Complement Factor B Level Assay 

Factor B (MW: 93 kDa) is a complement protein unique to the alternative pathway (AP). In the presence of C3b, FB binds to C3b to form the pre-convertase (C3bB). Factor D cleaves factor B releasing Ba (MW: 33 kDa) to generate the active proteolytic enzyme Bb (MW: 66kDa). The Bb subunit is the catalytically active site of the C3bBb C3 convertase complex and cleaves new C3 to C3a and C3b. If C3bBb recruits additional available C3b, the C5 convertase, C3bBbC3b, forms launching terminal pathway activation. C3 convertase can be dissociated by spontaneous decay or complement regulators (factor H, CR1). It can also be inactivated by factor I-mediated C3b cleavage in presence of cofactors.

The common pathophysiological basis of both Dense Deposit Disease (DDD) and C3 Glomerulonephritis (C3GN) is dysregulation of the AP. Consumption of AP complement components is dependent on the degree of dysregulation of the C3 and C5 convertases. In both DDD and C3GN, patients often have lower levels of factor B as compared to controls (p<0.001) consistent with dysregulation of the C3 convertase in both diseases (see Zhang et al. Defining the complement biomarker profile of C3 glomerulopathy, CJASN 2014).

Indications for screening 
Screening is appropriate in patients with complement-mediated renal disease.

MORL screening methodology 
Radial immunodiffusion (RID)

Turnaround time 
Turnaround time is ~2 weeks

Sample Required 
1 ml frozen EDTA plasma (see testing requisition for specimen handling).

Cost & CPT Codes

The Clinical Diagnostics Service of the Molecular Otolaryngology and Renal Research Laboratories is a Joint Commission-approved CLIA-accredited diagnostic laboratory.