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Swelling of the endothelium of glomeruli with narrowing of their capillary lumina, notably associated with pre-eclampsia.^[ref]

An albuminous urine specimen can reflect renal leakage, altered renal function, or added blood plasma proteins below the kidney. Finding serum albumin in urine identifies a phenomenon, not a single disease.

In the historical setting, the important group was the renal disorders gathered under Bright's disease: acute inflammatory disease, including that following scarlet fever, and chronic diseased or contracted kidneys. In current terms, glomerulonephritis damages the glomerular filtration barrier, permitting increased passage of serum albumin into the filtrate. Red blood cells and urinary casts may accompany the loss. Different modern glomerular diseases can produce the same finding; the nineteenth-century umbrella does not identify one modern microscopic diagnosis.^[ref]

In renal amyloidosis, deposition in the glomeruli disrupts their filtering structure; heavy albuminuria and nephrotic syndrome may result. Diabetic kidney disease and hypertensive kidney disease provide further modern examples of chronic injury to the filtering apparatus. These modern classifications distinguish mechanisms.^[ref]

The heart failure group includes renal venous congestion and impaired effective kidney perfusion. Altered pressures and injury to the filtering apparatus can increase albuminuria; an albuminous specimen does not by itself prove destructive nephritis. Fever and severe acute illness can also produce transient proteinuria through reversible changes in renal function. In pre-eclampsia, injury to the filtering apparatus, including glomerular endotheliosis, can cause albuminuria.^[ref] Pregnancy alone does not make albuminuria normal, and pressure from a gravid uterus is not an adequate general explanation of this disorder.^[ref]

Finally distinguish renal loss from addition below the filtering apparatus. Haematuria, pyelonephritis, cystitis and contamination by inflammatory or genital material can add blood plasma proteins, including serum albumin, to a collected specimen. Microscopy and a clean sample help distinguish these possibilities. Tubulointerstitial nephritis reduces proximal tubule recovery of filtered proteins, but predominantly causes loss of smaller proteins. Likewise, overflow of immunoglobulin light chains in multiple myeloma is proteinuria, not necessarily albuminuria. Haematuria alone can contribute blood plasma proteins, so modest protein positivity need not locate the bleeding in the kidney.^[ref] The distinction matters because an old protein-precipitation test did not identify the molecular species.^[ref]

A useful diagnostic comparison is repeated urinalysis, with microscopy of the sediment and a modern urine albumin-to-creatinine ratio. Persistence, red blood cell casts, and the clinical setting distinguish sustained glomerular disease from a transient change or lower-tract addition. A positive protein test alone cannot make that distinction.^[ref]