Sm (Smith) Ab in serum
Indications
- systemic lupus erythematosus (30-40%)
Reference interval
- Normal: Negative
Principle
Highly purified SM antigen complex is purified from calf thymus, bound to microwells & stabilized for extended shelf life. Diluted patient sera are placed in the microwells & incubated. If anti-Sm antibodies are present, they will bind to the antigen in the microwell. The microwells are then washed to remove residual sample & a second incubation with anti-human IgG conjugated to alkaline phosphatase is carried out. The conjugate will bind immunologically to the anti-SM IgG of the antigen- antibody complex, forming a 'sandwich' consisting of:
Conjugate (Enzyme-labeled Anti-human IgG)
Well Coated with SM antigen
Unbound conjugate is removed in the subsequent washing step. Enzyme substrate is then added to the microwell & if bound conjugate is present, the colorless substrate (p-nitrophenyl phosphate), will be hydrolyzed to form a yellow end product, (p-nitrophenol). The reaction is then stopped & the color fixed. The intensity of the color is measured photometrically at 405 nm & is proportional to the concentration of anti-SM present in the patient sample. (The anti-Sm antibody value in EU/mL is subtracted from the anti-RNP EU/mL to give the anti-RNP value when anti-RNP Microassay is run.
Clinical significance
Anti-Smith antibodies are directed against a small nuclear RNP associated with the spliceosome (anti U1-RNP is also directed against a small nuclear RNP associated with the spliceosome)
Antibodies to Sm are present in approximately 30 to 40% of patients with systemic lupus erythematosus (SLE) & are considered to be highly specific markers for this disease since they have not been detected in normal sera or in patients with rheumatoid arthritis, Sjogren's syndrome, scleroderma, mixed connective tissue disease, dermatomyositis, or drug-induced LE. Certain very rare exceptions have been encountered, but in these instances an overlap with other types of connective tissue disease is a distinct possibility. Antibodies to Sm may be present in the absence of antibodies to double-stranded DNA (also highly specific for SLE), thus underscoring the potential diagnostic significance of antibody to Sm.
In addition to their diagnostic significance, the appearance of antibodies to Sm is an important tool in the prognosis, monitoring and management of SLE patients. Anti-Sm levels appear to fluctuate & rising titers have been found to correlate with clinical exacerbations of CNS & non-CNS disease in 60% of SLE patients; rising titers on anti-Sm antibodies also predicted disease relapse in 50% of SLE patients.
Specimen
Serum is separated from the clot & refrigerated, 2-8 degrees C for short term storage or stored frozen, -20 degrees C, for long term storage. Avoid freeze-thaw cycles. CAUTION: Serum samples should not be heat inactivated, as this may cause false positive results.
Interpretation
- < 20 EU/mL: Negative for antibodies to RNP.
- 20-25 EU/mL: Equivocal for antibodies to RNP. Before equivocal results are reported, retest the sample with Microassay. If repeated Microassay results are still equivocal, the test sample has no significant antibodies to RNP, & should be reported as negative.
- >25 EU/mL: Positive for antibodies to RNP.
More general terms
More specific terms
- Sm (Smith) B Ab in serum
- Sm (Smith) D Ab in serum
- Sm (Smith) D IgG in serum
- SM+RNP Ab in serum
- Smith extractable nuclear IgG antibody
Additional terms
Component of
References
- ↑ Henry, John Bernard, Clinical Diagnosis amd Management by Laboratory Methods, W. B. Saunders Co., Philadelphia, 1991. pp 891-892.
- ↑ The Physicians Guide to Anti-DNA Antibody Testing, Diamedix Corporation, Miami, Aug. 1989. pp 1-6.
- ↑ Summary of Procedure. DiaMedix Corporation, Miami, June 1991. pp 1-8.