Anti-U1 RNP antibody test
Quick answer: This blood test looks for antibodies against U1 ribonucleoprotein (RNP), a protein complex in the cell nucleus. Doctors usually order it when mixed connective tissue disease or lupus is suspected, often after a positive ANA.
- SampleBlood
- TakesAbout 5 minutes
- Before the testNo special preparation is usually needed.
How the test is done
- 1You are seated and asked to make a loose fist
- 2A vein on your arm is cleaned with a swab
- 3A needle draws blood into a tube
- 4The lab looks for antibodies to U1 RNP in your serum
What your results can mean
Reading your report
A positive result does not by itself confirm a disease, because some healthy people test positive; your doctor reads it with your symptoms and other tests.
If the result is outside the usual range
A high level can be seen with mixed connective tissue disease, and lower levels with lupus, scleroderma or other autoimmune conditions. A negative result makes mixed connective tissue disease unlikely.
Who usually has this test
People with a speckled ANA, swollen fingers, Raynaud's phenomenon or features of more than one connective tissue disease.
Risks and aftercare
A small bruise can appear on the arm, and it usually fades within a few days.
Names and options
Also called: U1 RNP, anti-RNP, U1 snRNP
Listed by the hospital as
- ANTI U1 RNP
- U 1-SN RNP, SERUM
Questions about this test
What is mixed connective tissue disease?
It is a condition with overlapping features of lupus, scleroderma and myositis, such as puffy hands, Raynaud's and joint pain. A high U1 RNP antibody is a key part of the diagnosis.
Is U1 RNP the same as RNP/Sm?
They are closely related but not identical. U1 RNP tests one target, while RNP/Sm tests the combined complex. Labs and doctors may use either, so check your report.
Can U1 RNP antibodies be positive in lupus?
Yes. Many people with lupus have these antibodies. Your doctor reads the result alongside other antibodies such as anti-dsDNA and anti-Sm.
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This page explains each test in general terms so you know what to expect. It is not medical advice: your doctor decides which tests you need and what your results mean. Reference ranges differ between laboratories. Updated 4 November 2026.