Nanoparticle Tracking Analysis of Urine to Detect Exosomes Can Be Confounded by Albuminuria

Letter to the Editor: In the article by Lv et al. published in the Journal of the American Society of Nephrology, the authors describe enhanced urinary exosomal release in experimental models of acute and chronic renal injury. They also describe increased exosomal release in patients with IgA nephropathy and a correlation between exosomal concentration and proteinuria. These findings are of clear interest, but we have reservations about some of the methodology used to ascertain this observation. They use nanoparticle tracking analysis (NTA) of urine and describe enhanced particle concentration in patients who are proteinuric. We have recently reported similar observations using NTA in 32 patients with varying levels of albuminuria. We found that albuminuria confounded particle analysis by NTA, because albumin-containing solutions contain particles of exosome-like size, and such particles can mimic exosomes in standard NTA. Immunodepletion of albumin from proteinuric urine resulted in a substantial reduction in the concentration of particles detected by NTA as did urine purification using iodixanol density gradients. More importantly, NTA measurements of pure albumin solutions yielded a substantial number of particles with a diameter of 105 nm. This reinforces the need for great caution in the interpretation of NTA results in fluids that have significant levels of protein. It also emphasizes the importance of stringent methods of purification, such as density gradients, in exosome analysis as recommended by the International Society for Extracellular Vesicles.