Complement C4 Test · Blood

If you have lupus, hereditary swelling episodes, or a family history of autoimmune disease, this number can tell you whether your immune system is actively consuming its own defenses. Complement C4 drops when your body is fighting itself, and that drop often shows up in blood work before you feel the flare.

Even outside autoimmune disease, your C4 level carries information. It is shaped by your genes more than almost any other blood protein, and people walk around with dramatically different baselines depending on how many copies of the C4 gene they inherited. Knowing your own number, and tracking it over time, gives you a window into classical complement pathway health that no other single test provides.

What Complement C4 Actually Is

Complement C4 (complement component 4) is a large glycoprotein, a sugar-coated protein that circulates in your blood as part of the complement system. The complement system is a group of about 30 proteins that work together as one of your body's oldest immune defenses, helping to tag invaders for destruction, punch holes in bacterial membranes, and clear away dead cells and immune debris.

C4 operates specifically in the classical and lectin pathways, two of three routes the complement system uses to activate. When your immune system detects a threat, whether it is a bacteria, a virus, or a cluster of antibodies stuck to your own tissues, C4 gets cleaved into two fragments. One fragment, C4b, sticks to nearby surfaces and helps build the molecular machinery that amplifies the immune attack. The other fragment, C4a, can increase leakage of blood vessels and contribute to inflammation.

C4 is produced mainly by the liver, though small amounts are made in the brain and other tissues. What makes C4 unusual is that it is encoded by two genes, C4A and C4B, and people inherit wildly different numbers of copies of each. You might carry anywhere from 2 to 10 total C4 gene copies, and this inherited variation is the single largest determinant of your baseline C4 blood level.

Autoimmune Disease and SLE

The strongest clinical use for C4 is in systemic lupus erythematosus, commonly called lupus or SLE (an autoimmune disease where the immune system attacks the body's own tissues). In active lupus, the classical complement pathway is driven into overdrive by immune complexes, clusters of antibodies bound to the body's own molecules. C4 gets consumed as part of this process, and its blood level drops.

In a study of 150 SLE patients, about half had low C4 levels, and there was a statistically significant negative correlation between C4 and disease severity. Lower C4 tracked with more severe flares. In lupus nephritis (kidney inflammation from lupus), low C4 at the time of kidney biopsy helped distinguish the more dangerous forms of the disease (proliferative lupus nephritis) from milder types.

Tracking C4 over time matters more than any single reading. In pooled data from two large randomized trials of belimumab (a biologic drug targeting B cells) involving over 1,600 SLE patients, those whose C4 normalized early, within the first 8 weeks, had a significantly lower risk of severe flares over the following year. The C4 trend was a better predictor of clinical course than the C4 level at any single time point.

In lupus pregnancies, C4 normally rises during the first trimester, just as it does in healthy pregnancies. When it fails to rise, that flat trajectory predicts flares and obstetric complications. In a study of 246 SLE pregnancies, low C4 before conception was an independent risk factor for flare during pregnancy, with about 14 times the odds compared to women with normal C4. The lesson: even a C4 that looks technically "normal" can be abnormal if it is not following the expected upward trajectory.

Hereditary Angioedema

The second major clinical use for C4 is screening for hereditary angioedema (HAE), a genetic condition that causes episodes of severe swelling in the skin, gut, and airways. In HAE, a deficiency in a regulatory protein called C1 inhibitor allows continuous low-level activation of the classical pathway, which steadily consumes C4.

During an acute HAE attack, C4 is almost always very low. Between attacks, C4 is low in most HAE patients but not all. In a study of 49 HAE patients, the sensitivity of low C4 for detecting HAE was 81% among untreated patients, meaning about 1 in 5 had normal C4 despite confirmed disease. A separate study in Danish children found sensitivity of 75%. This means a normal C4 does not rule out HAE, but a very low C4 in someone with unexplained recurring swelling is a strong signal to investigate further with C1 inhibitor testing.

One practical trap: C4 behaves as a mild acute phase protein, meaning it can rise during infection or inflammation. In HAE patients, a mild illness like an upper respiratory infection can temporarily push C4 into the normal range, masking the underlying deficiency. This has been documented in case series where HAE patients showed normal C4 during concurrent infections.

Other Disease Associations

Beyond lupus and HAE, abnormal C4 levels appear in several other conditions. In microscopic polyangiitis (MPA), a type of small-vessel inflammation in the kidneys, higher C4 at diagnosis predicted worse kidney outcomes. Patients with C4 above 29.6 mg/dL combined with elevated creatinine had 0% five-year survival free from end-stage kidney disease, compared to 96.2% for those with neither risk factor.

In newly diagnosed multiple myeloma (a blood cancer), low C4 below 0.095 g/L (about 9.5 mg/dL) was an independent predictor of shorter survival, with about 3.6 times the risk of death compared to patients with higher levels. In intracerebral hemorrhage (bleeding in the brain), plasma C4 was significantly higher in patients than in healthy controls, averaging 40.5 versus 35.3 mg/dL, and higher C4 correlated with larger bleeds and worse neurological outcomes.

Genetic variation in C4 also connects to neuropsychiatric disease. Higher C4A gene copy number is linked to increased schizophrenia risk, likely through excessive pruning of brain connections during development. Lower total C4 copy number increases risk of multiple sclerosis.

Reconciling High and Low C4 Risks

At first glance, the evidence can seem contradictory: low C4 is dangerous in lupus and HAE, but high C4 predicts worse outcomes in kidney vasculitis and brain hemorrhage. The resolution is that C4 is not a simple "higher is better" or "lower is better" marker. It reflects the state of your classical complement pathway. When C4 is low from consumption, it signals that immune complexes are driving tissue damage (as in lupus). When C4 is high, it can signal acute-phase inflammation or active complement contributing to tissue injury (as in vasculitis or hemorrhagic stroke). The clinical context, not the number alone, determines what a C4 result means for you.

Reference Ranges

C4 reference ranges vary by laboratory, assay method, and population. The ranges below come from large population studies using standard immunological measurement techniques that are the most common in clinical labs. Because C4 is strongly influenced by your inherited gene copy number and your body mass, these ranges are orientation, not absolute targets. Always compare your results within the same lab over time.

The most commonly cited general reference range is 10 to 40 mg/dL (0.10 to 0.40 g/L). Some labs use a wider range. If your result falls below 10 mg/dL, your physician should consider genetic complement deficiency, active autoimmune complement consumption, or HAE. Results above 40 mg/dL may reflect acute-phase elevation, high BMI, or high C4 gene copy number.

When Results Can Be Misleading

C4 has moderate within-person biological variation, estimated at about 5 to 7% from week to week in healthy adults measured under standardized fasting morning conditions. This means a change of less than about 15 to 20% between two draws could simply be normal fluctuation rather than a real biological shift.

• Gene copy number: Your inherited number of C4A and C4B gene copies is the largest single factor determining your baseline. Someone with 2 copies will have a lifelong low-normal C4 that does not indicate disease. Someone with 6 or more copies will run naturally high. A single reading cannot distinguish genetic baseline from active complement consumption without clinical context.
• BMI: C4 correlates positively with body mass index, especially in people with 4 or more C4 gene copies. Weight gain can push C4 up, and significant weight loss can drop it, independent of immune activity.
• Acute illness or infection: C4 rises modestly during inflammation. In HAE patients, a concurrent infection can temporarily normalize a usually low C4, hiding the underlying deficiency.
• Intense exercise: Acute vigorous exercise can transiently raise or lower C4 depending on the type and duration of effort. Endurance exercise tends to raise C4, while short anaerobic bursts can lower it. These effects normalize within hours, so avoid strenuous exercise on the day of your blood draw.

Tracking Your Trend

A single C4 reading is a snapshot, and snapshots can mislead. In lupus, the trajectory of C4 over weeks to months is far more informative than any isolated value. A C4 that is dropping, even while still technically within the reference range, can signal a flare before symptoms arrive. A C4 that is climbing after starting treatment confirms the therapy is working.

Get a baseline reading when you are feeling well, fasting, in the morning. If you are making changes to your treatment or if you have an autoimmune condition, retest in 3 to 6 months. For ongoing monitoring of lupus or other autoimmune conditions, at least every 3 to 6 months is reasonable. For general preventive tracking in someone without known autoimmune disease, annual measurement alongside C3 is sufficient to establish your personal trend.

What to Do With an Abnormal Result

If your C4 is low, the first step is to retest to confirm it is genuinely low and not a one-time fluctuation. If confirmed low, the next question is whether C3 is also low. Low C4 with normal C3 points toward classical pathway consumption or HAE, and C1 inhibitor testing (both level and function) should be ordered. Low C4 and low C3 together suggest broader complement consumption, as seen in active lupus, and anti-dsDNA antibodies (antibodies against your own DNA) and an antinuclear antibody screen should be checked.

If your C4 is persistently low without a clear cause, consider C4 gene copy number testing to determine whether you simply inherited fewer C4 genes. This is especially worth doing if you have a family history of autoimmune disease, recurrent infections, or unexplained swelling. A rheumatologist or clinical immunologist can help interpret the combination of C4, C3, autoantibody results, and clinical history.

If your C4 is unexpectedly high, check whether you were acutely ill, had recent surgery, or have a high BMI. Persistent elevation without obvious cause, especially in combination with kidney dysfunction, may warrant evaluation for vasculitis or other inflammatory conditions.