C4 is a component of the classic complement pathway. Depressed levels usually indicate classic pathway activation.
Normal Range: 15-45 mg/dL
increased in: Various malignancies: not clinically useful.
Decreased by: Decreased
synthesis, increased catabolism (SLE, rheumatoid arthritis, proliferative
glomerulonephritis, hereditary angioedema), and increased loss (burns,
protein-losing enteropathies). Congenital deficiency.
Additional: Low
C4 accompanies acute attacks of hereditary angioedema, and C4 is used
as a first-line test for the disease. C1 esterase inhibitor levels are
not indicated for the evaluation of hereditary angioedema unless C4 is
low. Congenital C4 deficiency occurs with an SLE-like syndrome. Test as
usually performed is an immunoassay and not a functional assay.
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