverted "U" Type Correlation with Osteoporosis in Chinese Ankylosing Spondylitis Patients: A Retrospective Study

Abstract

BACKGROUND This study was to investigate the correlation between osteoporosis and serum uric acid in ankylosing spondylitis (AS) patients, and to further identify potential factors that might be associated with osteoporosis in AS patients. MATERIAL AND METHODS We included 182 AS patients, consisted of 143 male patients and 39 female patients, who visited our hospital from January 1, 2014 to December 31, 2018. We used dual-energy x-ray absorptiometry to measure bone mineral density (BMD) of orthotopic lumbar vertebrae in patients with AS. The gender, age, disease duration, BMD, T-score, Z-score, uric acid, erythrocyte sedimentation rate (ESR), C-reactive protein (CRP), blood platelet (PLT), and status of treatment with biologics of the patients were collected. Then, the Spearman correlation coefficient and multivariate liner regression analysis were applied to identify the relationship between the factors and BMD, T-score, and Z-score in AS patients. RESULTS Male AS patients between the ages of 16 and 30 years old had a higher risk of osteoporosis (P<0.05). AS patients with uric acid value between 300-360 μmol/L had the highest BMD, T-score, and Z-score. The BMD had a positive correlation with age and disease duration (P<0.01) while had a negative correlation with PLT (P<0.05). BMD in AS patients with elevated ESR was significantly (P<0.05) lower than in AS patients with normal ESR. There were no significant differences in BMD between AS patients with elevated CRP and the patients with normal CRP and PLT. Treatment with TNFi (tumor necrosis factor alpha inhibitor) did not improve BMD in AS patients. CONCLUSIONS The relationship between uric acid and BMD in AS patients was observed as inverted "U"-type. Keeping uric acid within 300-360 μmol/L might be helpful in preventing AS patients from developing osteoporosis.

Figure 1

Flowchart of inclusion and exclusion.

Figure 2

Medications used in ankylosing spondylitis patients.

Figure 3

Linear regression analysis between bone mineral density (at the lumbar spine) and uric acid (UA), age, erythrocyte sedimentation rate (ESR), C-reactive protein (CRP), blood platelet (PLT), and disease duration of ankylosing spondylitis patients. (A) UA; (B) Age; (C) ESR; (D) CRP; (E) PLT; and (F) Disease duration.

Figure 4

(A–C) Bone mineral density (BMD) in the lumbar spine according to serum uric acid (UA) tertiles. (A) BMD; (B) T-score; and (C) Z-score. (D–K) Differences in BMD, T-score, Z-score, UA, erythrocyte sedimentation rate (ESR), C-reactive protein (CRP), and blood platelet (PLT) between male and female ankylosing spondylitis patients.

Figure 5

The T-score in ankylosing spondylitis patients with different gender and age ranges.

Figure 6

Bone mineral density (BMD), T-score, and Z-score according to different levels of erythrocyte sedimentation rate (ESR), C-reactive protein (CRP), and blood platelet (PLT) in total AS patients. (A) BMD; (B) T-score; (C) Z-score; (D) BMD in male and female patients; (E) T-score in male and female patients; and (F) Z-score in male and female patients.

Figure 7

Bone mineral density (BMD), T-score, and Z-score according to whether treatment with biologic disease-modifying antirheumatic drugs (bDMARDs). (A) BMD in total patients; (B). T-score in total patients; (C) Z-score in total patients. (D) BMD in male patients. (E) T-score in male patients. (F) Z-score in male patients.