Anti-Glomerular basement membrane antibody (Anti-GBM antibody)

Sample

1. This test is done on the venous blood to get the serum.
2. The patient needs to fast for 8 hours.
3. Another sample is a tissue biopsy of the kidney (lung tissue).

Purpose of the test (Indications)

1. To diagnose autoimmune induced glomerulonephritis.
2. To differentiate from the other type of glomerulonephritis.
3. To diagnose Goodpasture’s syndrome.

Precaution

• Drugs like antibiotics may decrease the test result.

Pathophysiology

1. This is an autoimmune phenomenon.
   1. About 5% of the glomerulonephritis is Anti-GBM type.
   2. There are auto-antibodies which react with the basement membrane of the renal glomeruli capillaries.
      1. These autoantibodies react with small blood vessels of the kidney and lung.
      2. These autoantibodies react with pulmonary alveoli.
   3. When these antibodies react only with glomeruli basement then give rise to anti-GBM glomerulonephritis.

2. Mechanism of the tissue damage is a type III hypersensitivity reaction where immune complexes activate the complement system.
   1. Immune complexes give rise to:
      1. Glomerulonephritis which leads to hematuria.
      2. Pulmonary hemorrhage leads to hemoptysis.
      3. Positive autoantibodies against basement membranes.

3. This hypersensitivity reaction is complement-mediated.
   1. The damage is due to the deposition of immune complexes.
4. Goodpasture’s syndrome is autoimmune diseases and characterized by:
   1. When both kidney and lung are involved.
   2. Hematuria.
   3. hemoptysis (pulmonary hemorrhage).
   4. Antibodies to the basement membrane which acts as an antigen.
5. 60 to 70 % of the cases, both kidney, and lung are involved (Goodpasture’s syndrome).
   1. 20 to 40 % of the cases show the only involvement of the kidneys only then called anti-GBM glomerulonephritis.
6. When there is the involvement of the lung, the patient will have blood in a cough.

Diagnosis

1. The serum method is easy and faster and more reliable in patients where biopsy is difficult or contraindicated.
2. The high anti-GBM titer is suggestive of :
   1. Goodpasture’s syndrome.
   2. Lupus erythematosus.
   3. Anti-GBM nephritis.
3. Lung or renal biopsy demonstrates these antibodies by immunohistochemical technique.

Normal

Source 2

Blood (EIA enzymatic immunoassay):

• Negative = <20 units.
• Borderline = 20 to 100 units.
• Positive = >100 units.

Tissue biopsy:

• Negative = No immunofluorescence was seen on renal or lung biopsy.

Source 4

• Negative = <5 EU/mL by ELIZA
• Borderline = 5.1 to 20.0 EU/mL
• Positive = 20.1 to 400 EU/mL

Anti-GBM antibody seen in:

1. Autoimmune glomerulonephritis.
2. Goodpasture’s syndrome.
3. Tubulointerstitial nephritis.
4. Some patients with lupus nephritis.

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