Low C3 with Normal C4 and Negative Direct Coombs Test
This pattern strongly suggests alternative complement pathway dysregulation, most commonly seen in C3 glomerulopathy (C3G), and requires immediate evaluation for underlying renal disease, infection-related glomerulonephritis, or genetic/acquired complement abnormalities.
Understanding the Complement Pattern
The combination of isolated low C3 with normal C4 indicates selective activation of the alternative complement pathway rather than the classical pathway 1. This is a critical distinction because:
- Classical pathway activation (triggered by immune complexes) consumes both C3 and C4, resulting in low levels of both
- Alternative pathway activation primarily consumes C3 while sparing C4
- The negative direct Coombs test rules out autoimmune hemolytic anemia, which can also present with low complement levels 2
Primary Differential Diagnosis
1. C3 Glomerulopathy (Most Likely)
This umbrella term encompasses C3 glomerulonephritis and dense deposit disease, both characterized by alternative pathway dysregulation 1. The pattern suggests:
- In children: Genetic complement defects (mutations in C3, Factor H, Factor I, Factor B, CD46, or CFHR proteins)
- In adults: Acquired autoantibodies to complement factors (C3 nephritic factor [C3Nef], anti-Factor H antibodies) 1
2. Infection-Related Glomerulonephritis
Must be ruled out before diagnosing C3G 3. Screen for:
- Hepatitis B and C
- Chronic bacterial infections (endocarditis, visceral abscesses)
- Parasitic infections (malaria, schistosomiasis)
- Other chronic microbial infections 1
3. Complement Regulatory Protein Deficiency
Deficiency of Factor H, Factor I, or properdin can lead to uncontrolled alternative pathway activation with isolated C3 consumption 4.
Essential Workup Algorithm
Immediate Laboratory Evaluation:
Renal Function Assessment:
- Urinalysis (hematuria, proteinuria)
- Urine protein-to-creatinine ratio
- Serum creatinine and eGFR
- Blood pressure measurement
Infection Screening:
- Hepatitis B surface antigen, hepatitis C antibody
- Blood cultures if febrile
- Consider additional infectious workup based on clinical presentation 3
Autoimmune Evaluation:
- ANA, anti-dsDNA (to exclude SLE, which typically lowers both C3 and C4)
- ANCA (though ANCA vasculitis is typically pauci-immune)
Specialized Complement Testing (if available):
For adults ≥50 years:
- Serum and urine protein electrophoresis with immunofixation
- Serum free light chains (monoclonal proteins can affect complement cascade) 3
For suspected C3G:
- C3 nephritic factor (C3Nef)
- Anti-Factor H antibodies
- Genetic screening for mutations in: C3, Factor H, Factor I, Factor B, CD46, CFHR1-5 1
Note: Many specialized complement tests are only available through reference laboratories (University of Iowa, National Jewish Health) 1.
Kidney Biopsy Consideration:
Strongly indicated if:
- Proteinuria >500 mg/day
- Declining eGFR
- Active urinary sediment (RBC casts, dysmorphic RBCs)
- Persistent hypocomplementemia
Biopsy should be evaluated by experienced nephropathologists with immunofluorescence showing complement-dominant pattern (C3-positive, immunoglobulin-negative or minimal) 3.
Critical Clinical Pitfalls
Common Misinterpretations:
Complement consumption vs. deficiency: Low C3 with normal C4 can indicate consumption from alternative pathway activation OR primary deficiency of alternative pathway components 4. Measuring Factor B levels helps distinguish (low Factor B suggests consumption).
Timing of complement measurement: Complement components are unstable and degrade with warming. Blood specimens should be placed on ice immediately after drawing for accurate measurements 4.
Overlooking monoclonal gammopathy: In adults ≥50 years presenting with this pattern, always screen for paraproteins, as monoclonal proteins can trigger complement-mediated disease 3.
Assuming "idiopathic" too quickly: True idiopathic C3G is rare. Exhaust all diagnostic possibilities including comprehensive complement analysis before labeling as idiopathic 3.
Management Approach
Before specific treatment:
- Rule out active infection (treat if present)
- Optimize blood pressure control with RAS inhibition
- Quantify proteinuria severity
If C3G confirmed on biopsy:
- Supportive care with RAS inhibition for mild disease (proteinuria <3.5 g/day, normal eGFR) 3
- Consider immunosuppression for progressive disease with nephrotic-range proteinuria and declining kidney function
- Treat underlying cause if identified (infection, autoimmune disease, monoclonal gammopathy)
Consultation: Nephrology referral is essential for patients with evidence of kidney involvement. Consider referral to centers with expertise in complement-mediated diseases for complex cases requiring specialized testing 1.