Evaluation of Vitamin D Storage in Patients with Chronic Kidney Disease: Detection of Serum Vitamin D Metabolites Using High Performance Liquid Chromatography-tandem Mass Spectrometry

BACKGROUND Vitamin D (VitD) deficiency is extremely common in chronic kidney disease (CKD). However, the current clinical testing of vitamin D is based on the recommended serum 25-hydroxyvitamin D [25(OH)D]. The levels of VitD components in CKD patients are rarely reported. In this study, we tested various VitD components, and used different methods to evaluate the VitD status of CKD patients in vivo. METHODS Totally 173 CKD patients and 111 control individuals were enrolled. Serum levels of 25(OH)D2, 25(OH)D3, C3-epimers (C3-epi) and free 25(OH)D [f-25(OH)D] were measured. The 25(OH)D2/25(OH)D3 ratio, C3-epi/25(OH)D3 ratio, total 25(OH)D [t-25(OH)D], and bioavailable vitamin D (BAVD) were calculated, respectively. RESULTS The ratios of 25(OH)D2/25(OH)D3, C3-epi/25(OH)D3, and the level of C3-epi in CKD patients were significantly higher than those in the control group (all P < 0.05). The levels of t-25(OH)D, 25(OH)D3, C3-epi, f-25(OH)D and BAVD in patients with CKD stage 5 were significantly lower than those in stages 2, 3, and 4 (all P < 0.05). The calculated VitD storage according to Method 3 [25(OH)D2/3 + 25(OH)D3] was only 32.95 %, which was lower than the results of 53.76 % by Method 1 [25(OH)D2+ 25(OH)D3+C3-epi] and 48.56 % by Method 2 [25(OH)D2/3 + 25(OH)D3+C3-epi]. In addition, the VitD results calculated by three methods were positively correlated with f-25(OH)D and BAVD, while C3-epi levels were also positively correlated with f-25(OH)D and BAVD. CONCLUSION Serum levels of t-25(OH)D, 25(OH)D3, C3-epi, f-25(OH)D and BAVD in CKD patients gradually decrease with the progression of CKD stages. Though the results of VitD storage in CKD patients evaluated by different methods are different, simultaneous detection of 25(OH)D2, 25(OH)D3, C3-epi and f-25(OH)D levels and fully estimation of their respective biological activities could accurately evaluate the VitD storage in vivo.