Urinalysis, Casts & the Glomerular Filtration Barrier

The glomerular filtration barrier

Three layers. Plasma filtered into Bowman space crosses a fenestrated capillary endothelium, whose pores hold back blood cells and whose glycocalyx is negatively charged; then the glomerular basement membrane (GBM), a meshwork of type IV collagen and laminin studded with anionic heparan sulfate proteoglycans; and finally the podocyte layer, where interdigitating foot processes are bridged by slit diaphragms built from nephrin and podocin. The slit diaphragm is the final size barrier.

Size and charge selectivity. Water, electrolytes, glucose and urea filter freely. Permeability falls steeply as molecular radius increases, and fixed negative charges on the GBM and podocyte surface repel anionic proteins, so at any given size a cationic molecule filters more readily than an anionic one. Albumin (about 69 kDa and anionic) is therefore almost entirely retained. Smaller proteins such as β₂-microglobulin, retinol-binding protein, light chains and myoglobin are filtered and then reclaimed by proximal tubular endocytosis through megalin and cubilin.

Charge loss. In minimal change disease the barrier loses its negative charge and podocyte foot processes efface; albumin leaks while larger proteins such as IgG stay behind, producing highly selective proteinuria and nephrotic syndrome with normal light microscopy. Structural damage in other glomerular diseases lets larger proteins through as well.

Inherited barrier defects. Congenital nephrotic syndrome of the Finnish type results from autosomal recessive NPHS1 (nephrin) mutations: massive proteinuria begins in utero, the placenta is enlarged and steroids do not help. Autosomal recessive NPHS2 (podocin) mutations cause steroid-resistant nephrotic syndrome with FSGS in childhood. Type IV collagen defects strike the GBM instead, as in Alport syndrome (hematuria, sensorineural deafness, eye lesions), and the α3 chain of type IV collagen is the antigen in anti-GBM (Goodpasture) disease.

Proteinuria: four patterns

Healthy adults excrete less than about 150 mg of protein a day, much of it Tamm–Horsfall protein secreted by the tubule. Larger amounts follow one of four patterns.

What the dipstick misses. The dipstick protein pad reacts mainly with albumin, so Bence Jones proteinuria can read negative or trace despite grams of protein a day. Sulfosalicylic acid precipitates all proteins; a strongly positive result with a weak dipstick means nonalbumin protein, and urine protein electrophoresis with immunofixation (plus serum free light chains) identifies a monoclonal light chain. Moderately increased albuminuria (albumin-to-creatinine ratio 30–300 mg/g), the earliest sign of diabetic nephropathy, is also below the dipstick's threshold.

Filtered light chains can also combine with Tamm–Horsfall protein and plug distal tubules, producing myeloma cast nephropathy.

Benign proteinuria. Orthostatic proteinuria is common in adolescents and young adults: protein appears only when upright, a split day and night collection confirms it, and the prognosis is excellent. Fever and strenuous exercise cause similarly transient proteinuria.

Reading the dipstick

Blood pad. The pad detects the peroxidase activity of heme, whether in intact RBCs, free hemoglobin or myoglobin. Microscopy decides: many RBCs mean true hematuria, whereas a strongly positive pad with few or no RBCs means pigmenturia.

Leukocyte esterase and nitrite. Leukocyte esterase is released by neutrophils and signals pyuria. Nitrite appears when Enterobacterales such as E. coli, Proteus and Klebsiella reduce dietary nitrate, which needs several hours of bladder dwell; it is often negative with Enterococcus or Staphylococcus saprophyticus and in dilute, frequently voided urine. Pyuria with a negative routine culture (sterile pyuria) suggests chlamydial or gonococcal urethritis, renal tuberculosis, interstitial nephritis, stones or recent antibiotics.

Hematuria and urinary casts

Where is the blood from? RBCs that squeeze through a damaged GBM and then meet the osmotic and pH gradients of the tubule become dysmorphic; acanthocytes, ring-shaped cells with small blebs, are the most specific form. Dysmorphic RBCs with RBC casts and proteinuria point to glomerulonephritis, often with brown or cola-colored urine. Uniform (isomorphic) RBCs, clots and absent casts point to a lower-tract source such as stones, tumor, infection or the prostate; painless gross hematuria in an older smoker suggests bladder cancer.

How casts form. Tamm–Horsfall protein (uromodulin), secreted by thick ascending limb cells, gels in the distal tubule and collecting duct and traps whatever cells or debris are present. A cast is a mold of the tubule, so a cellular cast proves the process is inside the kidney: cystitis produces pyuria but never WBC casts.

Eosinophiluria. Urine eosinophils, shown with Hansel or Wright stain, suggest drug-induced acute interstitial nephritis (with fever, rash, eosinophilia and WBC casts, classically after β-lactams, NSAIDs or proton pump inhibitors) or atheroembolic disease, but the test is neither sensitive nor specific.