RESUMO
BACKGROUND: High-dose vitamin D supplementation has been recommended as treatment to several conditions; however, potential side effects such as hypercalcemia should be considered, plus the fact that high levels of 25(OH)D may interfere with potential 1,25(OH)2D measurements. Our study compared two methods of measuring 1,25-dihydroxyvitamin D [1,25(OH)2D] in samples with 25-hydroxyvitamin D [25(OH)D] levels above 150 ng/mL (375 nmol/L). METHODS: We studied serum samples referred to 25(OH)D and 1,25(OH)2D quantification. The concentrations of 25(OH)D and 1,25(OH)2D were measured using DiaSorin chemiluminescent assays (CLIA) in 213 samples (CLIA group), whereas in 357 samples 25(OH)D and 1,25(OH)2D were measured by DiaSorin CLIA and liquid chromatography-tandem mass spectrometry (LC-MS/MS), respectively (CLIA + MS group). RESULTS: Median concentrations of 25(OH)D and 1,25(OH)2D in the CLIA group were 371 ng/mL (928 nmol/L, range 154-856 ng/mL) and 350 pg/mL (875 pmol/L, range 41-1280 pg/mL), respectively, and correlated significantly (Spearman correlation coefficient rs = 0.8469, P < 0.001). In the CLIA + MS group, the median concentrations of 25(OH)D and 1,25(OH)2D were, respectively, 344 ng/mL (860 nmol/L, range 152-756 ng/mL) and 56 pg/mL (140 pmol/L, range 17-151 pg/mL), and were not correlated (rs = 0.0218, P = 0.6811). No significant difference was found in calcium, creatinine, and PTH serum values between the groups. CONCLUSION: Methods for measuring 1,25(OH)2D in patients with high levels of 25(OH)D may be susceptible to interference by 25(OH)D and its metabolites and should be validated carefully. In such cases, measurement of 1,25(OH)2D using LC-MS/MS is preferred.