The predictive factors of low serum 25-hydroxyvitamin D and vitamin D deficiency in patients with systemic lupus erythematosus.

Vitamin D is a steroid hormone with pleiotropic effects. The association between serum 25-hydroxyvitamin D level [25(OH) D] and lupus nephritis are not clearly known. We aim to determine serum 25(OH) D levels in patients with inactive SLE, active SLE without lupus nephritis (LN) and active SLE with LN and to identify clinical predictor of vitamin D deficiency. One hundred and eight SLE patients were included. Patients were classified as Group (Gr) 1, 2 and 3 if they had SLE disease activity index (SLEDAI) <3, ≥ 3 but no LN and ≥ 3 with LN. Important baseline characteristics were collected. 25(OH) D was measured by high performance liquid chromatography (HPLC). SLEDAI in Gr1, Gr2 and Gr3 was 0.7 (0.9), 5.6 (2.3) and 9.2 (5.2), respectively. 43.5 % had vitamin D insufficiency and 29.6 % had vitamin D deficiency. Mean 25(OH) D in each groups was 28.3 (8.0), 26.7 (9.5) and 19.9 (7.6) ng/ml (p < 0.001 comparing Gr1 and 3) (p = 0.003 comparing Gr2 and 3). Vitamin D deficiency was found in 11.1, 22.2 and 55.6 % of Gr1, 2 and 3. Linear regression analysis found that 25(OH) D was significantly correlated with serum albumin (r = 0.28, p = 0.004), inversely correlated with SLEDAI (r = -0.22, p = 0.03) and urinary protein creatinine index (UPCI) (r = -0.28, p = 0.005), but not with sun exposure score, body mass index and estimated GFR. Only UPCI was significantly inversely correlated with 25(OH) D (p = 0.02) from multiple linear regression. LN was a significant predictor of vitamin D deficiency from multivariate logistic regression (OR 5.97; p = 0.006). Vitamin D deficiency and insufficiency was found in 93 and 86 % of LN with proteinuria ≥ and <500 mg/day. We conclude that SLE patients with LN have significantly lower vitamin D level than inactive SLE and active SLE without LN. Hence, nephritis is a significant predictor of vitamin D deficiency in SLE patients.