Post-infectious glomerulonephritis: Case

This girl has typical post-streptococcal glomerulonephritis. The diagnosis rests on the acute nephritic syndrome of haematuria, oedema, and hypertension, arising two weeks after an untreated sore throat, with dysmorphic red cells and red cell casts, a low C3 with a normal C4, and a raised antistreptolysin O titre. The latency of two weeks after pharyngitis and the low-C3-normal-C4 pattern together confirm the diagnosis and distinguish it from IgA nephropathy and from the other hypocomplementaemic glomerulonephritides. ^[1]

Clinical findings

The pattern is unequivocally that of pharyngitis-associated PSGN. The temporal sequence is classic: a sore throat two weeks before the haematuria, which is the expected one to three week latency after throat infection. The smoky brown urine reflects glomerular bleeding with dysmorphic red cells and red cell casts, the periorbital and ankle oedema reflects salt and water retention from the nephritic syndrome, and the hypertension is volume-dependent and renin-mediated from the renal injury. The mild acute kidney injury with a creatinine of 88 is consistent with the typical course. ^[2]

The low C3 with a normal C4 is the serological fingerprint of PSGN. It reflects consumption of C3 downstream of the alternative-pathway C3 convertase, with the lectin pathway also contributing, while the classical pathway is relatively spared so that C4 is preserved. The raised antistreptolysin O titre confirms a recent group A streptococcal throat infection, even though the throat swab is negative, because the organism is often cleared by the time PSGN declares itself. The negative throat swab therefore does not exclude the diagnosis. ^[3]