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By Shailendra Singh
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Updated on 8 Apr 2025, 18:06 IST
The Complement Fixation Test, commonly abbreviated as CFT, is a classical serological technique used in medical diagnostics. This comprehensive guide explores what CFT stands for, how it works, and its applications in modern medicine.
The full form of CFT in medical terminology is "Complement Fixation Test." It's an immunological procedure used to detect the presence of specific antibodies or antigens in a patient's blood serum. The test relies on the complement system, a part of the immune system consisting of serum proteins that assist in the body's defense mechanisms against pathogens.
CFT is primarily used for the diagnosis of infectious diseases by detecting antibodies produced in response to specific pathogens. The test is particularly valuable when dealing with bacterial, viral, or parasitic infections where antibody detection is crucial for diagnosis. Healthcare professionals may recommend a CFT when they suspect infections like syphilis, brucellosis, or certain respiratory infections.
The concept of complement fixation was first described by Jules Bordet in the late 19th century. Bordet discovered that antibodies combined with their specific antigens could "fix" or bind complement proteins, preventing them from causing hemolysis (rupture of red blood cells). This fundamental discovery laid the groundwork for developing the CFT as a diagnostic tool.
Over the decades, the test methodology has evolved, becoming more standardized and reliable. The process has been refined to increase sensitivity and specificity, making it a valuable diagnostic tool throughout most of the 20th century.
| Timeline | Development | Significance |
| 1894-1895 | Jules Bordet discovers complement fixation principle | Fundamental discovery enabling serological testing |
| 1901 | Bordet and Gengou develop first practical CFT | First application for diagnostic purposes |
| 1906 | Wassermann, Neisser, and Bruck develop syphilis test | First widespread clinical application |
| 1909 | Jules Bordet awarded Nobel Prize | Recognition of the importance of complement studies |
| 1920s-1940s | Standardization of CFT procedures | Improved reliability and reproducibility |
| 1950s-1970s | Peak usage in clinical laboratories | Gold standard for many infectious disease diagnoses |
| 1980s onward | Gradual replacement by newer techniques | Decline in routine use as ELISA and PCR emerged |
August von Wassermann made a significant contribution to the field by developing the Wassermann reaction in 1906, a complement fixation test specifically designed for syphilis diagnosis. This application revolutionized the detection of syphilis and demonstrated the practical utility of the CFT method in clinical settings.
Other scientists further refined and expanded the applications of CFT, establishing it as a cornerstone of medical serology before the advent of more modern techniques.
The CFT is based on a complex interaction between three key components:
When antibodies bind to their specific antigens, they can fix or consume complement. This binding activity forms the basis of the test.
The CFT involves two stages:
The visual outcome—either hemolysis or no hemolysis—serves as the indicator for the test result.
Before conducting a CFT, several preparations are necessary:
| Component | Description | Critical Factors |
| Patient Sample | Blood drawn and serum separated | Must be fresh or properly stored; heat inactivation required |
| Antigen | Standardized pathogen-specific antigen | Concentration must be optimized; quality control essential |
| Complement | Usually guinea pig serum | Highly labile; requires titration and standardization |
| Indicator System | Sheep red blood cells (SRBC) sensitized with anti-SRBC antibodies | Freshness and standardization critical |
| Controls | Positive and negative controls | Required for validation of results |
| Equipment | Water baths, pipettes, test tubes, etc. | Precision equipment necessary for accurate results |
The laboratory must maintain precise temperature control, as complement activity is highly temperature-dependent.
For example, in a CFT for brucellosis, a positive result (no hemolysis) suggests the presence of anti-Brucella antibodies in the patient's serum, supporting a diagnosis of brucellosis.
The Complement Fixation Test has been used to diagnose various infectious diseases, including:
| Disease Category | Examples | Notes |
| Bacterial infections | Brucellosis, Whooping cough, Syphilis | The Wassermann reaction for syphilis was one of the first widely used applications |
| Viral infections | Influenza, Measles, Respiratory viruses | Important for viral diagnosis before PCR was available |
| Rickettsial diseases | Q fever, Typhus | CFT remains useful for some rickettsial diagnostics |
| Parasitic infections | Various helminthic and protozoal infections | Less commonly used but still applicable |
| Fungal infections | Histoplasmosis, Coccidioidomycosis | Used in specialized mycology laboratories |
CFT was particularly valuable before the development of more modern diagnostic techniques.
While CFT was once a cornerstone of diagnostic serology, newer methods have largely replaced it:
| Diagnostic Test | Principle | Advantages | Disadvantages | Compared to CFT |
| CFT | Complement fixation by antigen-antibody complexes | Cost-effective, historically validated | Labor-intensive, subjective reading | Baseline for comparison |
| ELISA | Enzyme-linked antibody detection | Higher sensitivity, automation, quantitative results | More expensive reagents | More sensitive and specific |
| PCR | Direct detection of pathogen genetic material | Extremely specific, rapid results | Expensive equipment, can't distinguish past vs. current infection | Directly detects pathogen rather than antibodies |
| Immunofluorescence | Fluorescent-labeled antibody binding | Better visualization, cellular localization | Requires fluorescence microscope | More visually definitive |
| Rapid tests | Various immunochromatographic methods | Fast results, point-of-care testing | Lower sensitivity | Much faster but less sensitive |
Despite these advances, CFT remains relevant in certain settings, particularly in resource-limited laboratories or for specific applications where newer methods haven't been fully validated.
| Advantage | Description |
| Cost-effective | Requires minimal specialized equipment compared to modern molecular techniques |
| Versatility | Can be adapted to detect various pathogens by simply changing the antigen used |
| Historical validation | Extensive historical use has validated its effectiveness for certain applications |
| Group screening | Useful for epidemiological studies and population-level screening |
| Breadth of detection | Can detect antibodies against diverse pathogens including bacteria, viruses, and fungi |
| Limitation | Description | Impact |
| Labor-intensive | Requires skilled technicians and meticulous technique | Higher personnel costs and training requirements |
| Time-consuming | Multi-step procedure takes significant time to complete | Longer turnaround time for results |
| Subjective interpretation | Results can be subject to observer variation | Potential inconsistency between laboratories |
| Limited sensitivity | May not detect low levels of antibodies | Possibility of false negatives |
| Cross-reactivity | May show positive results due to related but different pathogens | Possibility of false positives |
| Complex standardization | Difficult to standardize across different laboratories | Challenges in result comparability |
| Temperature sensitivity | Complement is highly sensitive to temperature changes | Requires strict environmental control |
CFT stands for Complement Fixation Test, a blood test used to detect the presence of specific antibodies in serum samples.
A positive result, like in the Wasserman reaction for syphilis, indicates the presence of antibodies, confirming an infection.
Sheep red blood cells (RBCs) are used as indicators. If the complement binds to the antigen-antibody complex, the RBCs remain unlysed and form a button at the bottom of the well.
CFT is used to detect specific antibodies in the blood, helping diagnose various infectious diseases.
The indicator system is inactivated by heating it to 56 degrees Celsius for about 30 minutes.
Complement fixation occurs when an antigen-antibody reaction leads to the formation of a heterogeneous phase, such as red cells, bacteria, or a precipitate.
Sheep RBCs, coated with anti-sheep antibodies, are used in the indicator system.
A positive result indicates the presence of antibodies, suggesting an infection like syphilis.
The full form of CFT is Complement Fixation Test.
The main purpose of CFT is to diagnose infectious diseases by detecting specific antibodies in blood serum.
CFT has been used to diagnose diseases such as tuberculosis, brucellosis, and various viral infections.
While newer tests are available, CFT is still valuable for specific diagnostic purposes due to its high specificity.
CFT can be time-consuming and requires specialized equipment and expertise. It may produce false positives or negatives and isn't suitable for all diseases.
Unlike tests like ELISA, which measure direct antibody-antigen binding, CFT measures complement fixation, offering high specificity and quantitative results.