A complement C5 test evaluates the amount or function of complement component 5, the protein that connects upstream complement activation with two major outcomes: release of the inflammatory fragment C5a and formation of the membrane attack complex beginning with C5b. C5 testing is usually not the first complement study ordered. It is most useful after CH50 and AH50 show a terminal-pathway problem, after recurrent Neisseria infections raise concern for inherited deficiency, or when a specialist needs to distinguish absent protein from impaired function or treatment effect.
Low or absent C5 can result from a rare inherited deficiency, intense complement consumption, reduced protein production, loss, or a medication that blocks C5 activation. The clinical implications differ sharply. Inherited C5 deficiency mainly increases susceptibility to invasive meningococcal and other neisserial infections, while acquired low C5 may reflect active systemic illness. This article explains C5 antigen and functional tests, pathway patterns, infection risk, confirmatory testing, and practical prevention without treating one laboratory result as a diagnosis.
- C5 is cleaved into C5a, a potent inflammatory signal, and C5b, the starting scaffold for the membrane attack complex.
- C5 antigen testing measures protein quantity; C5 functional testing assesses whether C5 can support pathway activity.
- An absent CH50 and AH50 with normal C3 and C4 can point toward a terminal component deficiency, including C5.
- Inherited C5 deficiency is strongly associated with recurrent invasive Neisseria infection.
- C5-inhibiting drugs intentionally create functional terminal-pathway blockade and require specific infection-prevention measures.
Table of Contents
- Why C5 is the transition to the terminal pathway
- C5 antigen, C5 function, and screening tests
- Inherited C5 deficiency and its clinical pattern
- Acquired causes of low or absent C5
- How C5-inhibiting medicines affect results and risk
- Interpreting C5 with CH50, AH50, C3, and C4
- Confirming a suspected C5 defect
- Infection prevention and urgent symptoms
Why C5 is the transition to the terminal pathway
Complement activation begins through the classical, lectin, or alternative pathway. Each route creates enzymes that cleave C3 and amplify the response. When sufficient C3b accumulates, the pathway forms a C5 convertase. This enzyme splits C5 into C5a and C5b, which have very different jobs.
C5a is a small anaphylatoxin and chemoattractant. It binds receptors on neutrophils, monocytes, endothelial cells, and other tissues, promoting cell recruitment, vascular changes, oxidative responses, and inflammatory signaling. C5a can be protective during infection, but excessive or poorly regulated signaling can contribute to tissue injury, thrombosis, and inflammatory disease.
C5b remains associated with the target surface and binds C6. The C5b-C6 complex then recruits C7, C8, and multiple C9 molecules to form the membrane attack complex, often abbreviated MAC or C5b-9. The complex can create a pore in susceptible membranes. It is particularly important for defense against Neisseria species, which explains the characteristic infection risk when any terminal component from C5 through C9 is absent.
C5 is therefore different from early complement components. Early classical component deficiencies often impair immune-complex clearance and are strongly associated with lupus-like autoimmunity. Complete C3 deficiency causes broad failure of opsonization and severe infections with encapsulated bacteria. Terminal deficiencies leave many upstream functions intact: antibody responses and C3b opsonization can still occur, but direct terminal killing is impaired.
That distinction shapes the clinical presentation. A person with C5 deficiency may appear healthy for years and then develop meningococcal disease, sometimes repeatedly. The first episode can occur later than infections caused by profound C3 deficiency. Some affected people are identified only after a second invasive neisserial infection or after a relative is diagnosed.
C5 concentration also differs from C5 activation. A routine C5 antigen result does not measure C5a or soluble C5b-9. Specialized activation markers can help in selected complement-mediated diseases, but they are method-dependent and answer questions about ongoing activation rather than inherited absence of C5.
C5 antigen, C5 function, and screening tests
A clinician rarely begins a complement-deficiency evaluation by ordering C5 alone. Functional pathway screens are more efficient because they show whether an entire sequence works.
The CH50 test assesses the classical pathway and the shared terminal components. The AH50 test assesses the alternative pathway and the same terminal sequence. Because both require C5, C6, C7, C8, and C9, complete absence of a terminal component usually makes both assays very low or undetectable.
Once that pattern is confirmed, individual components can be tested:
C5 antigen testing measures the concentration of immunologically recognizable C5 protein. One reference laboratory reports 10.6–26.3 mg/dL for its current serum assay, but ranges and units are method-specific. An absent antigen result with normal upstream components supports quantitative C5 deficiency. Detectable antigen does not guarantee function, because a variant protein may be present but unable to participate normally.
C5 functional testing places the patient’s sample in an assay system designed to determine whether C5 can restore or support complement-mediated activity. One current laboratory reports a reference threshold of at least 39 U/mL. The numerical value cannot be compared with an antigen concentration because the tests use different units and analytical principles.
C5 activation-marker testing may measure C5a, soluble C5b-9, or other products. These markers can increase during active complement-mediated inflammation and are not used as simple substitutes for C5 antigen or function. They are particularly sensitive to collection and handling because complement may activate after blood is drawn.
The report should be read for the exact name, specimen type, reference interval, and method. “C5 complement” in a portal may conceal whether the test was antigenic or functional. The laboratory catalog can clarify. An abnormal result also needs to be checked against treatment: a C5-blocking drug may suppress function even when C5 antigen remains measurable.
Special preparation is usually not required for routine antigen testing. Functional assays demand careful specimen processing and freezing because complement activity deteriorates in mishandled serum. A false-low pathway screen can lead to unnecessary component testing, so confirmation with a properly collected sample is a core part of the workup.
Inherited C5 deficiency and its clinical pattern
Inherited complete C5 deficiency is a rare inborn error of immunity, usually caused by pathogenic variants affecting both copies of the C5 gene. The inheritance is generally autosomal recessive. Parents may each carry one variant without having symptoms, while siblings have different probabilities of being affected, carriers, or unaffected.
The signature clinical concern is invasive neisserial infection. Neisseria meningitidis can cause meningitis or bloodstream infection. Neisseria gonorrhoeae can occasionally cause disseminated disease involving joints, skin, blood, or other sites. Terminal complement defects markedly increase the risk of these infections because serum bactericidal activity depends heavily on MAC formation.
Several features should raise suspicion:
- More than one episode of meningococcal disease.
- Meningococcal infection caused by an uncommon serogroup or at an unusual age.
- A family history of recurrent meningitis or documented terminal complement deficiency.
- Disseminated gonococcal infection, especially if recurrent or unexpectedly severe.
- An undetectable CH50 and AH50 when the sample was collected correctly and the patient is not receiving complement-inhibiting therapy.
Not every person with C5 deficiency has repeated infections. Exposure, vaccination, antibiotic use, antibody levels, and other immune defenses influence presentation. Some individuals remain asymptomatic into adulthood. Conversely, one meningococcal infection in an otherwise healthy person does not prove complement deficiency, although clinicians may screen when the epidemiology or severity is unusual.
C5 deficiency can sometimes be associated with autoimmune manifestations, but this is not as defining as the immune-complex disease seen with early classical component deficiency. The central management issue is preventing and recognizing neisserial infection.
A low C5 antigen alone does not establish inheritance. Severe illness can consume C5, liver disease can reduce production, and laboratory issues can lower function. Confirmation should show a coherent pattern: repeatedly absent or severely reduced C5, defective terminal activity, preserved relevant upstream proteins, and correction or localization in specialized assays. Molecular testing can then identify the gene defect.
After confirmation, genetic counseling helps families understand recurrence risk and which relatives may benefit from testing. Broad family testing before the index diagnosis is secure can create confusing borderline results and unnecessary anxiety.
Acquired causes of low or absent C5
C5 can be reduced without an inherited defect. The surrounding C3, C4, pathway activity, clinical illness, and treatment history usually make acquired causes more plausible.
Complement consumption
Severe infection, immune-complex disease, systemic inflammation, complement-mediated kidney or blood disorders, and widespread tissue injury can activate complement faster than proteins are replaced. In this setting, C5 may fall along with C3, C4, CH50, or activation markers. An absent C5 accompanied by low C3 and C4 favors consumption rather than isolated congenital C5 deficiency.
The exact pattern depends on which pathway is driving activation. Classical-pathway immune complexes may lower C4 and C3 before or along with C5. Alternative-pathway dysregulation may disproportionately affect C3. Terminal activation can be intense even when antigen concentrations remain within range, so a normal C5 antigen does not rule out biologically important C5 cleavage.
Reduced production
The liver produces much circulating complement. Advanced hepatic failure can lower several components, including C5. Low albumin, prolonged coagulation measures, jaundice, and other evidence of impaired synthesis support this mechanism. Mild liver abnormalities do not automatically explain an absent C5.
Protein loss and dilution
Severe protein-losing kidney or gastrointestinal disease, major burns, massive fluid resuscitation, or plasma exchange can alter complement concentrations. These situations generally affect multiple proteins. Albumin, total protein, urine protein, and treatment timing help identify the pattern.
Autoantibodies or acquired functional interference
Rarely, antibodies or other inhibitors interfere with a complement protein or assay. More commonly, therapeutic antibodies or small molecules intentionally block C5 cleavage or C5a signaling. When functional activity is absent but antigen is detectable, the clinician must distinguish genetic dysfunction from drug effect and other inhibitors.
Preanalytical loss of activity
Complement proteins can activate or degrade after collection. Functional C5 assays and CH50/AH50 are especially vulnerable to delays at room temperature, repeated freeze-thaw cycles, or improper serum handling. Antigen measurements may remain normal while function appears low. A fresh, correctly processed repeat sample is essential before diagnosing a rare deficiency.
Acquired low C5 is managed by treating or evaluating the underlying problem, not by attempting to raise the laboratory number directly. In an acutely ill patient, the immediate diagnosis may be infection, vasculitis, thrombotic microangiopathy, or liver failure; C5 serves as one piece of evidence.
How C5-inhibiting medicines affect results and risk
Several medicines inhibit C5 cleavage or the terminal complement pathway. They are used for conditions in which uncontrolled complement damages blood cells, kidney microvasculature, the neuromuscular junction, or other tissues. Examples include monoclonal antibodies that bind C5 and newer agents that interfere with terminal-pathway activation. Other drugs block C5a or its receptor rather than preventing MAC formation to the same degree.
Therapeutic C5 blockade can make CH50 and AH50 very low or undetectable. C5 functional testing may also be suppressed. C5 antigen can be measurable, increased, decreased, or assay-dependent because drug-bound C5 may still be recognized by some methods. A result obtained during therapy should never be interpreted as proof of inherited deficiency without discussing the assay and drug timing with the laboratory.
The biological effect creates a susceptibility resembling terminal complement deficiency. People receiving agents that inhibit C5 or related terminal components have a greatly increased risk of meningococcal disease. Vaccination reduces risk but does not eliminate it because complement is required for optimal bactericidal protection, and vaccines do not cover every strain perfectly.
Current U.S. public-health guidance recommends both MenACWY and MenB vaccination for people receiving complement inhibitors, with ongoing boosters according to the high-risk schedule. Ideally, vaccines are completed or updated before therapy begins, but urgent treatment may need to start sooner. Clinicians may prescribe antimicrobial prophylaxis in selected situations, depending on product labeling, timing, local practice, and individual risk.
Patients need explicit education because meningococcal illness can progress rapidly and may present atypically during complement blockade. Fever, headache, neck stiffness, muscle pain, vomiting, confusion, rash, or sudden severe malaise requires immediate assessment. A vaccinated patient can still develop disease. Medical-alert information should identify the complement inhibitor.
Do not delay a prescribed dose, stop therapy, or alter antibiotics based on a laboratory result without the treating specialist. Abrupt interruption can allow the underlying complement-mediated disease to recur. The purpose of monitoring differs by medication: some assays assess pharmacodynamic blockade, while others evaluate disease activity. The ordering clinician should specify the question to the laboratory.
Interpreting C5 with CH50, AH50, C3, and C4
C5 becomes clinically interpretable when it is placed into a pathway pattern.
| Findings | What the pattern suggests | Important alternative explanation |
|---|---|---|
| CH50 absent, AH50 absent, C3 and C4 normal, C5 absent | Complete C5 deficiency is likely | C5 inhibitor therapy or severe sample mishandling |
| CH50 absent, AH50 absent, C5 antigen detectable but C5 function absent | Dysfunctional C5 or an acquired inhibitor | Drug-bound C5 or assay-specific interference |
| CH50 and AH50 low, C3 and C4 low, C5 low | Broad complement consumption | Severe liver failure or protein loss |
| CH50 absent, AH50 normal | Early classical-pathway defect rather than C5 deficiency | Classical-pathway drug effect or specimen issue |
| CH50 normal, AH50 absent | Alternative-pathway defect rather than C5 deficiency | Alternative-pathway inhibitor therapy |
| C5 antigen normal, CH50/AH50 suppressed during C5-blocking treatment | Expected pharmacologic terminal blockade | Inadequate sample handling can compound the result |
| C5 low once during critical illness, then normal after recovery | Acquired consumption is more likely | Transient treatment or dilution effect |
A C3 result and C4 result are useful because normal concentrations make isolated terminal deficiency more plausible, while simultaneous reductions suggest upstream activation or systemic protein problems. They do not replace functional testing. A patient with terminal deficiency can have normal C3 and C4 because those proteins are produced and activated normally.
C5 cannot be distinguished from C6, C7, C8, or C9 deficiency by CH50 and AH50 alone. All are shared terminal components, so all can create a similar dual-absent pattern. Component-specific antigen and functional studies localize the defect.
Partial reductions require caution. Carriers of recessive C5 deficiency may have intermediate antigen concentrations but adequate function and no characteristic phenotype. A low-normal functional result near the cutoff can reflect specimen quality. The diagnosis should not be made by assigning genetic meaning to one borderline number.
Confirming a suspected C5 defect
Confirmation proceeds from broad function to specific mechanism.
First, repeat CH50 and AH50 using a properly collected and rapidly processed sample. Confirm that the patient is not receiving a complement inhibitor and has not recently received plasma exchange or plasma products. If the repeat pathway tests are normal, the earlier abnormality was probably transient or preanalytical.
Second, measure C5 antigen and C5 function. Absent antigen and function suggest quantitative deficiency. Detectable antigen with absent function suggests a dysfunctional protein, drug effect, or inhibitor. Laboratories may use reconstitution studies in which purified component or deficient serum helps identify which terminal protein restores activity.
Third, review other components and clinical data. Normal C3 and C4 support isolated terminal deficiency. Low upstream components point toward consumption, liver disease, or protein loss. A complete blood count, immunoglobulins, liver tests, kidney tests, and infection records may reveal a broader disorder.
Fourth, seek clinical immunology input. Genetic testing can confirm pathogenic C5 variants, but results may include variants of uncertain significance. The functional phenotype remains important. Genetic counseling should occur before predictive testing in relatives and when reproductive implications are discussed.
Fifth, document the diagnosis clearly. The patient, primary clinician, emergency department, and vaccination service should know the specific defect. “Low complement” is too vague; infection risk and prevention differ among early, central, alternative, and terminal pathway defects.
Testing after meningococcal disease may be affected by acute consumption. Screening can begin during hospitalization, but abnormal results may need repetition after recovery. Antibiotics and vaccination should not be delayed while waiting for genetic confirmation.
Infection prevention and urgent symptoms
Prevention for confirmed C5 deficiency centers on meningococcal risk while maintaining broader routine care. The plan should be created with immunology or infectious-disease specialists and aligned with national recommendations.
Vaccination: High-risk schedules generally include quadrivalent meningococcal conjugate vaccine covering serogroups A, C, W, and Y, plus a serogroup B vaccine. Booster doses are needed while risk persists. Pneumococcal and other routine immunizations should also be current. Exact products and intervals change over time and depend on age and country, so current public-health guidance is essential.
Antibiotic strategy: Some clinicians recommend continuous prophylactic antibiotics, especially after invasive disease, in children, or when vaccine coverage is incomplete. Others emphasize rapid access to treatment. The decision balances breakthrough risk, adherence, resistance, allergies, and local epidemiology. Patients should never use leftover antibiotics as a substitute for urgent assessment.
Emergency recognition: Meningococcal infection can progress over hours. Emergency symptoms include fever with severe headache, neck stiffness, confusion, light sensitivity, vomiting, cold hands and feet, severe limb or muscle pain, rapid breathing, or a purple/non-blanching rash. Rash may be absent early. Anyone with known terminal complement deficiency or C5 blockade should tell emergency clinicians immediately.
Household and sexual health: Close contacts of a person with meningococcal disease may need public-health-directed antibiotic prophylaxis. C5 deficiency does not itself spread between people. Because disseminated gonococcal infection can occur, barrier protection, prompt sexually transmitted infection testing, partner treatment, and symptom evaluation are important.
Medical identification: Carry a diagnosis card or medical-alert entry naming C5 deficiency or the C5-inhibiting medication. Include vaccine history, prophylactic antibiotic, allergies, and specialist contact information. Travel planning should account for access to urgent care and region-specific meningococcal recommendations.
The main purpose of the C5 test is not merely to label a low protein. It is to identify a pathway failure that changes infection prevention—or to show that C5 reduction is part of an acquired illness requiring a different response. Correct assay selection, repeat confirmation, medication review, and a coherent CH50/AH50 pattern turn the result into actionable information.
References
- C5 Complement, Functional, Serum. 2026. Laboratory test guidance.
- C5 Complement, Antigen, Serum. 2026. Laboratory test guidance.
- Complement deficiencies and infections. 2026. Review article.
- Clinical Guidance for Managing Meningococcal Disease Risk in Patients Receiving Complement Inhibitor Therapy. 2026. U.S. Centers for Disease Control and Prevention guidance.
- Immunodeficiency: Complement disorders. 2024. Clinical review.
- The complement system: A key player in the host response to infection. 2024. Review article.
Disclaimer
This article is for general education and does not replace individualized diagnosis, treatment, vaccination advice, or emergency care. Complement tests must be interpreted with specimen handling, medications, infection history, and related pathway studies. People with confirmed terminal complement deficiency or complement-inhibitor treatment should seek immediate medical care for possible meningococcal symptoms even if fully vaccinated.
