Reference Range
Because normal and abnormal ranges depend on the laboratory used, it is important to analyze results according to guidelines established by the laboratory running the sample. For example, according to one laboratory, the following parameters are accepted:
A negative antinuclear ribonucleoprotein (anti-RNP) antibody result is defined as less than 20 U based on enzyme-linked immunoassay (ELISA).
A borderline result is defined as 20-25 U.
A positive result is defined as more than 26 U. [1]
Interpretation
A normal anti-RNP antibody value is negative and virtually excludes the diagnosis of mixed connective-tissue disease (MCTD). [2, 3, 4]
An elevated value is associated with different connective-tissue diseases to varying extents. An elevated anti-RNP level is necessary for the diagnosis of MCTD; [5] it has a sensitivity of 95%-100%, especially if it is found in high titers (ie, 1:1,000,000) and in isolation (ie without other extractable nuclear antigens such as anti-double stranded DNA). Low titers are associated with other rheumatologic diseases.
Sensitivities of anti-RNP antibody in various rheumatologic diseases are as follows: [6]
-
MCTD: 95%-100%
-
Systemic lupus erythematosus (SLE): 38%-44%
-
Discoid lupus erythematosus: 20%-30%
-
Scleroderma: 20%-30%
-
Rheumatoid arthritis: 10%
A study by Ordoñez-Cañizares et al indicated that in persons with SLE, anti-RNP antibodies are associated with polyautoimmunity, the odds ratio (OR) being 5.095. [7]
Collection and Panels
Collect blood in a serum-collection (red-top) tube.
Allow the blood to clot; separate the serum by centrifugation.
Although hemolysis and lipemia do not interfere with this assay, they are best avoided.
Anticoagulants and an elevated bilirubin levels also do not interfere with results.
Specimens may be stored at 2-8°C for up to 5 days or -20°C for up to 6 months. [8]
This test is associated with the antinuclear antibody test; if this shows a speckled pattern, the specimen may be reflexively sent for anti-RNP antibody, anti-Sm antibody, anti-SSA antibody, and anti-SSB antibody. [9] If the laboratory does not have such a protocol in place, it is prudent to send the positive sample for these studies.
Background
Description
Anti-RNP antibodies are directed against the U1 particle of an extractable nuclear antigen (ENA) named ribonucleoprotein, [10] which is made up of a complex of proteins and a small nuclear RNA named U1. [11] Ribonucleoprotein is part of the splicosome and also includes Smith (Sm) antigen, which is the target of the anti-Smith antibody and is frequently positive in SLE. The anti-RNP test usually involves ELISA for the quantitative measurement of immunoglobulin G (IgG) class autoantibodies.
While the detection of anti-RNP antibodies is consistent with MCTD, it is not diagnostic; a patient may have high levels of anti-RNP and present with no clinical signs of disease.
Indications/Applications
Anti-RNP antibody levels are elevated in 100% of patients with MCTD and 38%-44% of patients with SLE. [12] Anti-RNP antibody testing is usually ordered after a positive antinuclear antibody (ANA) finding in a patient with high clinical suspicion for MCTD or SLE. [13]
MCTD is characterized by overlapping clinical features of SLE, systemic sclerosis, polymyositis/dermatomyositis, and the presence of anti-RNP antibodies, as well as Reynaud phenomenon. [14] It is still unclear if MCTD is a distinct disease entity, a syndrome, or a subset of SLE or scleroderma. [15, 16, 17] In any case, the presence of anti-RNP antibodies in SLE has been associated with a lower prevalence of Raynaud phenomenon and milder renal involvement. [18]
There are no conditions under which this test should not be done. The test will be of very low yield if the patient has a negative ANA. [13]
Considerations
The degree of positivity or the titer of anti-RNP antibody does not indicate the severity or duration of disease.
