A 28-year-old man presents with sudden onset hematuria, hypertension (160/100 mmHg), and mild edema 10 days after a sore throat. Serum creatinine is 1.2 mg/dL. Urinalysis shows RBC casts and dysmorphic RBCs. Which investigation is most specific for confirming the diagnosis of post-streptococcal glomerulonephritis?

## Diagnostic Approach to Post-Streptococcal GN ### Why Anti-DNase B and ASO Titers Are Most Specific **Key Point:** Serum anti-DNase B and anti-streptolysin O (ASO) titers are the most specific investigations for confirming recent streptococcal infection as the causative event in post-streptococcal glomerulonephritis (PSGN). Together, these serologic markers provide direct evidence of antecedent Group A β-hemolytic Streptococcus (GABHS) infection, which is the defining etiologic requirement for PSGN. ### Serologic Markers in PSGN | Marker | Sensitivity | Clinical Utility | |---|---|---| | **ASO titer** | ~70–80% (pharyngitis) | Elevated 1–3 weeks post-infection; confirms pharyngeal strep | | **Anti-DNase B** | ~90% (pharyngitis) | More sensitive and specific; persists longer than ASO | | **Combined (ASO + anti-DNase B)** | >95% | Near-definitive serologic confirmation of recent GABHS | **Clinical Pearl:** Anti-DNase B is particularly valuable because it remains elevated for months after infection and is more sensitive than ASO for pharyngeal streptococcal disease. Using both together achieves >95% sensitivity for confirming recent GABHS infection (Harrison's Principles of Internal Medicine, 21st ed.). ### Why Other Options Are Less Specific **High-Yield:** - **Kidney biopsy (Option B):** Although biopsy with EM showing subepithelial "humps" is pathognomonic for PSGN, biopsy is **not** the first-line or routine investigation in a classic, uncomplicated presentation. It is reserved for atypical cases, rapidly progressive disease, or failure to recover. In the clinical scenario described (classic post-pharyngitic nephritic syndrome with RBC casts), biopsy is not indicated and is not the "most specific" investigation of choice. - **Serum C3 level (Option C):** Depressed C3 is supportive of PSGN (seen in ~90% of cases) but is non-specific — it is also low in lupus nephritis, MPGN, and other complement-mediated GN. It does not confirm streptococcal etiology. - **Renal ultrasound with Doppler (Option D):** Structural imaging assesses kidney size and vascularity but cannot differentiate GN subtypes or confirm the diagnosis of PSGN. ### Clinical Context **Warning:** Do not confuse "most specific for confirming the diagnosis" with "gold standard histologic test." In the clinical context of a classic PSGN presentation, serologic confirmation of recent GABHS infection (ASO + anti-DNase B) is the most specific and appropriate investigation. Biopsy is reserved for atypical or severe presentations. **Tip:** The classic triad for PSGN diagnosis is: (1) nephritic syndrome 1–3 weeks post-pharyngitis, (2) low C3, and (3) elevated streptococcal antibody titers. Recovery of renal function within weeks further supports the diagnosis without requiring biopsy (Robbins & Cotran Pathologic Basis of Disease, 10th ed.).