Direct measurement of serum free testosterone by ultrafiltration followed by liquid chromatography tandem mass spectrometry

BackgroundCurrently there is no reliable method suitable for routine measurement of serum free testosterone (FT).AimTo develop such a method involving liquid chromatography tandem mass spectrometry (LC-IDMS/MS) that directly detects and quantifies the FT present in serum.MethodsUltrafiltrate testosterone obtained from 0.5 mL of serum was partially purified by liquid/liquid extraction and quantified using an Agilent 1200 Series HPLC system coupled to an API 5000 mass spectrometer equipped with an atmospheric pressure chemical ionization ion source. Using split samples serum free testosterone was compared between direct ultrafiltration (UF) coupled LC-MS/MS, analogue FT immunoassay, free testosterone calculated from mass action equations (cFT) and with equilibrium dialysis (ED) coupled LC-MS/MS.ResultsTotal imprecision determined over twenty runs was < 6% at 67 pmol/L and 158 pmol/L FT. The dynamic response was linear up to at least 2500 pmol/L while physical LLOQ (18 % CV) equaled 16 pmol/L. The UF method agreed poorly with analogue immunoassay (correlation coefficient 0.667; bias − 81%), somewhat better against cFT when total testosterone was determined by immunoassay (correlation coefficient 0.816, bias 21% ) and still better yet against cFT when total testosterone was determined by LC-MS/MS (correlation coefficient 0.8996, bias 10%). Agreement was closest with ED method (correlation coefficient 0.9779, bias 2.4%).ConclusionWe present a relatively simple UF coupled LC-MS/MS definitive method that measures serum free testosterone. The method is relatively fast, reliable and is suitable for the routine clinical laboratory practice.