Measurements of serum complement components C3 is useful in the diagnosis and monitoring of immune complex disease e.g. SLE (Systemic lupus erythematosus) and some blood associated infectious diseases. Complement concentrations are acute phase proteins and may be normal, despite complement consumption, in some inflammatory and infective disorders. C3 alone is often decreased in infectious disease (septicaemia, endocarditis).
Measurement of serum complement is useful in the monitoring of specific immune complex diseases e.g SLE and infectious diseases post streptococcal disease, subacute bacterial endocarditis.
Though genetic deficiencies of C3 are exceedingly rare, deficiencies in other components which are more common (though still very rare) can result in low C3.
The normal adult range for C3 is 90-180mg/dl.
Values <1mg/dl can be found in active immune complex disease, septicaemia, bacteraemia, post streptococcal glomerulonephritis or diseases associated with nephritic factor. 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 (Systemic lupus erythematosus). Low levels of C3 associated with normal levels of C4 demonstrate alternative pathway activation suggestive of infectious disease or nephritic factor activity.
Although in some long standing SLE patients C3 levels remain low, which does not necessarily denote active disease, a sudden fall in levels does usually indicate exacerbation of disease activity and a risk of renal damage.
Values up to twice this upper limit are found in acute phase.
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