A complement deficiency genetic test looks for DNA variants that reduce, eliminate, or dysregulate proteins in the complement system. Complement is a network of blood and cell-surface proteins that helps kill microbes, clear immune complexes, and control inflammation. Inherited defects can cause recurrent meningococcal or pneumococcal infections, lupus-like autoimmunity, kidney disease, atypical hemolytic uremic syndrome, hereditary angioedema, or other inflammatory problems.
Genetic testing usually follows functional blood tests such as CH50 and AH50, measurements of individual complement proteins, and a detailed clinical history. A low complement level does not automatically mean an inherited deficiency; active autoimmune disease, severe infection, liver disease, protein loss, and certain treatments can consume or reduce complement. When a pathogenic variant confirms an inherited disorder, the result may guide vaccination, antibiotic prevention, emergency planning, family testing, and condition-specific treatment. A negative genetic result does not exclude complement dysfunction because some causes remain technically difficult or scientifically unresolved.
- Complement genetic testing is most informative after CH50, AH50, and individual-protein tests suggest a persistent pathway defect.
- Recurrent Neisseria infections strongly raise concern for terminal complement deficiencies involving C5 through C9 or properdin.
- Early classical pathway deficiencies can cause invasive bacterial infection and a high risk of lupus-like autoimmune disease.
- Low C3 or C4 during active illness may reflect complement consumption rather than an inherited gene defect.
- A confirmed hereditary deficiency can lead to vaccines, antibiotic planning, family testing, and rapid evaluation of fever.
Table of Contents
- What Complement Genetic Testing Evaluates
- Complement Genes and Clinical Patterns
- When Genetic Testing Is Considered
- Functional Complement Tests Before Genetics
- Testing Methods and Result Interpretation
- How a Confirmed Result Can Change Care
- Limitations and Acquired Causes of Low Complement
- Family Testing and Follow-Up
What Complement Genetic Testing Evaluates
Complement genetic testing examines genes that encode complement components, receptors, and regulatory proteins. The goal is to determine whether an inherited variant explains an abnormal complement pathway or a clinical pattern strongly suggestive of complement disease.
The complement system can be activated through three pathways:
- The classical pathway responds to antibody-bound targets and helps clear immune complexes.
- The lectin pathway recognizes carbohydrate patterns on microbes.
- The alternative pathway provides continuous low-level surveillance and amplifies complement activation.
All three converge at C3 and then activate C5 through C9, which form the membrane attack complex. Regulatory proteins prevent complement from damaging the body’s own cells. A deficiency can therefore create two opposite-looking problems: too little defense against infection or too much uncontrolled complement activity against tissues.
A genetic test may focus on one suspected gene or use a panel covering dozens of genes. Commonly included genes encode C1q subunits, C1r, C1s, C2, C3, C4, factor B, factor D, properdin, C5 through C9, factor H, factor I, membrane cofactor protein, decay-accelerating factor, CD59, mannose-binding lectin, C1 inhibitor, and other regulators.
The test is not the first step for every low C3 or C4 result. Complement proteins are consumed during active lupus, immune-complex disease, severe infection, and some kidney disorders. Levels can also be affected by liver production, protein loss, sample handling, and complement-blocking medications. Persistent pathway-specific abnormalities are more suggestive of inherited deficiency than a single low measurement during illness.
Genetic testing is different from the autoimmune disease genetic risk test. Complement panels generally search for rare variants with potentially large functional effects, not merely common alleles that slightly alter susceptibility.
Complement Genes and Clinical Patterns
The likely symptoms depend on where the defect occurs. The pattern is not absolute, but it can direct laboratory and genetic testing.
| Defect group | Examples of genes | Typical concerns |
|---|---|---|
| Early classical pathway | C1QA, C1QB, C1QC, C1R, C1S, C2, C4A, C4B | Encapsulated bacterial infection, immune-complex disease, lupus-like autoimmunity |
| Central component | C3 | Severe recurrent bacterial infection, kidney disease, impaired opsonization |
| Alternative pathway | CFB, CFD, CFP | Neisseria infection, recurrent invasive bacterial disease |
| Terminal pathway | C5, C6, C7, C8A, C8B, C8G, C9 | Recurrent meningococcal or gonococcal infection |
| Alternative pathway regulation | CFH, CFI, CD46, DGKE and related genes | Atypical hemolytic uremic syndrome, C3 glomerulopathy, variable infection risk |
| Cell-surface regulation | CD55, CD59 | Protein-losing enteropathy, complement-mediated hemolysis, neurologic or infectious features |
| C1 inhibitor pathway | SERPING1 | Recurrent bradykinin-mediated swelling rather than recurrent infection |
Early classical pathway proteins are important for clearing apoptotic material and immune complexes. Complete deficiencies of C1q, C1r, C1s, C2, or C4 can predispose to systemic lupus erythematosus or lupus-like disease, sometimes beginning in childhood. Infection risk also occurs, especially with encapsulated bacteria.
C3 sits at the center of complement activation. Complete C3 deficiency is rare and can cause severe, recurrent pyogenic infections from early life. Partial or regulatory defects may instead present with kidney disease or episodic complement activation.
Deficiencies of C5 through C9 impair the membrane attack complex. The classic clue is invasive or recurrent infection with Neisseria meningitidis. Disseminated gonococcal infection can also occur. These infections may arise in otherwise healthy adolescents or adults, so a late presentation does not rule out an inherited defect.
Properdin, encoded by CFP, is X-linked. Affected males can have severe meningococcal disease even when routine screening patterns are not identical to those of terminal component deficiencies. Family history may show affected maternal male relatives.
Variants in complement regulators such as CFH, CFI, and CD46 can have incomplete penetrance. A carrier may remain healthy until pregnancy, infection, surgery, transplantation, or another trigger shifts complement activation. In these disorders, the variant may confer susceptibility rather than guarantee disease.
Mannose-binding lectin deficiency is common and often causes no illness by itself. An MBL2 result should be interpreted cautiously, particularly in adults without other immune abnormalities. It may act as a modifier in infants or people with additional immune defects, but it is not a universal explanation for frequent colds.
When Genetic Testing Is Considered
Testing is considered when clinical history and functional studies suggest a primary complement disorder. The strongest indications involve severe or unusual infection, early autoimmunity, characteristic kidney disease, or a confirmed deficiency in a relative.
Clinical clues include:
- One or more episodes of meningococcal disease, especially recurrent episodes
- Disseminated gonococcal infection without another clear risk factor
- Recurrent invasive pneumococcal or Haemophilus influenzae infection
- Severe bacterial infection beginning in childhood
- Systemic lupus or lupus-like disease at a very young age
- Lupus accompanied by recurrent serious infection
- Persistent absence of a complement pathway on repeated testing
- Atypical hemolytic uremic syndrome or C3 glomerulopathy
- Unexplained complement-mediated hemolysis
- Recurrent angioedema without hives and with abnormal C1 inhibitor studies
- A known pathogenic complement variant in a biological relative
- Parental consanguinity or several affected siblings
A single invasive meningococcal infection can justify complement screening because the consequence of missing a terminal pathway deficiency is substantial. Genetic testing may follow if CH50 or AH50 is persistently abnormal or if the clinical suspicion remains high.
Kidney presentations require a different approach. Genetic analysis may be combined with complement protein levels, autoantibody testing, biopsy findings, and functional assays. Variants in CFH, CFI, CD46, C3, CFB, and related genes can influence atypical hemolytic uremic syndrome or C3 glomerulopathy, but penetrance is incomplete and multiple risk factors may interact.
For recurrent swelling, the main tests are C4, C1 inhibitor antigen, and C1 inhibitor function. A hereditary angioedema genetic test is interpreted differently from infection-focused complement panels because the disease mechanism is excessive bradykinin rather than failure to kill bacteria.
Testing healthy people without symptoms or abnormal functional studies is rarely useful unless a familial pathogenic variant is known. Broad sequencing can uncover uncertain variants that create confusion without improving care.
Functional Complement Tests Before Genetics
Functional testing usually provides the map that guides genetic analysis. CH50 and AH50 measure whether the classical and alternative pathways can complete the complement cascade in a laboratory assay.
CH50, or total hemolytic complement, evaluates the classical pathway and the shared terminal pathway. It depends on functional C1 through C9. A near-zero CH50 can suggest a complete deficiency of a classical or terminal component, although poor sample handling can produce a falsely low result.
AH50 evaluates the alternative pathway and shared terminal components. Interpreting CH50 and AH50 together helps localize the problem:
| CH50 | AH50 | Possible pattern |
|---|---|---|
| Low or absent | Normal | Early classical pathway defect, such as C1, C2, or C4 |
| Normal | Low or absent | Alternative pathway defect, such as factor B, factor D, or properdin |
| Low or absent | Low or absent | C3 or terminal pathway defect, broad consumption, or specimen problem |
| Normal | Normal | Major complete pathway deficiency is less likely, though specific regulatory defects may remain possible |
The pattern must be confirmed. Complement proteins can activate after blood collection if the sample is warm or processing is delayed. Laboratories often require serum to be separated and frozen promptly. Repeat testing when the person is clinically stable helps distinguish inherited absence from temporary consumption.
Individual component assays then measure antigen quantity or function. A protein may be present but dysfunctional, so normal antigen concentration does not always prove normal activity. Specialists may also order split products, soluble membrane attack complex, factor H antibodies, C1q antibodies, or pathway-specific assays.
C3 and C4 are useful but less comprehensive than CH50 and AH50. Low C3 with normal C4 can suggest alternative pathway activation. Low C3 and C4 together may occur with classical pathway consumption. Persistently normal C3 and C4 do not exclude a terminal complement deficiency.
Genetic results are strongest when they match the functional pattern. For example, biallelic C7 variants fit absent CH50 and AH50 with undetectable C7. A single uncertain C7 variant with normal pathway function is far less persuasive.
Age-specific interpretation is also important. Newborn complement concentrations can differ from adult values, and prematurity may lower several components without indicating a permanent genetic deficiency. In young children, the laboratory should use pediatric reference intervals and repeat borderline findings after recovery from infection. Conversely, a normal result years after an invasive infection does not erase the event; the clinician should confirm which assay was used and whether the sample was collected before vaccines, plasma products, or complement-active treatment.
Blood or plasma transfusion can temporarily supply missing soluble complement proteins and make an assay look less abnormal. That effect is usually short-lived, but the timing should be documented. In contrast, most inherited DNA results are unaffected by transfusion because the patient’s own leukocytes remain the main DNA source, although recent stem cell transplantation can make blood-derived genetic testing reflect donor cells. In that setting, a non-blood tissue such as cultured skin cells may be needed to determine the patient’s original germline genotype.
Testing Methods and Result Interpretation
Most laboratories use next-generation sequencing panels. These panels sequence coding regions and splice boundaries and may include deletion and duplication analysis. Some conditions require special methods because complement genes contain repeated regions, copy-number variation, pseudogenes, or complex structural changes.
Targeted testing is appropriate when a relative has a known variant. Broad panels are useful when the pathway is localized but the exact component is uncertain. Exome or genome sequencing may be considered when panel testing is negative, the phenotype includes other organ systems, or a novel disorder is suspected. A general multigene panel test can identify several candidate genes at once, but its coverage must be reviewed gene by gene.
Results commonly fall into four practical groups:
Pathogenic or likely pathogenic variant
A result can confirm an inherited deficiency when the variant, inheritance, and functional data align. Most complete component deficiencies are autosomal recessive, requiring pathogenic variants in both gene copies. Properdin deficiency is X-linked. Some regulatory disorders are autosomal dominant susceptibility conditions with incomplete penetrance.
The report should state whether two variants are in trans, meaning on opposite copies of the gene. Parental testing can establish phase. Two variants on the same copy may leave the other copy normal and may not cause a recessive deficiency.
Single carrier result
One pathogenic variant in a recessive complement gene usually means carrier status, not complete deficiency. However, partial effects can occur in some genes, and the finding must be compared with protein levels and pathway function. Carrier status may still matter for reproductive planning and relatives.
Variant of uncertain significance
A VUS is not a diagnosis. Functional assays, segregation in relatives, RNA studies, and future evidence may clarify it. Management should be based on the person’s clinical and complement findings rather than the uncertain sequence change alone.
Negative result
A negative panel does not exclude inherited complement disease. The variant may be outside the tested region, structural, mosaic, difficult to map, or in a gene not yet recognized. If functional evidence is convincing, the immunologist may pursue protein studies, research testing, genome sequencing, or periodic reanalysis.
Results should also distinguish loss-of-function deficiency from gain-of-function or regulatory dysregulation. Both can involve the same pathway but lead to different treatment. A variant that reduces factor H regulation can promote tissue injury, while complete absence of a terminal component mainly impairs bacterial killing.
How a Confirmed Result Can Change Care
A confirmed diagnosis can reduce the risk of another severe infection or complement-mediated episode. Management is individualized, but several actions are common.
Vaccination is central for people with infection-prone deficiencies. Clinicians usually ensure protection against meningococcal serogroups, pneumococcus, and Haemophilus influenzae type b according to age, country, and risk-based schedules. Complement-deficient patients may remain at increased meningococcal risk despite vaccination because vaccines do not cover every strain and antibody cannot fully compensate for a missing terminal pathway.
Some patients receive continuous antibiotic prophylaxis or keep emergency antibiotics available. The decision depends on the specific defect, prior infections, vaccine response, local resistance, age, and access to urgent care. Anyone with a terminal pathway or properdin deficiency needs a clear fever plan. Fever, severe headache, neck stiffness, a rapidly spreading rash, confusion, or sudden deterioration requires immediate medical evaluation.
Other care may include:
- Medical alert identification describing the complement deficiency
- Written emergency information for schools, travel, and emergency departments
- Education about meningococcal symptoms and the need for rapid treatment
- Screening relatives so preventive care can begin before infection
- Rheumatology follow-up for lupus-like disease
- Nephrology monitoring for complement-mediated kidney disorders
- Gene-specific discussions about complement inhibitors or replacement strategies
- Review of infection risk before starting a complement-blocking medication
Complement inhibitors can be lifesaving in diseases driven by excessive activation, but they also increase susceptibility to Neisseria. Vaccination and antimicrobial planning are required. The fact that a patient has a complement gene variant does not automatically mean an inhibitor is appropriate; treatment depends on disease mechanism and clinical evidence.
For C1 inhibitor deficiency, bradykinin-targeted on-demand and preventive therapies are used rather than standard allergy treatment. Antihistamines, corticosteroids, and epinephrine may not control isolated bradykinin-mediated attacks, although epinephrine remains appropriate when anaphylaxis is possible.
Limitations and Acquired Causes of Low Complement
Low complement results are common in acquired disease, while complete inherited deficiencies are rare. Interpreting genetics without considering consumption can lead to misdiagnosis.
Acquired causes include:
- Active systemic lupus or other immune-complex disease
- Severe bacterial infection or sepsis
- Membranoproliferative glomerulonephritis and other kidney disease
- Autoantibodies against complement components or regulators
- Liver failure that reduces protein synthesis
- Protein loss through kidneys or the gastrointestinal tract
- Hematologic malignancy
- Complement inhibitor therapy
- Acquired C1 inhibitor deficiency associated with lymphoproliferative or autoimmune disease
Timing matters. A low CH50 during acute sepsis may normalize after recovery. A person with inherited C6 deficiency, by contrast, typically has persistently absent terminal pathway activity even when well.
Genetic interpretation also faces incomplete penetrance and variable expressivity. Some CFH or CFI variants are risk factors whose effects depend on additional genetic and environmental triggers. Finding one does not prove that every kidney or inflammatory episode resulted from that variant. Population frequency, functional evidence, family segregation, and the person’s phenotype all matter.
Copy-number analysis of C4 is complex because people naturally vary in C4A and C4B copy numbers. Low copy number can influence autoimmune risk, but it is not the same as a complete monogenic deficiency. Reports should avoid turning common structural variation into a deterministic diagnosis.
Another limitation is that reference ranges differ by assay and laboratory. A result just below the range is not equivalent to absent activity. Repeat testing, clinical context, and direct component measurement are essential.
Family Testing and Follow-Up
Once a pathogenic variant is confirmed, targeted family testing is usually more useful than broad screening. The laboratory tests relatives for the exact variant, then clinicians measure complement function when appropriate.
For autosomal recessive disease, full siblings of an affected person generally have a 25% chance of being affected, a 50% chance of being carriers, and a 25% chance of inheriting neither familial variant, assuming both parents are carriers. For X-linked properdin deficiency, maternal male relatives may be at particular risk. Dominant regulatory disorders can be passed to each child with a 50% probability, but penetrance may be incomplete.
Testing children can be medically urgent when an undiagnosed deficiency creates a risk of meningococcal sepsis. Preventive vaccines, antibiotics, and emergency planning should not wait for symptoms if a familial complete deficiency is known.
After testing, families should ask:
- Which pathway and protein are affected?
- Is the deficiency complete, partial, or a susceptibility state?
- Does the genetic result match CH50, AH50, and protein measurements?
- Which vaccines and boosters are recommended?
- Is antibiotic prophylaxis advised?
- What symptoms require emergency care?
- Which relatives should receive targeted testing?
- Does the result affect pregnancy, kidney monitoring, or medication choices?
Genetic counseling can explain reproductive options, carrier testing, and the uncertainty of incompletely penetrant variants. A recessive inheritance result has different family implications from an X-linked or dominant regulatory condition.
Long-term follow-up should remain with clinicians familiar with complement disorders. Vaccination records, antibiotic plans, and emergency letters need periodic updates. Functional testing may be repeated if results were borderline, the clinical picture changes, or a complement-targeted therapy begins.
The most reliable diagnosis combines three forms of evidence: a characteristic clinical pattern, a reproducible functional defect, and a molecular result that explains both. When all three align, genetic testing can turn a rare laboratory abnormality into a practical plan that protects the patient and relatives.
References
- Complements and Their Role in Systemic Disorders 2024 (Review)
- Inherited defects in the complement system 2022 (Review)
- Genetic workup as a complementary tool for the diagnosis of complement deficiencies 2022
- Clinical Outcome and Underlying Genetic Cause of Complement Deficiency in the United Kingdom 2022
- Complement Genetics for the Practicing Allergist/Immunologist 2022 (Review)
- European Society for Immunodeficiencies (ESID) and European Reference Network on Rare Primary Immunodeficiency, Autoinflammatory and Autoimmune Diseases (ERN RITA) Complement Guideline: Deficiencies, Diagnosis, and Management 2020 (Guideline)
Disclaimer
Complement results require specialist interpretation because low levels may reflect inherited deficiency, active disease, medication effects, or specimen handling. Do not change vaccines, antibiotics, or complement-targeted treatment based only on a genetic report. Suspected meningococcal infection or rapidly worsening fever requires emergency medical care.
