A 28-year-old man presents with sudden onset haematuria, mild periorbital oedema, and hypertension (BP 148/92 mmHg) for 3 days. Urine dipstick shows 3+ protein and 4+ blood. Serum creatinine is 1.2 mg/dL (baseline 0.8 mg/dL). Complement C3 levels are low. Which investigation is most specific for confirming the diagnosis of post-infectious glomerulonephritis?

Explanation

## Investigation Most Specific for Confirming Post-Infectious Glomerulonephritis **Key Point:** In the clinical context of suspected post-streptococcal glomerulonephritis (PSGN), **serum ASO titre and throat culture** are the most specific investigations for *confirming* the diagnosis by establishing a preceding streptococcal infection — the defining etiological requirement for PSGN. ### Why ASO Titre + Throat Culture Is the Answer The question asks for the investigation most specific for **confirming the diagnosis of post-infectious GN** — not for characterising renal pathology in general. PSGN is defined by: 1. Clinical/laboratory features of nephritic syndrome (haematuria, hypertension, oedema, low C3) 2. **Evidence of a preceding streptococcal infection** — this is the specific confirmatory step that distinguishes PSGN from other causes of nephritic syndrome | Investigation | Role | Specificity for PSGN | |---|---|---| | **ASO titre / throat culture** | Confirms streptococcal aetiology | **High** — directly links syndrome to causative organism | | Renal biopsy | Characterises glomerular pathology | Moderate — subepithelial humps are suggestive but not exclusive to PSGN | | 24-hr urinary protein | Quantifies proteinuria | Non-specific | | Renal ultrasound + Doppler | Structural assessment | Non-specific | **Clinical Pearl (Harrison's Principles of Internal Medicine, 21st ed.):** PSGN is a *clinical diagnosis* supported by serological evidence of streptococcal infection. ASO titre is elevated in ~80% of pharyngitis-associated PSGN; anti-DNase B and streptozyme tests increase sensitivity for skin infections. Renal biopsy is **not routinely indicated** in classic PSGN and is reserved for atypical presentations, rapidly progressive course, or diagnostic uncertainty. ### Why Renal Biopsy (Option B) Is NOT the Best Answer Here - Biopsy is the gold standard for characterising **glomerular pathology** but is not specific to PSGN — subepithelial "humps" on electron microscopy are suggestive but can resolve early and are not pathognomonic; other GN types can produce a similar picture. - In a **classic presentation** (young patient, haematuria, hypertension, oedema, low C3, recent infection), biopsy is not the first-line confirmatory step per standard guidelines. - The question specifically asks what is most specific for **confirming PSGN** — establishing streptococcal aetiology via ASO titre/throat culture fulfils this requirement directly and non-invasively. ### This Patient's Profile - 28-year-old male with acute nephritic syndrome (haematuria, periorbital oedema, hypertension) - Low C3 → complement-consuming process (consistent with PSGN) - ASO titre elevation would confirm recent streptococcal infection → **confirms PSGN diagnosis** **High-Yield:** ASO titre > 200 Todd units in adults, combined with the classic nephritic picture and low C3, is sufficient to confirm PSGN without biopsy in most cases (Harrison's, 21st ed.; Brenner & Rector's The Kidney, 10th ed.). **Mnemonic: PAST** — **P**SGN = **A**SO titre confirms **S**treptococcal **T**rigger