Analysis of molecular forms of urine Retinol-Binding Protein in Fanconi Syndrome and design of an accurate immunoassay

► 'Top-down' mass spectrometry quantitated forms of RBP in Fanconi urine. ► RBP4, desL-RBP4 and desLL-RBP4 were found in ratio 2:2:1. ► A validated dual mAb fluorescence immunoassay with RBP4 calibrant was devised. ► Assay overcame severe non-linearity seen on dilution of Fanconi urine in previous assays. ► Discrimination of elevated uRBP levels from normal was improved about 10-fold.