Transient synovitis is a temporary and harmless inflammation of the hip joint, while septic arthritis is a serious infection of a joint that requires immediate treatment.

Transient synovitis is an inflammation of the hip joint and is a common cause of hip pain in children. Transient synovitis is not harmful, and the condition will resolve by itself.

Transient synovitis can appear similar to other conditions, including septic arthritis, which can be serious without treatment. Septic arthritis is an infection of a joint and requires immediate treatment to prevent joint damage.

This article looks at the differences between transient synovitis and septic arthritis.

A woman with transient synovitis or septic arthritis striking a pose. -1Share on Pinterest
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The following table shows the similarities and differences between transient synovitis and septic arthritis.

Transient synovitisSeptic arthritis
Transient synovitis is inflammation of the hip joint, affecting the synovium. The synovium is the soft tissue lining of the joint surface. Septic arthritis is severe inflammation of a joint, affecting the synovium and joint space, which can lead to joint destruction.
It affects the hip joint.It usually affects one larger joint, such as the hip or knee, but may affect smaller or multiple joints. In children, septic arthritis most commonly affects the hip joint.
It is the most common cause of hip pain in children, usually occurring in children aged 3–10 years. In rare cases, it occurs in infants or adults.Septic arthritis is uncommon, but is more common in children than adults. It occurs most frequently in children aged 2–3 years. Being over the age of 80 is also a risk factor.
There is no known cause, but risk factors may include previous trauma, bacterial infection, or upper respiratory infection.Septic arthritis usually occurs from a bacterial infection. In some cases, a viral or fungal infection may cause septic arthritis.
Symptoms include hip pain, a limp, or refusal to bear weight. Symptoms include sudden and severe joint pain, refusal to move the affected joint, swelling, and fever.
Pain will resolve within 24–48 hours, and all symptoms will resolve within 1–2 weeks.Septic arthritis requires emergency treatment to prevent long-term complications.
Treatment is not necessary, but rest and nonsteroidal anti-inflammatory drugs (NSAIDs) can help in pain relief and recovery.Treatment includes antibiotics and fluid drainage of the affected joint.
It is low risk and goes away by itself. The main complication is possible recurrence. Transient synovitis may recur in up to 25% of cases.Early diagnosis and treatment are essential to protect joint function and prevent serious complications, such as sepsis.

According to a 2022 study, there are currently five criteria that may be effective in differentiating septic arthritis of the hip and transient synovitis.

The following may indicate septic arthritis:

  • fever above 101.3ºF (38.5ºC)
  • inability to bear weight
  • serum white blood cell count higher than 12,000 per cubic millimeter (mm3)
  • erythrocyte sedimentation rate (ESR) of 40 millimeters per hour (mm/h) or above, which doctors measure with a blood test to show inflammation
  • C-reactive protein levels above 2 milligrams per deciliter (mg/dL)

Doctors use these tests to rule out septic arthritis and other potential conditions to diagnose transient synovitis.

In addition, doctors may use MRI scans, ultrasound, and removal of joint fluid for testing to diagnose septic arthritis or transient synovitis.

Transient synovitis and septic arthritis can share similar symptoms, but certain symptoms may indicate one condition over the other.

Septic arthritis

Symptoms of septic arthritis may include:

Transient synovitis

Symptoms of transient synovitis may include:

  • hip pain
  • refusal to bear weight
  • reduced range of motion
  • in children, agitation or crying more than usual

A recent case of upper respiratory tract infection or injury may also indicate transient synovitis.

People will need to contact a doctor if they have symptoms of either septic arthritis or transient synovitis.

Although transient synovitis is harmless and does not require treatment, symptoms overlap with septic arthritis and a doctor will need to carry out tests to rule out similar, more serious conditions.

This section answers some frequently asked questions about transient synovitis and septic arthritis.

What is the CRP criteria for septic arthritis?

C-reactive protein (CRP) is a protein the liver makes, and levels increase if there is inflammation in the body.

Doctors measure CRP levels through a blood test to check for septic arthritis. A CRP level of more than 2 mg/dL is a strong risk factor for septic arthritis.

Researchers have found that elevated CRP levels are the strongest predictor of septic arthritis of the hip. Elevated CRP levels, in combination with an inability to bear weight, are strong predictors for septic arthritis of the knee.

What are the Kocher criteria for transient synovitis?

According to a 2022 study, the Kocher criteria identify four factors that differentiate septic arthritis of the hip from transient synovitis:

  • fever above 101.3ºF (38.5ºC)
  • inability to bear weight
  • serum white blood cell count above 12,000/mm3
  • ESR of 40 mm/h or above

Kocher created the criteria in 1999, and the initial study found these four factors were 99.6% predictive of septic arthritis of the hip. Later research has not found the criteria as effective.

In 2006, researchers added a fifth criterion, with a level of CRP greater than 2 mg/dL being a strong risk factor for septic arthritis of the hip. The research found that all five factors could predict septic arthritis in 98% of cases.

The 2022 study found that the updated Kocher criteria effectively diagnose septic arthritis of the hip, with elevated CRP levels being the most significant risk factor.

Transient synovitis is an inflammation of the hip joint, while septic arthritis is a bacterial infection causing inflammation of a joint, such as the hip or knee.

A doctor must carry out tests to differentiate transient synovitis from septic arthritis.

Transient synovitis is harmless and goes away on its own, while septic arthritis requires immediate treatment to prevent long-term complications.