The anti-CCP (anti-Cyclic Citrullinated Peptide) antibody test is a blood test that looks for anti-CCP antibodies (also called anti-citrullinated protein antibodies or ACPAs). These are a type of autoantibody: an antibody produced by the immune system that targets something that the body’s own tissues are made of. ACPAs target a type of protein that is called a citrullinated protein that can be found in some people’s joints.
The anti-CCP antibody test can be helpful in diagnosing Rheumatoid Arthritis (RA). ACPAs are present in 60-70% of people who get RA. Since not everyone with RA will test positive, doctors interpret the results of this test in context with their patient’s symptoms and the results of other tests.
This test’s results are important to consider because:
- In a healthy person without joint problems, a positive anti-CCP significantly increases the odds that the person will get RA in the future
- In a person in the early stages of arthritis, a positive anti-CCP significantly increases the odds of developing RA (even more so than having another type of antibody associated with arthritis, called Rheumatoid Factor or “RF”).
- In a person who has RA, a positive anti-CCP is a good predictor for erosive disease (a more severe condition where the bones can start to erode away).
The anti-CCP antibody test can help distinguish RA from other possible types of arthritis. The ACPAs it looks for are almost always associated with RA, they are not associated with many other types of arthritis, and they are only rarely found in certain other autoimmune conditions.
Science Behind this Test
The body is made up of amino acids. These are the building blocks of all of the proteins in the body.
Standard Amino Acids
There are 20 standard amino acids that make up all of the body’s various protein structures. It’s like the body is made out of 20 blocks from a very special LEGO-brand building block set. When amino acids are linked together, they can make peptides (short chains) or proteins (long chains). Examples of a few standard amino acids include: arginine, cysteine, glutamine, and tyrosine.
Non-Standard Amino Acids
The body contains a number of non-standard amino acids. They are created when a standard amino acid is modified. These modifications can be essential for the function and regulation of a protein (the protein itself would be constructed out of standard amino acids). However, many non-standard amino acids that can possibly exist in chemistry are never normally found in the body’s proteins.
Citrullinated Peptides or Proteins
Arginine is a standard amino acid. Arginine can be converted to the non-standard amino acid citrulline by an enzyme called Peptidyl-Arginine-Deiminase (PAD). When arginine is converted to citrulline, the resulting protein or peptide is said to be citrullinated.
Immune Response and Joint Involvement
Citrullinated proteins are understood to be one of the drivers of the immune process in rheumatoid arthritis. They are not normally found in the body. If these proteins do exist, the body’s immune system thinks they are foreign and starts to attack them. This becomes a problem when citrullinated proteins are found in the joints. When the immune system starts attacking, the joints can become involved too.
The immune system’s attack is very harsh. Its response is like an air force dropping a bomb on a building because it wants to get rid of a target inside. Other things in and around the building are also likely to get damaged or destroyed.
Instead of bombs, the immune system makes antibodies that travel through the body in the blood. The antibodies that specifically attack citrullinated proteins are called anti-cyclic citrullinated peptide antibodies (anti-CCP antibodies) or anti-citrullinated protein antibodies (ACPAs). The antibodies’ attack can also damage any joints that might contain these proteins.
The name of the test, the anti-CCP antibody test, now makes sense: it is the test that is used to tell if these antibodies exist in someone’s blood.
People who have anti-CCP antibodies have significantly increased odds of developing RA, and people with RA who have them are more likely to have worse forms of the disease.