Overview
The Complement Fixation Test (CFT) is one of the oldest serological techniques used to detect specific antibodies or antigens in a patient's serum by exploiting the "fixation" (consumption) of complement during an antigen-antibody reaction. It was developed from the pioneering work of Jules Bordet, who discovered that a heat-labile, non-specific serum component — later called complement — was needed, along with a specific antibody, for bacteriolysis and haemolysis to occur.
CFT became historically important in syphilis serology after August von Wassermann adapted it in 1909 (the Wassermann test) and was later extended to viral and other bacterial infections. Although newer, more sensitive methods like EIA and RIA have largely replaced it, understanding CFT gives students a strong foundation in classical immunology and complement biology.
Learning Objectives
After this lesson you will be able to…- Describe the term "complement" and its role in immune defence
- Explain the principle underlying the Complement Fixation Test
- Describe the two steps involved in performing a CFT
- List the practical uses/applications of CFT
- Describe key modifications of the CFT (indirect CFT, conglutinating complement absorption test, immune adherence, immobilisation test, cytolytic tests)
Clinical Story
Why This MattersA young man presents to the clinic with a painless genital ulcer that appeared weeks earlier. His physician suspects syphilis and orders serological testing. Decades ago, this sample would have been tested using the Wassermann Complement Fixation Test — the technician needed to titrate fresh guinea pig complement, prepare sensitised sheep red cells, and carefully read whether haemolysis occurred to report a positive or negative result.
Core Concepts
Complement (originally called "alexine" by Bordet) is a heat-labile, non-specific substance present in all normal animal serums. It is destroyed by heating serum to 55°C. When combined with a specific antibody bound to its antigen, complement is "fixed" (used up), producing lysis of the target cell (bacteria or red blood cells). Guinea pig serum is the most common source of fresh complement because it is rich in complement activity and must be collected freshly before testing.
In 1894, Richard Pfeiffer showed that cholera bacteria injected into an immunised guinea pig's peritoneum caused rapid bacteriolysis. Jules Bordet found this lysis required two components: a specific heat-stable antibody and a heat-labile, non-specific substance (complement). He later applied the same principle to haemolysis of red blood cells, forming the basis of the CFT.
CFT consists of a test system (patient's serum + known antigen) and an indicator system (sheep red blood cells coated with rabbit anti-sheep antibody, called amboceptor/haemolysin). Both systems can activate complement, so the reaction becomes a competition — if the test system uses up the complement first, the indicator system shows no lysis (positive test).
One unit (MHD) is the highest dilution of guinea pig serum that lyses one unit volume of washed sheep red blood cells in the presence of excess amboceptor in 30 or 60 minutes at 37°C. Both complement and amboceptor must be titrated for haemolytic activity before use.
Laboratory Principle
CFT depends on the fact that complement is consumed ("fixed") whenever an antigen-antibody complex forms. In Step 1, a known antigen is incubated with the patient's (heat-inactivated) serum plus a standardised amount of complement. If the serum contains the specific antibody, an antigen-antibody complex forms and fixes the complement. In Step 2, sensitised sheep red cells (indicator system) are added. If complement was already fixed in Step 1, none remains free to lyse the indicator cells — no haemolysis = positive test (antibody present). If no antibody was present in Step 1, complement remains free and lyses the sheep red cells — haemolysis = negative test.
Equipment Required
Reagents & Materials
| Reagent / Material | Concentration / Grade | Purpose | Storage |
|---|---|---|---|
| Guinea pig complement | Titrated to 1 MHD | Fixed by antigen-antibody complex | Fresh; or lyophilised/frozen with preservative |
| Sheep red blood cells | 5% washed suspension | Indicator system | 4°C, use within a few days |
| Haemolysin (amboceptor) | Titrated rabbit anti-sheep RBC antibody | Sensitises sheep RBCs | 4°C |
| Physiological saline (with Mg²⁺/Ca²⁺) | Isotonic | Diluent for titration and test | Room temperature |
| Known antigen | As per test kit | Test system component | As per manufacturer |
Step-by-Step Procedure
Heat the patient's serum at 56°C for 30 minutes to destroy any native complement, so only the test complement added later is measured.
Mix known antigen, inactivated patient's serum, and a standardised limited amount of complement (1 MHD). Incubate at 37°C for 30–60 minutes to allow antigen-antibody-complement complex formation.
Add sensitised sheep red blood cells (coated with amboceptor) to each well/tube. Incubate again at 37°C for 30–60 minutes.
Observe for haemolysis. No haemolysis (intact red cell button) = complement was fixed by antibody = positive test. Haemolysis (pink/red supernatant) = complement remained free = negative test.
Run antigen and serum controls to confirm they are not anti-complementary, confirm the correct complement dose was used, and ensure sheep red cells do not undergo autolysis.
Flow Diagram
Quality Control
Complement and amboceptor must both be titrated for haemolytic activity before every run to confirm the correct 1 MHD dose is used. Antigen and serum controls must confirm they are not anti-complementary on their own, and sheep red cells must be checked for spontaneous autolysis.
Laboratories performing CFT for syphilis or viral serology should participate in national/regional external quality assessment schemes and compare acute versus convalescent phase sera (a four-fold rise in titre is considered diagnostically significant for infection).
Reference Values
Normal Ranges⚠️ Reference ranges may vary between laboratories. Always apply your laboratory's established reference intervals.
Clinical Interpretation
| Finding | Possible Significance | Action / Follow-up |
|---|---|---|
| No haemolysis | Complement fixed by antigen-antibody complex — specific antibody present (positive CFT) | Report as positive; correlate clinically |
| Complete haemolysis | No antigen-antibody complex formed — antibody absent (negative CFT) | Report as negative; consider window period if suspicion is high |
| 4-fold rise between acute and convalescent titres | Indicates recent/active infection | Supports diagnosis of current infection |
Common Errors & How to Avoid Them
Cause: Complement is heat-labile and degrades quickly at room temperature.
Prevention: Collect guinea pig serum fresh, keep on ice, and titrate before each run; use preserved/lyophilised complement if fresh is unavailable.
Cause: Some sera or antigens can fix complement non-specifically, giving false positives.
Prevention: Always run serum-only and antigen-only controls to rule out non-specific fixation.
Cause: If native complement in the patient's serum is not destroyed by heating at 56°C, results become uninterpretable.
Prevention: Always heat-inactivate serum for exactly 30 minutes at 56°C before testing.
Laboratory Tips from the Bench
Always titrate both complement and amboceptor together on the same day of testing — their combined activity, not just individual titres, determines correct dosing.
Keep sheep red blood cells refrigerated and use within the manufacturer's expiry window — aged cells lyse spontaneously and give false results.
Remember: "No lysis = Yes antibody." A clear button of intact red cells (no haemolysis) means the complement was used up by a positive antigen-antibody reaction.
Important Notes
Although CFT is conceptually simple, it involves many variables (complement titre, amboceptor titre, incubation time/temperature) that must be tightly controlled — small deviations can produce false results.
CFT is being progressively replaced by more direct, sensitive, and rapid techniques such as RIA and EIA, though it remains historically and conceptually important.
Interactive Quiz
Test Your KnowledgeFlashcards
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Clinical Case Study
Apply Your KnowledgeRohan presents with a painless genital ulcer of 3 weeks' duration and mild inguinal lymphadenopathy. He reports unprotected sexual contact 6 weeks ago. His physician orders serological testing for syphilis using a complement fixation-based assay.
Absence of haemolysis in the test well, with normal control results, indicates the complement was fixed by an antigen-antibody complex — meaning specific antibodies to the syphilis antigen are present in the patient's serum. This is a positive CFT.
- →No haemolysis in a properly controlled CFT = positive test (antibody present).
- →Controls must always show expected haemolysis to validate the run.
- →Confirmatory treponemal-specific tests (e.g. TPI/FTA-ABS) are recommended to confirm CFT-based screening results.
Frequently Asked Questions
The patient's own serum may contain native complement, which would interfere with the standardised complement added in the test. Heating at 56°C for 30 minutes destroys this native complement so only the measured complement is used.
CFT is not very sensitive, is time-consuming and labour-intensive, and can show non-specific cross-reactivity (e.g. between HSV and VZV). Newer techniques like EIA and RIA are more sensitive, rapid, and easier to standardise.
It is used when the test serum itself does not fix guinea pig complement well. A standard antiserum known to fix complement is added after step 1 in a duplicate set — if the antigen was already used up by antibodies in the test serum, no reaction occurs with the standard antiserum, and haemolysis indicates a positive result (reversed interpretation compared to standard CFT).
Quick Revision
10-Minute ReviewKey Takeaways
- CFT was developed from Bordet's discovery that complement plus specific antibody together lyse bacteria/red cells.
- The test consists of two systems — a test system and an indicator system — both capable of activating complement.
- Absence of haemolysis in the indicator system signals a positive result (antibody present).
- Guinea pig serum is the standard fresh complement source and must be freshly titrated.
- CFT has several modifications tailored to different diagnostic needs (indirect CFT, immobilisation test, immune adherence, cytolytic tests).
- Though largely superseded by EIA/RIA, CFT remains foundational to understanding classical serology.
Competency Checklist
Track Your MasteryReferences
- NIOS Microbiology Module — Lesson 61: Complement Fixation Test.
- Bordet J. Studies on complement and its role in immunity. Historical publications, early 1900s.
- Standard textbooks of Medical Microbiology and Immunology (Ananthanarayan & Paniker; Baveja).