Validation of 5 routine assays for serum free testosterone with a candidate reference measurement procedure based on ultrafiltration and isotope dilution-gas chromatography-mass spectrometry. - Van Uytfanghe,K.; Stöckle,D.; Kaufman,J.M.; Fiers,T.; De Leenheer,A.; Thienpont,L.M. (Clin Biochem, 2005)