Albuminuria is the hallmark of glomerular disease. The oncometric assay of glomerular albumin permeabilityA has been used to measure glomerular reflection coefficient to albumin (σalb) in clinically‐relevant circumstancesA. We have previously reported theoretical limitations of this assayB, and here report practical assay limitations.Single glomeruli, isolated by graded sieving, were held on an aspiration pipette. Glomerular volume (V) was recorded during incubation in BSA solution (Valb) of known oncotic pressure. The incubating solution was exchanged for an iso‐oncotic solution of dextran molecules (>100kDa MW after dialysis), and glomerular volume was remeasured (Vdex). Vdex/Valb describes σalb.σalb was measured in healthy mouse glomeruli before and after exposure to pronase and protamine. No changes were observed (pronase: pre 0.99±0.02; post 0.99±0.01 (7); p>0.9) (protamine: pre 0.95±0.01; post 0.95±0.01 (7); p>0.8).In addition, σalb was not different between glomeruli from mice with (0.95±0.03 (30)) or without (0.95±0.02 (24)) streptozotocin‐induced diabetic albuminuria (p>0.7), nor between glomeruli from rats with (1.00±0.01 (9)) or without (0.99±0.01 (8)) spontaneous proteinuria (p>0.6).We conclude that this assay is inadequate for glomerular albumin permeability measurements.