C4A and systemic lupus erythematosus: Patients with low levels of both C3 and C4 are more readily diagnosed with SLE than patients who exhibitlow C3 or C4, and patients with either low C3 or low C4 together with a positive ANA test showed 94.3% specificity for an SLE diagnosis, while patients with simultaneously low C3 and C4 levels combined with a positive ANA showed 97.6% specificity for an SLE diagnosis [42].