Table 3. Multiple linear regression analysis of independent predictors of serum 25(OH)D concentration in patients with psoriasis.

Predictors	β coefficient	P value	95% CI
Arthritis
Absent *
Present	−0.266	0.759	−1.97 – 1.44
Severity
Mild *
Moderate-severe	−6.712	0.000 £	−8.51 – −4.91
Family history positive	0.194	0.797	−1.29 – 1.68
Age at diagnosis	−0.024	0.383	−0.007 – 0.030
Duration of psoriasis	0.006	0.474	−0.06 – 0.08
Season of the year
Winter £
Autumn	3.525	0.017	0.61 – 6.43
Spring	4.512	0.002	1.58 – 7.44
Summer	8.708	0.000 £	6.07 – 11.33
Skin phototype
III*
IV	1.556	.035	1.10 a 3.01
V	−3.783	.014	−6.89 a −0.95

^*Reference category: 25(OH)D = 25-hydroxyvitamin D; CI = Confidence Interval

^££P < 0.0001; adjusted R2 = 0.31, P < 0.0001.