title: "All that glisters is not gold": Ultrafiltration and free thyroxine measurement With apologies to W Shakespeare.
abstract: OBJECTIVE
  To examine the merits of measuring free analytes by ultrafiltration using either diluted or undiluted serum.
  
  
  RESULTS
  Confidence in the accuracy of measurements is affected both by problems identified in current systems using semipermeable membranes, the sensitivity of the system to artefacts and comparisons with other imperfect assays.
  
  
  CONCLUSIONS
  All "gold standard" methods must robustly obey sound physicochemical principles if valid conclusions are to be drawn.
authors.forename: John E M
authors.initials: JE
authors.lastname: Midgley
mesh_terms: D003956:Dialysis; D006801:Humans; D006435:Renal Dialysis; D044967:Serum; D013960:Thyroid Function Tests; D013974:Thyroxine / Q000097:blood; D014284:Triiodothyronine / Q000097:blood; D014462:Ultrafiltration
chemical_list: D014284:Triiodothyronine; D013974:Thyroxine