Reagents for Complement Fixation Tests

SERION CFT

The Complement Fixation Test (CFT) is a serological method for the detection of antibodies against pathogens in accordance with the KOLMER technique, World Health Organization (WHO) guidelines as well as the National Health Service (NHS, Great Britain and Northern Ireland).

Complement fixation tests can be used as screening tests for acute infections. Positive CFT titers are often an indication for the presence of IgM antibodies or very high IgG antibody titers and are therefore an indication for an acute or recent infection, respectively. The CFT is especially helpful for acute respiratory infections: The, in general, strong IgG booster reaction is very well represented whereas residual titers of past infections are screened out. The distinction between IgG and IgM antibodies is not possible.

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Our erythrocyte suspensions as well as the ready-to-use Hemolytic system is available from Labor Dr. Merk & Kollegen GmbH (Ochsenhausen, Germany; www.labormerk.com).

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Highlights

  • Cost-effective due to standardized reagents
  • Lyophilized antigens, control antigens as well as positive and negative control sera simplify storage
  • Suitable screening test to identify acute infections
  • Compensation of seroprevalences allow high specificity

Test principle

A pathogen-specific antigen is mixed with the test serum. In the presence of specific antibodies, immune complexes form. Added complement is activated by these immune complexes and, due to its labile nature, inactivated during the subsequent incubation step. In the absence of specific antibodies immune complexes are not formed and consequently the complement components remain in their non-fixed state. In order to detect the presence of specific immune complexes a hemolytic system (HS) consisting of antibody-coated erythrocytes is added. If the complement has been fixed by antigen-antibody complexes in the serum, the erythrocytes will remain intact as complement is no longer available to react with the HS [Ab–Ag] (inhibition of hemolysis). In contrast, the erythrocytes will be lysed if complement is accessible. After centrifugation, unlysed erythrocytes form a button on the bottom of the microtiter plate. To improve button formation sedimentation should be supported by centrifugation. Titer determination is enabled by serial dilution of the serum.