Rheumatoid arthritis (RA), is a relatively common disorder, but diagnosis remains difficult, especially early in the disease before all its diagnostic features have developed. Early diagnosis is important, because evidence indicated that early and aggressive treatment improves long-term outcomes, especially erosions and joint destruction. However, the only “specific” diagnostic test is rheumatoid factor (RF), which was first discovered more than 50 years ago. However this test lacks both sensitivity and specificity, particularly early in disease. After many years, a new diagnostic test, antibody to cyclic citrullinated peptide (CCP) has become available, that adds significantly to our capacity for early disease diagnosis. Citrulline is the name given to the amino acid arginine after it has been post translationally modified by the enzyme peptidyl arginine deiminase

A number of comprehensive studies have attested to the usefulness of the CCP antibody test in the diagnosis of early rheumatoid arthritis. Anti-CCP antibodies can be detected in approximately 50% of patients with RA at a mean 4.5 years prior to onset of the disease. Data on the sensitivity and the specificity of this test varies a little depending on the details of the cohort understudy. But most studies suggest a very high specificity (about 90%), with a sensitivity of 60% to 80%.  Anti CCP antibodies, occur in both rheumatoid factor positive and rheumatoid factor negative rheumatoid arthritis.  As suggested by the specificity, anti CCP antibodies in significant titre are uncommon in conditions other than rheumatoid arthritis.

The expected frequency of positive results at levels >20u and >40u in various patient groups is shown in the table below.