U1 RNP Ab in serum
Etiology
- systemic lupus erythematosus (46%)
- mixed connective tissue disease (high titer)
- scleroderma
- rheumatoid arthritis
- discoid lupus
- Sjogren's syndrome
Reference interval
- Normal: negative for antibodies to RNP [<20 EU/mL]
Principle
Highly purified SM/RNP 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-RNP (or 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 and/or anti-RNP IgG of the antigen-antibody complex, forming a 'sandwich' consisting of:
Conjugate (Enzyme-labeled Anti-human IgG)
Human anti-SM and/or anti-RNP (IgG)
Well Coated with SM/RNP 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 and the color fixed. The intensity of the color is measured photometrically at 405 nm & is proportional to the concentration of anti-SM and/or anti-RNP present in the patient sample. The anti-Sm antibody value in EU/mL is then subtracted from the test EU/mL to give the anti-RNP value.
Clinical significance
- 35-46% sensitivity for SLE[4]
- high titers characteristic of MCTD
- titer does not correlate with disease activity[4]
Antibodies to RNP are detected at high frequency (up to 46%) in patients with systemic lupus erythematosus (SLE) either alone or in parallel with Sm antigen. The RNP antigen is very closely associated with the Sm antigen. However, in contrast to anti-Sm, anti-RNP is found in patients with a variety of connective tissue diseases, including scleroderma, rheumatoid arthritis, discoid lupus & Sjogren's syndrome. A very high titer of anti-RNP, in the absence of other antibodies, is highly characteristic of mixed connective tissue disease (MCTD).
SLE patients who produce RNP antibodies in the absence of anti-Sm antibodies have a low frequency of nephritis & antibodies to DNA whereas patients who produce both RNP & Sm have a higher frequency of nephritis & anti-DNA antibodies. The comparative data on anti-Sm & anti-RNP presence/absence & levels can thus have diagnostic significance.
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
- >25 EU/mL: positive for antibodies to RNP
More general terms
More specific terms
Additional terms
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.
- ↑ 4.0 4.1 4.2 Medical Knowledge Self Assessment Program (MKSAP) 14, 16, 17 American College of Physicians, Philadelphia 2006, 2012, 2015