Determination of Urine Ig and Clinical Significance
The Ig content in the urine of normal individuals is very low. When there is mild damage to the filtration membrane, IgG is primarily filtered and increased in the urine. When the damage to the filtration membrane is severe, in addition to IgG, larger molecular weight IgM also begins to filter into the urine. Clinically, the measurement of transferrin (TRF) and IgG levels in urine and blood is commonly used to calculate the Selective Proteinuria Index (SPI) to assess the degree of damage to the glomerular filtration membrane and to observe treatment effects and prognosis. The formula for calculating SPI is: SPI = (urine IgG / serum IgG) / (urine TRF / serum TRF). An SPI ≤ 0.1 indicates that the kidneys are selectively excreting smaller molecular weight proteins. An SPI ≥ 0.2 indicates that the kidneys are non-selectively excreting larger molecular weight proteins. Most cases of minimal change disease have a highly selective SPI (SPI ≤ 0.1). Membranous nephropathy, membranoproliferative glomerulonephritis, and nephrotic syndrome usually have SPI ≥ 0.2.
Urine IgA is highest in primary glomerulonephritis and chronic nephritis, with mild increases in chronic nephritis hypertensive type and ordinary type, while very little is present in latent nephritis and acute nephritis. Urine IgG is higher in primary glomerulonephritis and chronic nephritis, with only mild increases in other types of glomerular diseases. Urine IgM is only present in chronic nephritis, while it is very low in primary glomerulonephritis and latent nephritis.
Determination of Cerebrospinal Fluid Ig and Clinical Significance
The concentrations of IgG, IgA, and IgM in CSF decrease in that order. Clinically, the degree of blood-brain barrier damage is mainly reflected by measuring the albumin quotient (Alb quotient, QALB), which is the ratio of albumin in CSF (AlbCSF) to serum albumin (AlbS). The formula for calculating the albumin quotient is: Albumin Quotient = [AlbCSF / AlbS × 1000]. When the quotient is <9, it suggests no significant damage to the blood-brain barrier; 9-15 indicates mild damage; 15-33 indicates moderate damage; 33-100 indicates severe damage; >100 indicates complete rupture. Mildly elevated QALB is commonly seen in acute and chronic viral infections, multiple sclerosis, neuroviruses, herpes zoster neuralgia, brain atrophy, and other neurological diseases. Moderately elevated QALB is often seen in acute neurospirochetosis, opportunistic meningitis, Guillain-Barré syndrome, etc. Severely elevated QALB is commonly seen in purulent meningitis, herpes simplex encephalitis, tuberculous meningitis, and other severe bacterial infectious diseases.
The method for determining cerebrospinal fluid IgG uses rate scattering immunoturbidimetry, and the CSF should be centrifuged before measurement. An increased IgG index and 24-hour IgG synthesis rate are often seen in multiple sclerosis. In cases of purulent meningitis and tuberculous meningitis, both IgG and IgA are increased. In cerebrovascular diseases such as cerebral thrombosis and subarachnoid hemorrhage, IgG is significantly elevated. In SLE encephalopathy, neurosyphilis, and myasthenia gravis, IgG is significantly elevated. In schizophrenia, both IgG and IgM are markedly elevated. In neurological tumors, IgA and IgM are primarily elevated.
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