A 28-year-old woman presents with photosensitive malar rash, arthralgia of small joints, and oral ulcers for 3 months. Laboratory findings show ANA positivity (1:640, homogeneous pattern) and low complement levels (C3 and C4). Which investigation is most specific for confirming the diagnosis of SLE?

Question

Accepted Answer

B. Anti-dsDNA antibodies and anti-Smith (anti-Sm) antibodies. ## Diagnosis Confirmation in SLE

**Key Point:** Anti-dsDNA and anti-Sm antibodies are highly specific for SLE and are part of the ACR/EULAR classification criteria.

### Why Anti-dsDNA and Anti-Sm?

**High-Yield:** Anti-Sm antibodies are 99% specific for SLE (though only 20–30% sensitive), making them the most specific single test. Anti-dsDNA antibodies are present in 60–70% of SLE patients and correlate with lupus nephritis activity.

| Antibody | Sensitivity | Specificity | Clinical Significance |
| --- | --- | --- | --- |
| Anti-Sm | 20–30% | 99% | Highly specific; part of SLE criteria |
| Anti-dsDNA | 60–70% | 95% | Correlates with nephritis; prognostic value |
| ANA | 95–98% | 70–80% | Sensitive but not specific |
| Anti-Ro/SSA | 40–60% | 90% | Associated with photosensitivity |

**Clinical Pearl:** The combination of ANA positivity (homogeneous pattern), low complement (C3/C4), and clinical features (malar rash, arthralgia, oral ulcers) strongly suggests SLE. Anti-dsDNA and anti-Sm antibodies confirm the diagnosis and help distinguish SLE from other autoimmune conditions.

### Diagnostic Algorithm

```mermaid
flowchart TD
  A[Clinical suspicion of SLE]:::outcome --> B[ANA screening]:::action
  B --> C{ANA positive?}:::decision
  C -->|Yes| D[Check anti-dsDNA and anti-Sm]:::action
  C -->|No| E[SLE unlikely]:::outcome
  D --> F{Anti-Sm or anti-dsDNA positive?}:::decision
  F -->|Yes| G[SLE confirmed]:::outcome
  F -->|No| H[Consider other autoimmune disease]:::outcome
  D --> I[Check complement C3, C4]:::action
  I --> J[Low complement = active disease]:::outcome
```

**Mnemonic:** **SLAM** criteria include **S**mith antibodies (anti-Sm) — the most specific marker for SLE diagnosis.

Answer

A. Rheumatoid factor and anti-CCP antibodies

Answer

C. Anti-tissue transglutaminase (anti-tTG) IgA antibodies

Answer

D. Anti-centromere antibodies and anti-topoisomerase-1 antibodies

A 28-year-old woman presents with photosensitive malar rash, arthralgia of small joints, and oral ulcers for 3 months. Laboratory findings show ANA positivity (1:640, homogeneous pattern) and low complement levels (C3 and C4). Which investigation is most specific for confirming the diagnosis of SLE?

Explanation

Diagnosis Confirmation in SLE

Why Anti-dsDNA and Anti-Sm?

Diagnostic Algorithm

Practice similar questions

Join our NEET PG community

Antibody	Sensitivity	Specificity	Clinical Significance
Anti-Sm	20–30%	99%	Highly specific; part of SLE criteria
Anti-dsDNA	60–70%	95%	Correlates with nephritis; prognostic value
ANA	95–98%	70–80%	Sensitive but not specific
Anti-Ro/SSA	40–60%	90%	Associated with photosensitivity