From the Guidelines
RNP antibody is a significant diagnostic marker for mixed connective tissue disease (MCTD) and other autoimmune conditions, with its presence indicating immune system attack on the body's own ribonucleoproteins. The RNP antibody test is used to detect autoantibodies targeting ribonucleoproteins, which are crucial for protein synthesis. According to the international recommendations for the assessment of autoantibodies to cellular antigens referred to as anti-nuclear antibodies 1, the detection of RNP antibodies is recommended in cases of clinical suspicion of MCTD. Key points about RNP antibody include:
- It is primarily used to help diagnose MCTD, but can also be relevant in other autoimmune conditions like SLE, scleroderma, and polymyositis.
- A positive RNP antibody test, particularly at high titers, is strongly associated with MCTD.
- The test is typically ordered as part of an autoimmune panel when patients present with symptoms like joint pain, muscle weakness, Raynaud's phenomenon, swollen hands, or lung problems.
- Results are usually reported as positive or negative, sometimes with titer levels indicating antibody concentration.
- While a positive result suggests autoimmune activity, diagnosis should never be based solely on this test but considered alongside clinical symptoms and other laboratory findings, as stated in the recommendations for ANA testing 1.
- The presence of these antibodies indicates that the immune system is mistakenly attacking the body's own ribonucleoproteins. It is essential to note that the method used for detecting RNP antibodies should be reported, and in case of discrepancy with clinical suspicion, the use of an additional method should be considered, as recommended in the guidelines 1.
From the Research
Significance of Rheumatoid Nucleoprotein (RNP) Antibody
The presence of RNP antibody is significant in the diagnosis and differentiation of various connective tissue diseases, including mixed connective tissue disease (MCTD) and systemic lupus erythematosus (SLE).
- RNP antibody is a key criterion for the diagnosis of MCTD, but it is also found in patients with SLE and other connective tissue diseases 2, 3, 4, 5.
- The clinical significance of RNP antibody includes its association with certain clinical manifestations, such as Raynaud's phenomenon, myositis, and sclerodactyly 3, 4, 5.
- The presence of RNP antibody can also be used to distinguish between MCTD and SLE, with patients meeting MCTD criteria more likely to display scleroderma features, such as swollen hands, sclerodactyly, and esophageal reflux 4.
Diagnostic Challenge
The diagnosis of patients with anti-RNP antibodies can be challenging, as these antibodies are found in both MCTD and SLE, and the clinical manifestations of these diseases can overlap 2, 4, 5.
- Studies have shown that the analysis of the proportion of different autoantibodies directed against the U1-snRNP complex can be more informative than the analysis of each autoantibody separately in distinguishing between MCTD and SLE 2.
- The presence of scleroderma features, such as swollen hands and sclerodactyly, is significantly associated with the diagnosis of MCTD, while the absence of these features suggests the diagnosis of another connective tissue disease, such as SLE 4.
Association with Microvascular Changes
RNP antibody has been associated with distinct microvascular changes, as assessed by nailfold capillaroscopy (NFC), in patients with SLE and MCTD 6.
- Studies have shown that RNP-positive SLE patients have more frequent giant capillaries, enlarged capillaries, and ramified capillaries compared to RNP-negative SLE patients 6.
- The capillary density is lower in patients with MCTD compared to patients with RNP-positive SLE and RNP-negative SLE, and the reduction in capillary density is less severe in patients with RNP-negative SLE compared to RNP-positive SLE 6.