Anti-RNP Antibody

Anti-RNP Antibody

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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 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]

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%

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.^[7]

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.^[8]If the laboratory does not have such a protocol in place, it is prudent to send the positive sample for these studies.

Anti-RNP antibodies are directed against the U1 particle of an extractable nuclear antigen (ENA) named ribonucleoprotein,^[9]which is made up of a complex of proteins and a small nuclear RNA named U1.^[10]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.

Anti-RNP antibody levels are elevated in 100% of patients with MCTD and 38%-44% of patients with SLE.^[11]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.^[12]

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.^[13]It is still unclear if MCTD is a distinct disease entity, a syndrome, or a subset of SLE or scleroderma.^[14]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.^[15]

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.^[12]

The degree of positivity or the titer of anti-RNP antibody does not indicate the severity or duration of disease.

Ed. Frances T. Fischbach. “Immunodiagnostic Studies.” Manual of Laboratory & Diagnostic Tests. 7th ed. Philadelphia: Lippincott Williams & Wilkins; 2004. 590. 302 Found. Ovid. Web. 19 Mar. 2012. [Full Text].

Nicoll Diana, McPhee Stephen J, Pignone Michael, Lu Chuanyi Mark. “Ribonucleoprotein Antibody.” Pocket Guide to Diagnostic Tests. 5th ed. [Full Text].

Kiani IG, Qureshi SH, Shah F. Depression and seizures as the main neuropsychiatric manifestation of mixed connective tissue disorder. J Coll Physicians Surg Pak. 2014 May. 24 Suppl 2:S141-3. [Medline].

Maezawa R, Kurasawa K, Arai S, Okada H, Owada T, Fukuda T. Positivity for anti-RNP antibody is a risk factor for adverse effects caused by trimethoprim-sulfamethoxazole, a prophylactic agent for P. jiroveci pneumonia, in patients with connective tissue diseases. Mod Rheumatol. 2013 Jan. 23(1):62-70. [Medline].

Imboden JB, Hellmann DB, Stone JH. Chapter 3. Laboratory Diagnosis. CURRENT Rheumatology Diagnosis & Treatment. 2nd ed. New York: McGraw-Hill; 2007.

Nicoll Diana, McPhee Stephen J, Pignone Michael, Lu Chuanyi Mark. “Ribonucleoprotein Antibody.” Pocket Guide to Diagnostic Tests. 5th ed. [Full Text].

Fischbach , Frances, Marshall B. Dunning. “Antibodies to Extractable Nuclear Antigens.” A Manual of Laboratory and Diagnostic Tests. Philadelphia: Wolters Kluwer/Lippincott Williams & Wilkins; 2009. 625. Print.

Immunometric Enzyme Immunoassay for the quantitative determination of IgG autoantibodies to RNP/Sm” Immuno-biologic Laboratories, Inc. 2004. [Full Text].

Firestein. “DISEASES ASSOCIATED WITH ANTINUCLEAR ANTIBODIES,” Kelley’s Textbook of Rheumatology. 8th ed.

Williamson, Mary A., L. Michael. Snyder, and Jacques B. Wallach. “Autoimmune and Miscellaneous Diseases.” Wallach’s Interpretation of Diagnostic Tests. Philadelphia: Wolters Kluwer/Lippincott Williams & Wilkins; 2011. 1035. Print.

Imboden JB, Hellmann DB, Stone JH. Chapter 3. Laboratory Diagnosis. CURRENT Rheumatology Diagnosis & Treatment. 2nd ed. New York: McGraw-Hill; 2007. [Full Text].

Tsai YY et al. Fifteen-year experience of pediatric-onset mixed connective tissue disease. Clin Rheumatol. 2010 Jan. 29(1):53–8. [Full Text].

Hellmann, David B, John B. Imboden. “Mixed Connective Tissue Disease & Overlap Syndromes.” Current Medical Diagnosis and Treatment. [Full Text].

Migliorini P, Baldini C, Rocchi V, Bombardieri S. Anti-Sm and anti-RNP antibodies. Autoimmunity. 2005 Feb. 38(1):47-54. [Medline].

Asma Al-Zougbi, MD Resident in Internal Medicine, St Louis University Hospital

Asma Al-Zougbi, MD is a member of the following medical societies: American College of Physicians

Disclosure: Nothing to disclose.

Eric B Staros, MD Associate Professor of Pathology, St Louis University School of Medicine; Director of Clinical Laboratories, Director of Cytopathology, Department of Pathology, St Louis University Hospital

Eric B Staros, MD is a member of the following medical societies: American Medical Association, American Society for Clinical Pathology, College of American Pathologists, Association for Molecular Pathology

Disclosure: Nothing to disclose.

Anti-RNP Antibody

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