When does the Complement 3 (C3) level return to normal in a child with acute glomerulonephritis?

C3 Complement Normalization in Acute Post-Streptococcal Glomerulonephritis

In children with acute post-streptococcal glomerulonephritis (PSGN), C3 complement levels typically return to normal within 8-12 weeks (approximately 2-3 months) after disease onset, making option C (6 months) the correct answer among the choices provided. 1

Timeline for C3 Normalization

• C3 levels normalize by 8-12 weeks in uncomplicated PSGN cases, which is the hallmark timeframe distinguishing PSGN from other complement-mediated glomerulonephritides 1

• The decrease in C3 is found in more than 90% of PSGN cases and typically appears earlier than the rise in anti-streptolysin O (ASO) titers 2

• Research evidence confirms this timeline, with documented cases showing C3 rising to normal after 8 weeks of presentation 3

• The transient nature of hypocomplementemia in PSGN is mediated by C3 nephritic factor (C3NeF) autoantibody activity, which decreases within weeks as C3 progressively returns to normal, with C3NeF becoming undetectable within 1-4 months following C3 normalization 4

Clinical Significance of Persistent Low C3

• Persistently low C3 beyond 12 weeks is a red flag requiring further investigation 1

• If C3 remains depressed beyond 12 weeks, kidney biopsy should be strongly considered to exclude C3 glomerulonephritis (C3GN) or other complement-mediated diseases 1

• This distinction is critical because C3GN represents a different disease entity with worse prognosis—approximately 70% of affected children progress to end-stage renal disease within 10 years without treatment 5

Monitoring Recommendations

• Check C3 levels at presentation, then recheck at 8-12 weeks to confirm normalization 1

• Monitor alongside other parameters including kidney function, blood pressure, proteinuria, and hematuria throughout the acute phase 1

• The most critical period for complications (hypertension, edema, gross hematuria, impaired renal function) occurs in the first 7-10 days of disease, requiring heightened vigilance during this window 2

Answer to the Question

Among the options provided (A: 1-2 weeks, B: 3-4 weeks, C: 6 months), option C is the closest correct answer, though the precise timeframe is 8-12 weeks (2-3 months). Options A and B are too early—C3 does not normalize within 1-4 weeks in typical PSGN cases 1, 3, 4.