Background

Anti-Smith Ab or Anti-sm antibodies target their own proteins from the body. It is mainly found in the patients with lupus. It is present in conditions like mixed connective-tissue disease (MCTD) and systemic lupus erythematosus (SLE). Smith antigens are one of the extractable nuclear antigens (ENA). These proteins are resistant to the ribonuclease.  

Around 30% of the patients with lupus have Anti-Smith Ab. Around 60% of the patients with SLE have Anti-Smith Ab. This is more common in Asian and black patients with SLE than the white patients with SLE. 

The smith antigens contain E, B1, and D proteins. Small nuclear RNAs contain Smith antigens and RNP antigens. In SLE condition, antibodies level against these antigens increased.  

Indications/Applications

• Anti-Smith antibody test is performed in the patients who have conditions like MCTD or SLE. The symptoms may include:  
• Malar or discoid rash 
• Mucosal ulcer 
• Photosensitivity 
• Anemia 
• Avascular necrosis 
• Arthritis 
• Glomerulonephritis 
• Serositis 

Reference Range

Anti-Smith antibodies present in conditions like mixed connective-tissue disease (MCTD) and systemic lupus erythematosus (SLE). 

The normal test result should be negative. 

Interpretation

Anti-Smith Ab has high specificity and low sensitivity for SLE. Patients with SLE have a 30% increased level of Anti-Smith Ab. Patients with MCTD have an 8% increased level of Anti-Smith Ab.  

Specificity and sensitivity may vary from laboratory to laboratory, and it is also depend on the technique. Immunoprecipitation and immunoblotting are more sensitive than the passive hemagglutination, immunodiffusion, and counter immune electrophoresis.  

As per the study, patient with CLE (cutaneous lupus erythematosus) and later progresses to SLE have Anti-Smith Ab than the patient who has only CLE. Patients with SLE have 30% increased level of Anti-Smith Ab. Patients with MCTD have 8% increased level of Anti-Smith Ab.  

Collection And Panels

Sample type: Serum sample 

Sample collection container: Red-top tube 

Unacceptable samples: Samples which are hemolyzed or lipemic samples 

Sample volume: 1 ml, minimum volume: 0.3 ml 

Sample stability: Samples stable for 4 days at room temperature, 7 days in refrigerator, and 30 days in frozen condition. 

Panels: Anti-Smith Ab test is a part of ENA (Extractable Nuclear Antigens) panel. The other tests include in this panel are Anti-Ribonucleoprotein test, anti-RO test, and Anti-La test.  

References

https://pubmed.ncbi.nlm.nih.gov/31552434/