title: The relative dose response test based on retinol-binding protein 4 is not suitable to assess vitamin A status in very low birth weight infants.
abstract: BACKGROUND
  The relative dose response (RDR) test, which quantifies the increase in serum retinol after vitamin A administration, is a qualitative measure of liver vitamin A stores. Particularly in preterm infants, the feasibility of the RDR test involving blood is critically dependent on small sample volumes.
  
  
  OBJECTIVE
  This study aimed to assess whether the RDR calculated with retinol-binding protein 4 (RBP4) might be a substitute for the classical retinol-based RDR test for assessing vitamin A status in very preterm infants.
  
  
  METHODS
  This study included preterm infants with a birth weight below 1,500 g (n = 63, median birth weight 985 g, median gestational age 27.4 weeks) who were treated with 5,000 IU retinyl palmitate intramuscularly 3 times a week for 4 weeks. On day 3 (first vitamin A injection) and day 28 of life (last vitamin A injection), the RDR was calculated and compared using serum retinol and RBP4 concentrations.
  
  
  RESULTS
  The concentrations of retinol (p < 0.001) and RBP4 (p < 0.01) increased significantly from day 3 to day 28. On day 3, the median (IQR) retinol-RDR was 27% (8.4-42.5) and the median RBP4-RDR was 8.4% (-3.4 to 27.9), compared to 7.5% (-10.6 to 20.8) and -0.61% (-19.7 to 15.3) on day 28. The results for retinol-RDR and RBP4-RDR revealed no significant correlation. The agreement between retinol-RDR and RBP4-RDR was poor (day 3: Cohen's κ = 0.12; day 28: Cohen's κ = 0.18).
  
  
  CONCLUSIONS
  The RDR test based on circulating RBP4 is unlikely to reflect the hepatic vitamin A status in preterm infants.
authors.affiliation: Department of Physiology and Pathophysiology, University of Potsdam, Nuthetal, Germany.
authors.forename: Bettina
authors.initials: B
authors.lastname: Schmiedchen
authors.forename: Ann Carolin
authors.initials: AC
authors.lastname: Longardt
authors.forename: Christoph
authors.initials: C
authors.lastname: Bührer
authors.forename: Jens
authors.initials: J
authors.lastname: Raila
authors.forename: Andrea
authors.initials: A
authors.lastname: Loui
authors.forename: Florian J
authors.initials: FJ
authors.lastname: Schweigert
mesh_terms: D001800:Blood Specimen Collection / Q000379:methods / Q000592:standards; D019411:Clinical Laboratory Techniques / Q000379:methods* / Q000592:standards; D004305:Dose-Response Relationship, Drug; D005240:Feasibility Studies; D005260:Female; D006304:Health Status; D006801:Humans; D007231:Infant, Newborn; D007235:Infant, Premature, Diseases / Q000097:blood / Q000175:diagnosis*; D019102:Infant, Very Low Birth Weight / Q000097:blood*; D008099:Liver / Q000378:metabolism; D008297:Male; D054839:Retinol-Binding Proteins, Plasma / Q000032:analysis*; D014801:Vitamin A / Q000008:administration & dosage* / Q000097:blood / Q000378:metabolism*; D014802:Vitamin A Deficiency / Q000097:blood / Q000151:congenital* / Q000175:diagnosis*
chemical_list: C101326:RBP4 protein, human; D054839:Retinol-Binding Proteins, Plasma; D014801:Vitamin A