What's on this Page
Lab findings of poststreptococcal glomerulonephritis
What laboratory findings are characteristic of poststreptococcal glomerulonephritis?
The development of glomerulonephritis results from immune complex deposition in the basement membrane of the glomerulus, leading to loss of the filtration area and increased permeability to protein.
These immune complexes consist of an antigen from the infection and usually an immunoglobulin G (IgG) antibody. The damage to the basement membrane from these complexes is often a result of local complement activation exclusively through the alternative pathway.
This pathway leads to C3 cleavage and then the production of the membrane attack complex C5-9. Bypassed in this pathway is the activation of C4, which is used only in activation of the classic complement pathway.
Therefore when PSGN is suspected, the measurement of serum complement both C3 and C4 is crucial. In PSGN, the C3 level will be low, but the C4 level will typically be normal. If both C3 and C4 are low, the diagnosis of PSGN is suspect, and another source of immune complexes—such as SAAG, cryoglobulinemia, or systemic lupus erythematosus—may be present that activates the classic complement pathway.
Microbiologic and radiologic tests for infection are frequently negative but should still be done. These studies include a chest x-ray as well as blood, sputum, pharyngeal, and urine cultures in addition to a careful examination of the skin, anogenital region, and oral cavity for signs of inflammation. If a streptococcal infection is suspected, specific serologic assays are available, such as the anti-streptolysin O (ASO) titer. However, this test may be falsely negative and lacks the sensitivity to be used as a stand-alone test for streptococcal infection. A more sensitive screen for a recent streptococcal infection is the streptozyme test, which includes five different antibodies that develop to this infection. These include ASO titer, antihyaluronidase (AHase), antistreptokinase, anti–nicotinamide adenine dinucleotidase, and anti-DNAse B antibodies. This test is important because the source of the streptococcal infection will determine what pattern of antibodies will develop; for skin infections, only the anti-DNAse and AHase are positive, whereas for a pharyngeal infection, all five antibodies are usually present in various concentrations. An ASO test alone would miss the previous presence of a streptococcal skin infection.
