C4A and systemic lupus erythematosus: Serum Complement protein levels (C3 and C4) exhibited poor sensitivity for SLE at 95-96% specificity for SLE vs. AHV, with sensitivity for SLE of 4.8% (95% CI: 1.6-10.8%) and 9.5% (95% CI: 4.7-16.8%), respectively.