How is reactive arthritis diagnosed?

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Reactive arthritis (also known as Reiter’s syndrome) is a type of inflammatory arthritis that typically develops after an infection, often affecting the joints, eyes, and urinary system. It is part of a group of conditions known as spondyloarthropathies, which affect the spine and other joints. The condition is characterized by the inflammation of the joints, often triggered by an infection in another part of the body, such as the urinary tract, gastrointestinal tract, or respiratory system.

Diagnostic Process for Reactive Arthritis:

Diagnosing reactive arthritis involves a combination of medical history, symptom assessment, physical examination, and specific tests to rule out other causes and confirm the diagnosis. Since there is no single test that definitively diagnoses reactive arthritis, doctors usually rely on a combination of methods.

1. Medical History:

Recent infections: The physician will first inquire about any recent infections, particularly in the urinary tract, gastrointestinal tract, or respiratory system. Common infections that may trigger reactive arthritis include:
- Chlamydia trachomatis (urinary or genital infection)
- Salmonella, Shigella, Campylobacter, or Yersinia (gastrointestinal infections)
- Mycoplasma pneumoniae (respiratory infection)
Family history: The doctor may also ask about any family history of conditions like psoriasis, ankylosing spondylitis, or other spondyloarthropathies, as these may be related to genetic susceptibility.

2. Physical Examination:

The doctor will assess for the classic symptoms of reactive arthritis, which typically include:
- Arthritis: Swelling, pain, and stiffness, particularly in the knees, ankles, and feet. The inflammation is often asymmetric, meaning it typically affects one side of the body more than the other.
- Enthesitis: Inflammation at the points where tendons and ligaments attach to bones, such as the Achilles tendon or the plantar fascia.
- Dactylitis: Swelling of an entire finger or toe, often referred to as “sausage digits.”
- Urethritis: Pain or discomfort during urination, suggesting a urinary tract infection.
- Conjunctivitis or uveitis: Inflammation of the eyes, leading to redness, pain, and blurred vision.

3. Laboratory Tests:

While there are no definitive blood tests for reactive arthritis, several lab tests can help rule out other conditions and support the diagnosis:

HLA-B27 test: The HLA-B27 gene is present in about 50–80% of people with reactive arthritis, though not all individuals with the gene will develop the condition. Testing for HLA-B27 is often performed to help identify people at higher risk for spondyloarthropathies, including reactive arthritis.
Blood tests: These can help confirm the presence of inflammation and rule out other conditions:
- Erythrocyte sedimentation rate (ESR): Elevated ESR indicates inflammation in the body.
- C-reactive protein (CRP): This is another marker of inflammation.
- Complete blood count (CBC): This may show mild anemia or elevated white blood cells, which may be signs of an ongoing infection or inflammation.
- Rheumatoid factor (RF) and anti-citrullinated protein antibodies (ACPA): These tests are used to rule out rheumatoid arthritis.
Urine tests: If a urinary infection is suspected as the trigger, urine tests may identify bacteria or other signs of infection, particularly Chlamydia.

4. Microbiological Tests:

Since reactive arthritis often follows an infection, identifying the triggering infection is important for diagnosis:

Stool culture: For gastrointestinal infections, stool samples may be tested for pathogens like Salmonella, Shigella, Campylobacter, or Yersinia.
Urine culture: In cases where Chlamydia or other urinary tract infections are suspected, a urine culture may be used to identify the pathogen.
Throat culture: In rare cases, throat cultures may be used if a respiratory infection is suspected.

5. Imaging Studies:

X-rays: While there are no specific X-ray findings for reactive arthritis, imaging may be used to detect joint damage or inflammation, particularly in the spine, hips, and sacroiliac joints. The presence of sacroiliitis (inflammation of the sacroiliac joints) may suggest a spondyloarthropathy like reactive arthritis.
Ultrasound: In some cases, ultrasound can detect joint inflammation or fluid accumulation in the joints or tendons.

6. Differential Diagnosis:

Since reactive arthritis shares symptoms with other conditions, such as rheumatoid arthritis, psoriatic arthritis, and gout, it’s essential to rule out other potential causes of symptoms. Key differences include:

Rheumatoid arthritis: Usually affects the same joints on both sides of the body (symmetrical), whereas reactive arthritis is often asymmetrical and affects large joints like the knees and ankles.
Psoriatic arthritis: Typically associated with psoriasis and can involve the skin and nails.
Gout: Gout is usually characterized by sudden, intense joint pain, often affecting the big toe, and is linked to high uric acid levels.

Diagnostic Criteria:

While there is no single test to definitively diagnose reactive arthritis, the diagnosis is generally based on the following:

Recent infection (usually gastrointestinal, urinary, or respiratory)
Symptoms of arthritis (joint pain, swelling, and stiffness)
Positive HLA-B27 test (though not definitive)
Absence of other causes of arthritis (such as rheumatoid arthritis or gout)
Other signs: Such as conjunctivitis, urethritis, or enthesitis, which are common features of reactive arthritis.

Conclusion:

Diagnosing reactive arthritis involves a comprehensive approach that combines a detailed medical history (especially recent infections), physical examination, laboratory tests (including HLA-B27 and markers of inflammation), and microbiological tests to identify the underlying infection. Imaging studies may be used to assess joint damage or inflammation. Since many conditions can mimic reactive arthritis, it is essential to rule out other causes of joint pain and inflammation to arrive at an accurate diagnosis.

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