Laboratory testing is an important part of forming a complete picture that will help determine your diagnosis. There are certain substances that tend to be present in the blood of patients with RA or elevations of common components of blood that are suggestive of inflammatory disease. Laboratory tests for RA commonly look at:
- Rheumatoid factor (RF)
- Anti-citrullinated protein antibodies (ACPA) (including anti-cyclic citrullinated peptide [anti-CCP] and anti-mutated citrullinated vimentin [MCV] antibody tests)
- Erythrocyte sedimentation rate (ESR)
- C-reactive protein (CRP)
- Antinuclear antibody (ANA)
- Complete blood count (CBC)
What is rheumatoid factor (RF)?
RF is an autoantibody that is found in 75% to 80% of patients with RA. Your immune system (specifically B-cells) create antibodies against viruses and bacteria and other organisms that invade the body. Autoantibodies are created in autoimmune diseases when the immune system malfunctions and attacks healthy tissues in your body.2
RF has been shown to be useful in diagnosing RA. Even as diagnostic criteria have changed over time and now emphasize another autoantibody called anti-citrullinated peptide antibodies (ACPA), RF continues to be important. Measurement of both RF and ACPA improves the accuracy of diagnosis of RA. A patient may be positive for RF even before the signs and symptoms of RA appear.2
RF levels will decrease with effective treatment of RA with disease-modifying anti-rheumatic drugs (DMARDs) and newer biologic treatments.2
Additionally, RF is a strong predictor of severe RA and the likelihood of extra-articular manifestations. Specifically, RF can predict increased risk for2:
- More rapid radiographic progression of RA
- More bone erosions, functional disability, and disease activity
- RA nodules
- Vasculitis and interstitial lung disease
RF can be used to predict the response to treatment with certain biologic treatments, such as Rituxan (rituximab) which inhibits B-cells. The presence of RF is also associated with other diseases, including systemic lupus erythematosus (SLE) and primary Sjögren's syndrome, as well as certain infections (eg. malaria, rubella).2
RF is tested by taking a blood sample. There is no preparation necessary before your blood is drawn.
RF testing is typically reported as either units per milliliter (u/mL) or as a titer, which means how much a blood sample can be diluted before RF is no longer detectable. A titer is expressed as a ratio, such as 1:80 units. Normal values for RF vary somewhat among different laboratories, but normal measurements are 14-60 units/mL or 1: 20-80 units.
What is anti-citrullinated peptide antibody (ACPA)?
ACPA testing is a common and accurate laboratory test for RA. Nine out of ten or 90% of people with RA test positive for ACPA and the test is useful for distinguishing RA from other forms of arthritis. Similar to RF, a patient may test positive for ACPA even before the signs and symptoms of RA appear.2
There are a few different ACPA tests used, including tests for anti-cyclic citrullinated peptide (anti-CCP) antibodies and anti-mutated citrullinated vimentin (anti-MCV) antibodies.2
As with RF, levels of ACPA will decrease with effective treatment of RA with disease-modifying anti-rheumatic drugs (DMARDs) and newer biologic treatments.2
Although ACPA testing is a very specific predictor of RA, a patient may also be ACPA-positive with other diseases, including autoimmune diseases such as SLE and primary Sjögren's syndrome, tuberculosis, and chronic lung disease.2
Similar to RF, ACPA is useful in predicting the course of RA. An ACPA-positive test early in the course of RA may predict increased risk of joint damage, including erosive damage.2
What is erythrocyte sedimentation rate (ESR)?
ESR is a test that measures how quickly erythrocytes (red blood cells) fall through plasma (the colorless fluid part of blood). Because ESR can be influenced by all of the cells and chemicals that are found in plasma, results of ESR may not be precise.2
However, despite limitations associated with the test, ESR rates are typically elevated in patients with RA and do predict increased risk for joint damage as revealed by x-ray and increased disease activity and severity. Consequently, ESR is a valuable test for monitoring response to RA treatment. Treatments that result in decreases in ESR may decrease risk for later joint damage.2
ESR is tested by taking a blood sample. There is no preparation necessary before your blood is drawn.
Normal ranges for ESR have been determined for different age groups and genders, but may vary from laboratory to laboratory.
- Men less than 50 years of age: less than 15 millimeters (mm)/hour (h)
- Men over 50 years of age: less than 20 mm/hr
- Women less than 50 years of age: less than 20 mm/hr
- Women over 50 years of age: less than 30 mm/hr
- Children (newborn to puberty): 3-13 mm/hr
ESR of 50-80 mm/hr is not uncommon in patients with severe RA and in patients with limited disease (only a few joints with mild to moderate symptoms) ESR of 20-30 mm/hr may be observed.1
What is C-reactive protein (CRP)?
CRP is a protein (part of a family of substances called acute phase reactants) found in blood. Levels of CRP rise when inflammation is present in the body. CRP has been shown to be a useful measure of RA disease activity. However, the test is somewhat limited in that, while it can detect inflammation, it cannot determine the location of that inflammation.2
Elevated ESR and CRP, taken together, provide a better picture of joint damage than CRP alone. However, one study conducted in patients with early RA who had had symptoms for less than a year, those with high levels of CRP had increased likelihood of disease progression with joint damage visible on x-ray.2
CRP is tested by taking a blood sample. There is no preparation necessary before your blood is drawn.
What is Antinuclear antibody (ANA)?
ANA testing is a DNA test that can be useful in determining whether a patient has RA or help exclude another rheumatic disease such as SLE. Up to one-third of patients with RA test positive for ANA.1
ANA testing is done by taking a blood sample. No special preparations are required for the test. However, certain medications may interfere with the test results, so make sure that your doctor knows what medications you are taking.
ANA is reported as a titer, which means how much a blood sample can be diluted before ANA is no longer detectable. A titer is expressed as a ratio, such as 1:80 units. Typically a normal test will show no ANA present in the blood. However, some people will have a low titer (ranging from 1:40-60) without having any disease.
What is a complete blood count (CBC)?
A CBC will test various levels of cells and chemicals present in your blood, including red and white blood cells, platelets, markers of liver and kidney function, and uric acid. Patients with RA often have an abnormal CBC, with anemia (decreased red blood cells or hemoglobin) and thrombocytopenia (decreased platelets).1