Proteinuria: Glomerular Vs Tubular Mechanisms &

The glomerular filtration barrier consists of three layers: fenestrated endothelium, glomerular basement membrane (GBM — type IV collagen, laminin, proteoglycans), and podocyte foot processes with slit diaphragms (nephrin, podocin proteins). This barrier restricts filtration based on molecular size (proteins >69 kDa are largely excluded) and charge (negatively charged GBM repels anionic albumin). Proteinuria occurs through several mechanisms: (1) Glomerular proteinuria — damage to the filtration barrier allows albumin and larger proteins to pass; this is the most clinically significant type and occurs in diabetic nephropathy (GBM thickening, podocyte loss), glomerulonephritis (immune complex deposition, GBM damage), and focal segmental glomerulosclerosis (podocyte injury). (2) Tubular proteinuria — normal filtered low-molecular-weight proteins (β2-microglobulin, retinol-binding protein) are not reabsorbed by damaged proximal tubules (Fanconi syndrome, aminoglycoside toxicity, interstitial nephritis). (3) Overflow proteinuria — excessive production of small proteins overwhelms normal tubular reabsorption (Bence Jones protein in multiple myeloma, myoglobin in rhabdomyolysis). The UACR (urine albumin-to-creatinine ratio) quantifies glomerular proteinuria: A1 (<30 mg/g — normal), A2 (30-300 mg/g — moderately increased/microalbuminuria), A3 (>300 mg/g — severely increased/macroalbuminuria). Proteinuria itself accelerates CKD progression through tubular toxicity,...