Diagnosis and monitoring of immune complex deposition, Septicaemia or Bacteraemia.  They are useful in monitoring immune complex disease eg SLE or Glomerulonephritis.

Low levels of C3 associated with low levels of C4 demonstrate classical pathway activation and strongly suggest immune complex disease.  They are found most commonly in cases of active SLE.  Other associations include sepsis and liver failure.

Low levels of C3 associated with normal levels of C4 demonstrate alternative pathway activation suggestive of infectious disease or nephritic factor activity.

C3 is dramatically depleted in acute bacterial infection.

C3 is also depleted in Glomerulonephritis or partial lipodystrophy associated with nephritic factor.

In some long standing SLE patients C3 levels remain low.  This does not necessarily denote active disease.