The content of immunoglobulin (Ig) in the urine of healthy individuals is minimal. When there is slight damage to the filtration membrane, IgG is primarily filtered and increases in the urine. When there is severe damage to the filtration membrane, in addition to the filtered IgG, larger molecular weight IgM also begins to appear in the urine.
Clinically, the measurement of transferrin (TRF) and IgG levels in urine and blood is often selected to calculate the Selective Proteinuria Index (SPI), which is used to evaluate the degree of glomerular filtration membrane damage and to observe treatment effects and prognosis.
SPI=(Urinary IgG/Blood IgG)/(Urinary TRF/Blood TRF).
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SPI≤0.1 indicates that the kidneys are highly selectively excreting smaller molecular weight proteins.
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SPI≥0.2 indicates that the kidneys are non-selectively excreting larger molecular weight proteins.
Minimal Change Disease typically has an SPI that is highly selective (SPI≤0.1). Membranous Nephropathy, Membranoproliferative Glomerulonephritis, and Nephrotic Syndrome usually have SPI≥0.2.
Urinary IgA is highest in primary glomerulonephritis and chronic nephritis, with mild increases in chronic nephritis with hypertension and the common type, while very little is found in latent nephritis and acute nephritis.
Urinary IgG is higher in primary glomerulonephritis and chronic nephritis, with only slight increases in other types of glomerular diseases.
Urinary IgM is only present in chronic nephritis, while primary glomerulonephritis and latent nephritis show very little content.
Source: Laboratory Starry Sky
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