Design: A prospective, single-arm, multicenter, real-world study that does not interfere with the patient's treatment plan

Primary Objective:

1. To evaluate the effectiveness and safety of subcutaneous Methotrexate (MTX) in RA patients in a real-world setting;

Exploratory Objectives:

1. To assess the safety and effectiveness of subcutaneous MTX in RA patients with interstitial lung disease (ILD) or interstitial lung abnormalities (ILAs), and stable coronary artery disease (SCAD) in a real-world setting;
2. To evaluate the effectiveness and safety of subcutaneous MTX in RA patients with different clinical subtypes.

The study includes adult RA patients treated with subcutaneous MTX, divided into the following four cohorts based on comorbidities and clinical subtypes:

Cohort 1: Chinese RA patients receiving subcutaneous MTX treatment (8,000 cases) Cohort 2: Chinese RA patients with ILD or ILAs receiving subcutaneous MTX treatment (200 cases) Cohort 3: Chinese RA patients with clinical subtype results at enrollment, receiving subcutaneous MTX treatment (1,500 cases) Cohort 4: Chinese RA arthritis patients with SCAD receiving subcutaneous MTX treatment (300 cases)

Inclusion Criteria:

1. Adult patients diagnosed with rheumatoid arthritis according to the 1987 ACR or 2010 ACR/EULAR classification criteria.
2. Patients who, after clinical evaluation, are starting or preparing to start subcutaneous methotrexate treatment, with expected benefits outweighing the risks.
3. Patients who agree to participate in the study, can comply with follow-up, and sign the informed consent form.

Additionally, subjects meeting the following inclusion criteria can be assigned to Cohort 2:

Inclusion Criteria:

1. Diagnosed with interstitial lung disease (ILD) or interstitial lung abnormalities (ILAs) prior to or at the time of enrollment;
2. Have baseline forced vital capacity (FVC) results at the time of enrollment, with FVC being 50% or more of the predicted value.

Subjects meeting the following inclusion criteria can be assigned to Cohort 3:

1. Have a clear clinical subtype result at the time of enrollment.

Subjects meeting the following inclusion criteria can be assigned to Cohort 4:

1. Diagnosed with stable coronary artery disease prior to or at the time of enrollment, including those with a history of coronary artery intervention or coronary artery bypass grafting for at least one year, or angiographic evidence of ≥50% stenosis in at least one coronary artery without the need for revascularization;
2. C-reactive protein (CRP) or high-sensitivity CRP (hsCRP) ≥2 mg/L.

Exclusion Criteria:

1. Pregnant or breastfeeding women;
2. Serum creatinine: Female patients with serum creatinine >1.4 mg/dL (124 μmol/L); male patients with serum creatinine >1.6 mg/dL (141 μmol/L); patients with alanine aminotransferase (ALT) or aspartate aminotransferase (AST) >1.5 times the upper limit of normal (ULN);
3. Platelet count <80×10^9/L; white blood cell (WBC) count <3.5×10^9/L; total bilirubin >1.5 times the ULN;
4. Presence of severe, uncontrolled comorbid conditions, such as (but not limited to) neurological, cardiovascular, hepatic, renal, gastrointestinal, or endocrine diseases, which in the investigator's judgment may interfere with the patient's participation in the study;
5. Patients with a history of malignancy within the past 5 years;
6. Cardiac diseases: decompensated heart failure or refractory hypertension (hypertension that cannot be controlled to target levels of systolic and diastolic blood pressure despite lifestyle modifications and adequate doses of at least three antihypertensive drugs, including diuretics);
7. Patients with significant or laboratory-confirmed immunodeficiency syndromes;
8. Patients with severe acute or chronic infections;
9. Patients with pre-existing hematologic disorders, such as myelodysplasia, leukopenia, thrombocytopenia, or anemia;
10. Patients allergic to the study-related drug;
11. Patients with a history of drug abuse, mental illness, or alcoholism, who cannot cooperate with clinical researchers;
12. Other patients deemed unsuitable for participation in the study by the investigator.

The study includes adult RA patients treated with subcutaneous MTX.