Total complement activity

Total Complement Activity (CH50) is a blood test that measures the activity of the complement system, a part of the immune system that helps the body fight infections. The complement system consists of a number of small proteins found in the blood, generally synthesized by the liver, and normally circulating as inactive precursors. When stimulated by one of several triggers, proteases in the system cleave specific proteins to release cytokines and initiate an amplifying cascade of further cleavages. The end-product of this activation cascade is the generation of a membrane attack complex that forms pores in the cell walls of bacteria, leading to their death.

Overview

The complement system plays a key role in the innate immune system, being involved in the clearance of pathogens. It can be activated through three pathways: the classical pathway, which is triggered by antibody-antigen complexes; the lectin pathway, which is activated by mannose-binding lectin binding to pathogen surfaces; and the alternative pathway, which can be activated spontaneously on pathogen surfaces. The total complement activity test, or CH50, assesses the overall functionality of the complement system by measuring its ability to lyse sheep red blood cells that have been coated with antibodies.

Indications

The CH50 test is used to diagnose and monitor diseases associated with abnormalities in the complement system. These can include autoimmune diseases, such as systemic lupus erythematosus (SLE) and rheumatoid arthritis (RA); certain types of kidney diseases, like glomerulonephritis; and inherited deficiencies in complement proteins. A decreased CH50 level can indicate a consumption of complement components due to ongoing activation (as seen in autoimmune diseases) or a primary deficiency. An increased level, although less common, can be seen in acute inflammatory conditions where complement activation is part of the body's defense mechanism.

Procedure

The CH50 test is performed on a sample of the patient's blood. The test measures the ability of the patient's serum to lyse a standard amount of erythrocytes that have been sensitized with a specific amount of antibodies. The degree of lysis is measured spectrophotometrically. The result is expressed as the volume of serum that causes 50% lysis of the erythrocytes, hence the name CH50.

Interpretation

- Decreased CH50: May indicate a deficiency in one or more of the complement components, which could be due to a genetic deficiency or consumption of complement in diseases. - Increased CH50: Less common, but may be observed in acute phase reactions where there is an increase in complement activity as part of the inflammatory response.

Limitations

The CH50 test has its limitations. It is a global assessment and cannot pinpoint which specific component of the complement system is defective. If an abnormal CH50 result is obtained, further testing of individual complement components (C3, C4, etc.) may be necessary to identify the specific deficiency or abnormality.

Conclusion

The total complement activity test is a valuable tool in the diagnosis and monitoring of diseases associated with the complement system. It provides a broad overview of the functionality of the complement system, but further testing may be required to identify specific component deficiencies or abnormalities.