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Standard complement blood tests measure the total supply of immune proteins sitting in your bloodstream. But supply alone does not tell you whether those proteins are being actively consumed in an immune attack right now. That is the gap C4a fills. When your immune system's classical defense chain fires, the C4 protein is cleaved in two, and the smaller piece, C4a (complement component 4a), is released into circulation as a direct byproduct. Measuring C4a is like checking for spent shell casings: even if the ammunition supply looks full, the casings tell you shots have been fired.
This makes C4a especially useful in autoimmune diseases like lupus, where the immune system chronically activates complement against the body's own tissues. It also rises sharply during severe infections and sepsis. Because C4a is not part of routine lab panels, most people with active complement-driven disease have never had it measured, even when standard complement levels appear reassuringly normal.
C4a is a 77-amino-acid protein fragment, roughly 9,000 daltons in size, generated when an enzyme called C1s (in the classical immune pathway) or MASP-2 (in the lectin pathway) cuts the larger C4 protein. The classical pathway is the branch of your immune system activated by antibodies bound to targets, while the lectin pathway responds to specific sugar patterns on bacteria and damaged cells. Both pathways converge on C4, and when C4 is split, C4a is released into the blood while the larger piece (C4b) attaches to the target surface to continue the immune attack.
C4a was long classified alongside C3a and C5a as an "anaphylatoxin," a type of inflammatory alarm signal. However, more recent research has reclassified its role. A 2015 review argued that C4a should not be grouped with the true anaphylatoxins because it does not bind to the same receptors that C3a and C5a use to trigger allergic-type inflammation. Instead, research published in 2017 found that C4a activates a different set of receptors on blood vessel walls (called PAR1 and PAR4), increasing the permeability of blood vessels and triggering cellular activation through a distinct mechanism.
The most commonly ordered complement tests, C3 and C4, measure the total amount of intact protein circulating in your blood. When these proteins are consumed during an immune attack, their levels drop. In theory, low C3 or C4 signals active complement consumption. In practice, this relationship breaks down for an important reason: C4 is also an acute-phase protein, meaning the liver ramps up production during inflammation. So the same disease process that consumes C4 also stimulates your liver to make more of it. The net result can be a normal-looking C4 level even while complement activation is ongoing.
C4a sidesteps this problem entirely. Because it is generated only when C4 is actively cleaved, a rising C4a level means complement activation is happening right now, regardless of what the total C4 level shows. In studies of lupus, the C4d/C4 ratio (C4d is a closely related activation product) correlated with disease severity across all activity levels, while intact C3 and C4 levels showed no such linear relationship. Standard C3 and C4 levels had a sensitivity of about 75% and 48%, respectively, for predicting lupus kidney flares, meaning they missed a substantial number of active disease episodes.
The strongest clinical evidence for C4a comes from systemic lupus erythematosus (SLE). In one study, C4a was elevated in 20 of 24 lupus patients (83%), and levels correlated with disease severity. Patients with more aggressive disease requiring stronger immunosuppressive medications like azathioprine had significantly higher C4a than those managed with steroids alone. By comparison, C3a (the activation fragment from a different complement protein) was elevated in only 2 of the same 24 patients (8%), suggesting C4a is a far more sensitive marker of the specific immune pathway activated in lupus.
C4 genetics add another layer. The C4 gene is the most variable (polymorphic) gene in the complement system, with people carrying anywhere from 2 to 5 copies. Complete deficiency of the C4A gene variant is one of the strongest single-gene risk factors for developing lupus. In one study, people with complete C4A gene deficiency had dramatically higher rates of lymphoma (12.5% versus 0.8% in controls) and celiac disease (12.5% versus 0%).
In severe infections, complement activation ramps up as the immune system fights invading organisms. A study of 48 patients with suspected sepsis found that elevated C4a at admission correlated with fatal outcomes. Among patients with elevated C3a above 14 nmol/L (a parallel complement activation marker), mortality was 86% compared to 33% in those with lower levels. The C4a findings tracked similarly, showing that higher complement activation on arrival predicted worse outcomes.
After major trauma, complement C4 activation peaks between 6 and 24 hours post-injury. Patients with tissue hypoperfusion (inadequate blood flow to organs, measured by a base deficit greater than 10 mEq/L) and those requiring massive blood transfusions showed greater C4 activation. These findings position C4a as a prognostic marker in acute care, reflecting the intensity of the immune response to injury.
Most cardiovascular outcome data involves total C4 protein rather than C4a specifically. In a study of 389 men with known or suspected coronary artery disease followed for 24 months, higher C4 protein levels independently predicted stroke risk (about 1.6 times higher risk per unit increase, after adjusting for other risk factors). Men with C4 above the median had 90.1% stroke-free survival compared to 96.1% for those at or below the median.
A separate study of 148 patients after acute coronary events found that the C3/C4 ratio, reflecting the balance between different complement pathways, predicted recurrent cardiovascular events. Those in the highest quartile had a 48% event rate compared to 24% in the two lowest quartiles combined, representing roughly three times the risk. These findings involve total C4 protein, not the C4a activation fragment specifically, so they should be interpreted as evidence that complement system activity broadly relates to cardiovascular risk rather than direct evidence about C4a.
Standardized clinical reference ranges for C4a do not exist in the way they do for established markers like cholesterol or blood sugar. This is a Tier 2/3 biomarker with assay-dependent values that vary substantially between laboratories and testing methods. The numbers below come from published research and should be treated as orientation points, not fixed clinical cutpoints.
C4a concentrations are heavily influenced by the collection method. Serum samples (collected in tubes without anticoagulant) show significantly higher C4a than EDTA plasma because the clotting process itself activates complement. For this reason, EDTA plasma is the correct specimen for measuring true in-body C4a levels. Any result from a serum sample may be falsely elevated.
The wide gap between 155 ng/mL and 488 ng/mL across methods shows why cross-lab comparisons are unreliable. Your own trend within the same lab, using the same assay, is far more informative than any single reading compared to a published reference.
Unlike most biomarkers, C4 has extraordinary genetic variability. People carry between 2 and 5 copies of the C4 gene, and these genes come in two functionally different forms: C4A (which binds proteins) and C4B (which binds sugars). Additionally, about 77% of C4 genes are "long" forms containing an ancient viral DNA sequence, while the rest are "short" forms. This genetic diversity directly influences how much C4 protein your liver produces and how much C4a is generated during immune activation.
Ethnicity also matters. Studies comparing Black African and White European men found higher C4 protein levels in Black African men. Asian-Indian Americans have distinct C4 gene copy number patterns compared to European Americans. These genetic and ethnic differences mean that a "normal" C4a level for one person may not be normal for another, reinforcing the value of tracking your own trend rather than comparing to population averages.
Because C4a lacks standardized cutpoints and has wide inter-individual genetic variation, a single reading tells you relatively little. The real value comes from serial measurements. A rising C4a over time may signal increasing complement activation before symptoms worsen. In lupus, serial C4a tracked disease activity and predicted the need for treatment escalation. In chronic Lyme disease with musculoskeletal symptoms, changes in C4a correlated with response to antibiotic therapy.
If you are monitoring a known autoimmune condition, consider testing every 3 to 6 months, or more frequently during treatment changes or symptom flares. If you are testing for the first time without a specific diagnosis, get a baseline, then retest in 3 to 6 months to establish your personal range. Always use the same lab and specimen type (EDTA plasma) for comparability. A single elevated reading should prompt a retest and clinical correlation, not immediate alarm.
Sample handling is the single largest source of error in C4a testing. If the blood sample clots before processing (serum instead of EDTA plasma), complement activates in the tube and produces falsely elevated readings. Freeze-thaw cycles also spike C4a levels, and thawing at body temperature (37 degrees Celsius) causes particularly dramatic increases. Samples should be processed within 4 hours of collection, stored at -80 degrees Celsius, and never repeatedly frozen and thawed.
Any acute illness, especially infection, can elevate C4a for days to weeks. A recent surgery, major injury, or severe infection will activate complement and produce high readings that reflect the acute event, not your baseline immune status. Wait at least 2 to 4 weeks after an acute illness before testing if you want a representative result.
Intense physical exercise temporarily elevates C4a. Maximal exercise raises C4a immediately afterward, with levels returning to normal within 30 to 60 minutes. Prolonged endurance exercise (over 2 hours) can keep C4a elevated for 1 to 3 hours post-exercise. Avoid strenuous exercise in the 24 hours before your blood draw.
Tocilizumab, an IL-6 receptor blocker used for rheumatoid arthritis and other inflammatory conditions, reduces C4 protein levels by 44 to 55% within 1 to 12 months of treatment, with over half of patients developing levels below normal. This reflects the drug's anti-inflammatory mechanism, not a complement deficiency, but it can make C4-based tests difficult to interpret. Metabolites of procainamide and practolol (older cardiac medications) can also interfere with C4 binding reactions.
One of the most persistent misconceptions about C4a is its use in diagnosing mast cell activation syndrome (MCAS). C4a is not a validated biomarker for MCAS and does not appear in any consensus diagnostic guidelines for the condition. The 2019 AAAAI guidelines specify that validated MCAS mediators include tryptase, histamine metabolites, prostaglandin D2 metabolites, and leukotriene E4. C4a is absent from all recommended panels. More recent 2024 and 2025 consensus statements continue to exclude it. If you are being tested for MCAS, ask about the validated mediators rather than relying on C4a.
Evidence-backed interventions that affect your Complement C4a level
Complement C4a is best interpreted alongside these tests.