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?

Question

Accepted Answer

A. Serum anti-DNase B and anti-streptolysin O titers. ## 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.).

Answer

B. Kidney biopsy with light, electron, and immunofluorescence microscopy

Answer

C. Renal ultrasound with Doppler

Answer

D. Serum complement C3 level

Explanation

Diagnostic Approach to Post-Streptococcal GN

Why Anti-DNase B and ASO Titers Are Most Specific

Serologic Markers in PSGN

Why Other Options Are Less Specific

Clinical Context

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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