Glomerulonephritis and Low Complement

Key points

• Glomerulonephritis with low complement has a differential diagnosis that includes infection-related GN, lupus nephritis and complement-mediated GN.
• Post-infectious/post-streptococcal GN commonly has low C3, with C4 often preserved; complement usually recovers during follow-up.
• Infection-related GN, including infective endocarditis-associated GN, can cause hypocomplementaemia; do not assume every low-complement GN is post-streptococcal.
• Systemic lupus erythematosus/lupus nephritis commonly causes low C3 and C4, particularly during active disease.
• MRCP discriminator: low C3 with delayed post-infectious nephritic syndrome → infection-related GN; low C3 + low C4 with SLE features → lupus nephritis; persistent isolated low C3 → consider complement-mediated disease.
• Exam trap: complement pattern supports but does not establish the diagnosis.