title: C-3 epimers can account for a significant proportion of total circulating 25-hydroxyvitamin D in infants, complicating accurate measurement and interpretation of vitamin D status.
abstract: BACKGROUND
  We have recently introduced liquid chromatography-tandem mass spectrometry (LC-MS/MS) for 25-hydroxyvitamin D(2) (25OHD(2)) and 25OHD(3) testing. During subsequent clinical use, we identified significantly elevated results in some infants. We hypothesized this might represent assay interference caused by C-3 epimers of 25OHD(2) or 25OHD(3).
  
  
  OBJECTIVE
  Our aims were to 1) determine the prevalence of C-3 epimers of 25OHD(2) or 25OHD(3) in human serum, and 2) identify the patient populations that might be affected.
  
  
  METHODS
  We modified our LC-MS/MS method to allow detection of C-3 epimers. We retested specimens from four patient groups with the new method and an extracted RIA: 1) children less than 1 yr old, 2) children 1-18 yr old, 3) adults aged 20-87 yr with liver disease, and 4) adults aged 19-91 yr without liver disease.
  
  
  RESULTS
  In 172 children from group 1 with detectable 25OHD(2) or 25OHD(3), we identified C-3 epimers in 39 (22.7%). The epimers contributed 8.7-61.1% of the total 25-OHD. There was an inverse relationship between patient age and epimer percentage (r = 0.48; P < 0.002). The RIA gave accurate 25-OHD results that correlated with the modified LC-MS/MS method. No C-3 epimers were detected in any of the other groups.
  
  
  CONCLUSIONS
  Significant concentrations of C-3 epimers of 25OHD(2) or 25OHD(3) are commonly found in infants. This can lead to overestimation of 25-OHD levels. Measurements in children less than 1 yr should therefore be performed with an assay that allows accurate detection of 25-OHD in the presence of its C-3 epimers.
authors.affiliation: Department of Laboratory Medicine, Mayo Clinic, 200 1st Street SW, Rochester, MN 55905, USA.
authors.forename: Ravinder J
authors.initials: RJ
authors.lastname: Singh
authors.forename: Robert L
authors.initials: RL
authors.lastname: Taylor
authors.forename: G Satyanarayana
authors.initials: GS
authors.lastname: Reddy
authors.forename: Stefan K G
authors.initials: SK
authors.lastname: Grebe
mesh_terms: D015652:25-Hydroxyvitamin D 2 / Q000097:blood* / Q000737:chemistry*; D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000375:Aging; D002112:Calcifediol / Q000097:blood* / Q000737:chemistry*; D002648:Child; D002675:Child, Preschool; D002851:Chromatography, High Pressure Liquid; D005260:Female; D006801:Humans; D007223:Infant; D007536:Isomerism; D008297:Male; D013058:Mass Spectrometry; D008875:Middle Aged; D009752:Nutritional Status; D011863:Radioimmunoassay; D012016:Reference Values; D012680:Sensitivity and Specificity
chemical_list: D015652:25-Hydroxyvitamin D 2; D002112:Calcifediol