Urinalysis - The Liquid Kidney Biopsy

Urinanalysis, considered the “liquid biopsy” of the kidney, is rapidly becoming a lost art among nephrologists. A well-performed urinanalysis provides information on the presence of kidney disease, identifies the affected compartment, and helps monitor disease activity. The three main components of urine analysis are gross examination, dipstick tests, and urine sediment examination under microscopy. The urine sediments should be analyzed under a bright-field, dark-field, phase-contrast, and polarization microscope for better visualization and categorization of cells, casts, and crystals. The Sternheimer-Malbin (SM) stain can enhance urine sediments under a microscope. The urine sediment, like in a kidney biopsy, should be systematically analyzed for evidence of glomerular (dysmorphic RBCs, acanthocytes, RBC casts, lipid casts) and tubulointerstitial (RTE cell casts, Granular casts, Leucocyte casts, broad waxy casts) injury. For crystal identification in sediments, knowledge of urinary pH, morphology, and birefringence features under polarized light is essential.