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Blood Test Tracker

Anti-U1-RNP Antibody (RNP)

Looks for antibodies to U1-RNP. They are most closely linked with mixed connective tissue disease and can also appear in lupus and related conditions.

AI-verified against cited sources. Awaiting expert reviewAlso called Anti-RNP, U1-RNP, nRNPLOINC 29958-6

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What it is

U1-RNP is a complex of RNA and proteins inside the nucleus of your cells. Anti-RNP antibodies are autoantibodies, which means your immune system makes antibodies that react with your own cell parts. The test is usually ordered after a positive ANA when a clinician is looking at connective tissue conditions such as lupus, scleroderma or myositis (muscle inflammation). Labs report a number plus a call of negative, equivocal or positive. The cutoffs depend on the lab's test kit.

Why it matters

Mixed connective tissue disease (MCTD) is an overlap condition with features of lupus, scleroderma and muscle inflammation, and it is characterized by anti-U1-RNP antibodies. Anti-RNP is also seen in lupus, systemic sclerosis (scleroderma) and myositis, so a positive result narrows the possibilities but does not point to one condition. Your clinician reads it with your symptoms, your ANA and your other antibody results, especially anti-Sm, which sometimes appears together with anti-RNP.

If it’s high

A positive result means your blood has antibodies to U1-RNP. These antibodies are most strongly linked with mixed connective tissue disease. They are also seen in lupus, scleroderma and myositis. A positive result by itself does not mean you have any of these. Your clinician puts it in context with your symptoms and other tests.

Common causes

  • Mixed connective tissue disease
  • Lupus (systemic lupus erythematosus)
  • Systemic sclerosis (scleroderma)
  • Myositis (muscle inflammation)

What can help

  • Ask your clinician how this result fits with symptoms such as fingers that turn white or blue in the cold (Raynaud's), joint pain or swelling, or muscle weakness
  • Ask whether anti-Sm, anti-dsDNA and other antibodies were checked too

What can affect your result

  • Lab and test kit: units and cutoffs differ between labs, so compare results only from the same lab
  • Report format: some labs report a combined Sm/RNP result, which can be positive because of anti-Sm alone, and some report U1-RNP parts such as 70 kD, A and C separately. Check which antibody your report names
  • Hemolyzed, severely lipemic or contaminated samples can be rejected or give unreliable results (ARUP specimen requirements).
  • Labs use different test kits, scales and cutoffs (for example AU/mL, antibody index or chemiluminescent units), so a number from one lab can't be compared with a number from another. Compare results from the same lab and test.

Not a short-term marker. These antibodies are usually checked to help sort out a condition, not to follow day-to-day changes.

When to talk to a clinician

A positive anti-RNP result is not an emergency by itself. Contact your clinician soon if you have fingers or toes that stay pale, blue or painful, finger sores, new joint swelling, muscle weakness or new shortness of breath. If you have severe chest pain, trouble breathing, confusion or fainting, call 911 or your local emergency number.

Questions you could ask

  • Why was the anti-RNP test ordered for me?
  • How does this result fit with my symptoms and my other lab results?
  • Would it help to see a rheumatologist, a specialist in autoimmune conditions?
  • Were anti-Sm and the other related antibodies checked, and what did they show?

Questions people ask

Does a positive anti-RNP result mean I have mixed connective tissue disease?

Not by itself. High anti-RNP levels are closely tied to that condition, but the antibody also appears in lupus, scleroderma and myositis. Your clinician looks at your symptoms and other tests before deciding what it means.

What is the difference between anti-RNP and anti-Sm?

Both target parts of the same kind of particle in the cell nucleus. Anti-Sm is much more specific for lupus. Anti-RNP is more closely tied to mixed connective tissue disease. Some people have both, so clinicians usually look at them together.

Show the science and sources

For clinicians and researchers

U1-RNP is a small nuclear ribonucleoprotein made of U1 RNA and proteins, including the 70 kD, A and C proteins that carry the RNP-specific antigens plus the Sm core proteins it shares with other snRNPs. Anti-U1-RNP antibodies characterize mixed connective tissue disease and are also seen in SLE, systemic sclerosis and myositis. In a South China SLE cohort of 917 patients, anti-RNP was linked with Raynaud phenomenon and photosensitivity, and anti-Ro, anti-Sm and anti-RNP formed one antibody cluster. ARUP Laboratories' Smith/RNP (ENA) IgG test (semi-quantitative multiplex bead assay, AU/mL; negative 29 or less, equivocal 30 to 40, positive 41 or more) notes that antibodies to the Smith/RNP complex also recognize Smith antigens, so the Smith result must be read with it; that test is a complex result and is tracked under anti-sm-rnp, not here. Other labs report anti-RNP by ELISA, chemiluminescent or line-blot methods with their own scales and cutoffs, and the numbers are not interchangeable.

LOINC 29958-6 (primary), 29374-6, 31588-7, 51928-0. Specimen: serum.

Sources

  1. Smith/RNP (ENA) Antibody, IgG (test 0050470), ARUP Laboratories
  2. Autoantibodies in systemic lupus erythematosus, Annals of the Academy of Medicine, Singapore (1988)
  3. Clinical and serologic correlations and autoantibody clusters in systemic lupus erythematosus: a retrospective review of 917 patients in South China, Medicine (Baltimore) (2010)
  4. Autoantibody Testing: MedlinePlus Medical Test, MedlinePlus (National Library of Medicine) (2025)

AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.

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