Rheumatoid arthritis (RA) is a chronic autoimmune disease characterized by inflammation that primarily affects the lining of the joints, leading to pain, swelling, and potential long-term damage. Since disease activity can fluctuate and the symptoms are often subjective, objective measurement is essential for effective management. Rheumatologists use a “Treat-to-Target” strategy, which relies on regularly monitoring disease activity to guide treatment decisions and adjust therapies until a low-activity goal or remission is achieved. This systematic approach helps to slow the progression of joint destruction and preserve function.
Assessing Systemic Inflammation Through Blood Tests
Measuring systemic inflammation provides objective data about the body’s overall inflammatory state, which is a hallmark of active RA. Two common laboratory tests quantify acute-phase reactants, which are proteins produced by the liver in response to inflammation. The Erythrocyte Sedimentation Rate (ESR) measures how quickly red blood cells settle in a test tube, with a faster rate indicating higher levels of inflammatory proteins in the blood.
C-reactive protein (CRP) is a more direct measure of systemic inflammation, often reflecting activity more rapidly than the ESR. High levels of both ESR and CRP generally correlate with increased disease activity, but they do not specifically indicate which joints are affected. These markers are essential inputs for calculating various composite disease activity scores.
Specific autoantibodies help confirm the RA diagnosis and predict the disease’s course. Rheumatoid Factor (RF) is an antibody found in a majority of people with RA. Anti-Cyclic Citrullinated Peptide (anti-CCP) antibodies are highly specific to RA and are a stronger predictor of a more aggressive, erosive disease course. The presence of high levels of these autoantibodies helps physicians determine the potential need for more intensive treatment strategies, even early in the disease.
Clinical Evaluation of Joint Activity
Physical assessments by a rheumatologist quantify the localized effects of inflammation, moving beyond the systemic data provided by blood tests. The Tender Joint Count (TJC) measures the number of joints that are painful or sore when the physician gently presses on them. The Swollen Joint Count (SJC) determines the number of joints exhibiting visible or palpable swelling due to inflammation of the joint lining.
These counts are typically performed on a standardized set of 28 joints, including those in the hands, wrists, elbows, shoulders, and knees. The resulting TJC28 and SJC28 values provide a localized, physical measure of disease burden. These counts are highly specific to RA activity and are combined with other measures to form a comprehensive picture.
Two important subjective measures reflect the patient’s and physician’s perceptions of the disease activity. The Patient Global Assessment (PGA) asks the patient to rate their overall disease activity, often on a 0-to-100 visual analog scale, which strongly reflects factors like pain, fatigue, and function. The Physician Global Assessment (PhGA) is the physician’s holistic rating, integrating the physical exam findings and laboratory results.
Calculating Disease Activity Scores
Combining these data points into a single number provides an efficient, actionable metric for monitoring RA over time. These composite indices are central to the Treat-to-Target strategy, allowing for standardized comparison of disease status across different visits and patients. The Disease Activity Score 28 (DAS28) is the most widely used index.
Components of DAS28
The DAS28 incorporates:
- Swollen Joint Count (SJC28)
- Tender Joint Count (TJC28)
- A measure of systemic inflammation (either ESR or CRP)
- The Patient Global Assessment (PGA)
The DAS28 produces a score ranging from approximately 0 to 10, where a higher number indicates greater disease activity. This score is used to categorize the patient’s current status: a score above 5.1 signifies high disease activity, 3.2 to 5.1 is moderate, and 2.6 to 3.2 is low activity. A score below 2.6 is generally considered to be in remission, representing the optimal goal for treatment.
Other indices are also frequently used in clinical practice due to their simplicity and ability to be calculated immediately. The Clinical Disease Activity Index (CDAI) is calculated by summing the TJC28, SJC28, PGA, and PhGA, notably excluding the need for immediate blood test results. The Simplified Disease Activity Index (SDAI) includes the same four clinical components as the CDAI but adds the C-reactive protein (CRP) value.
Evaluating Long-Term Joint Damage
Measuring current disease activity is distinct from assessing the long-term structural changes that RA can cause, which often require imaging techniques. Conventional Radiography, or X-rays, is the standard method for monitoring the progression of irreversible joint damage over time. X-rays can visualize two primary signs of structural damage: bone erosions and joint space narrowing.
Erosions are areas where the chronic inflammation has caused the bone structure near the joint to be dissolved, while joint space narrowing reflects the loss of joint cartilage. The modified Sharp score is a common method used to grade and track these changes numerically by assessing specific joints in the hands and feet. This score is most valuable for comparing images taken years apart to determine if the therapeutic regimen is successfully preventing structural damage.
More advanced imaging, such as Magnetic Resonance Imaging (MRI) and ultrasound, offers higher sensitivity for detecting early inflammatory changes that X-rays may miss. Ultrasound, particularly with Power Doppler technology, can identify active inflammation in the joint lining, known as synovitis, and can also detect early bone erosions. MRI is even more sensitive, able to detect bone marrow edema, a sign of inflammation within the bone itself that is considered a precursor to future erosions.