Serum C4 Testing Availability
Serum C4 testing is widely available in the United States and most developed countries through standard clinical laboratories, though the specific type of C4 test (complement C4 versus bile acid marker C4) must be clarified with your ordering laboratory. 1
Understanding Which C4 Test You Need
The term "serum C4" can refer to two completely different laboratory tests, and clarification is essential:
Complement C4 measures the C4 component of the complement immune system, typically reported in mg/dL or mg/L (normal range approximately 10-40 mg/dL or 100-400 mg/L), and is used to evaluate hereditary angioedema, autoimmune diseases like systemic lupus erythematosus, and complement deficiencies 1, 2
Serum C4 (7α-hydroxy-4-cholesten-3-one) is a bile acid synthesis marker reported in ng/mL (normal <30-52.5 ng/mL), used to diagnose bile acid diarrhea in patients with chronic diarrhea or irritable bowel syndrome 1, 2
Availability by Test Type
Complement C4 Testing
- Readily available at virtually all hospital and commercial laboratories in the United States and developed countries as part of standard complement testing panels 3, 4
- Can be ordered as an individual test or as part of complement panels (C3, C4, CH50) 5, 6
- Results typically available within 24-48 hours from most laboratories 4
Bile Acid C4 Marker Testing
- Limited availability - commercial assays are coming to market in some countries including the United States, but not yet widely available 1
- Requires specialized laboratories and may need to be sent to reference laboratories 1
- The American Gastroenterological Association notes these tests are "not widely available" as of 2019, though accessibility is improving 1
Critical Ordering Considerations
Always specify which C4 test you need when ordering, as the units of measurement and reference ranges differ dramatically between complement C4 (mg/dL) and bile acid C4 (ng/mL). 2
- Verify with your laboratory which C4 test they offer and confirm the units of measurement on the result 2
- For complement C4: ensure proper specimen handling with immediate refrigeration or placement on ice, as complement proteins degrade rapidly at room temperature, leading to falsely low results 5, 6
- For bile acid C4 marker: ensure timely laboratory processing to avoid degradation 2
When to Order Each Test
Order Complement C4 for:
- Suspected hereditary angioedema (typically low C4 with recurrent angioedema episodes) 1, 2
- Monitoring systemic lupus erythematosus activity (low C4 correlates with active disease, particularly lupus nephritis) 5, 7, 3
- Evaluation of recurrent infections suggesting complement deficiency 5
- Autoimmune disease workup 6, 3
Order Bile Acid C4 Marker for:
- Chronic diarrhea evaluation, particularly in patients with prior cholecystectomy, terminal ileum resection, Crohn's disease, or IBS-D 1, 2
- Suspected bile acid malabsorption or bile acid diarrhea 1
Common Pitfall
Do not assume the test name alone clarifies which marker was measured—always check the units (mg/dL versus ng/mL) and reference ranges on the laboratory report to confirm which C4 test was actually performed. 2