Which diagnostic test has the highest sensitivity and specificity for confirming myasthenia gravis in a seronegative patient?

Explanation

## Diagnostic Testing in Seronegative Myasthenia Gravis **Key Point:** Single-fiber electromyography (SFEMG) is the most sensitive test for MG overall, with sensitivity >95% in generalized MG and ~80% in ocular MG, regardless of serostatus. ### Sensitivity and Specificity Comparison | Test | Sensitivity (Generalized MG) | Sensitivity (Ocular MG) | Specificity | Notes | |------|------|------|------|----------| | **Anti-AChR antibodies** | 80–90% | 50% | ~99% | Gold standard for seropositive; misses 10–20% | | **Anti-MuSK antibodies** | 40% of seronegative | N/A | ~99% | Detects subset of seronegative cases | | **SFEMG** | >95% | ~80% | 80–90% | Most sensitive; abnormal in both seropositive and seronegative | | **RNS (decremental response)** | 60–70% (generalized) | 10% (ocular) | ~95% | Positive in ~2/3 of generalized; poor in ocular | | **Edrophonium test** | 90–95% (if done) | 90–95% | ~95% | Rarely used now (safety concerns); largely replaced by SFEMG | **High-Yield:** In seronegative MG (10–15% of cases), SFEMG is the most reliable confirmatory test because it detects neuromuscular junction dysfunction regardless of antibody status. RNS is less sensitive in seronegative disease. ### SFEMG Findings in MG **Increased jitter** → Variability in time interval between action potentials of muscle fiber pairs **Blocking** → Intermittent failure of neuromuscular transmission These findings reflect impaired neuromuscular transmission and are present in >95% of generalized MG, making SFEMG the most sensitive confirmatory test. **Clinical Pearl:** SFEMG is abnormal in nearly all patients with generalized MG (seropositive or seronegative) but is less sensitive in purely ocular MG (~80%). It is not specific for MG alone — similar findings occur in other neuromuscular disorders (LEMS, motor neuron disease, myopathy). **Warning:** Edrophonium testing, once the gold standard, is now rarely performed due to risk of cholinergic crisis and availability of safer alternatives (SFEMG, antibody testing). Do not choose edrophonium as a first-line diagnostic test in modern practice. ### Diagnostic Algorithm for Suspected MG ```mermaid flowchart TD A[Clinical suspicion of MG]:::outcome --> B[Anti-AChR antibody test]:::action B --> C{Positive?}:::decision C -->|Yes| D[Seropositive MG confirmed]:::outcome C -->|No| E[Anti-MuSK antibody test]:::action E --> F{Positive?}:::decision F -->|Yes| G[MuSK-seropositive MG]:::outcome F -->|No| H[Seronegative MG likely]:::outcome H --> I[SFEMG for confirmation]:::action I --> J{Abnormal jitter/blocking?}:::decision J -->|Yes| K[Seronegative MG confirmed]:::outcome J -->|No| L[Consider alternative diagnosis]:::urgent ```