Lyme Arthritis vs. Septic Arthritis in Children: Can MRI Help?

Lyme arthritis and septic arthritis can look remarkably similar
Symptoms, blood tests, and joint fluid findings may overlap
MRI and combinations of clinical findings can sometimes help distinguish them

How can doctors tell Lyme arthritis from septic arthritis in a child with a swollen knee? The distinction is not always easy. Both conditions can cause substantial joint swelling, difficulty bearing weight, fever, elevated inflammatory markers, and a high white blood cell count in the joint fluid.

The distinction is important because the treatments can be very different. Bacterial septic arthritis is a medical emergency that can rapidly damage a joint and may require urgent surgical drainage and antibiotics. Lyme arthritis generally does not cause the same rapid cartilage destruction and is usually treated with antibiotics without surgery.

A 2023 current concept review examined the evolving evidence clinicians use to distinguish pediatric Lyme arthritis from septic arthritis, including the history and physical examination, inflammatory markers, Lyme disease testing, joint fluid findings, prediction models, and MRI.¹

The review reinforces an important point: there is considerable overlap between the two conditions, and no single symptom or laboratory result can reliably distinguish every case.

Why Lyme Arthritis and Septic Arthritis Can Be Difficult to Distinguish

Lyme arthritis commonly affects a large joint, particularly the knee. Children may develop a substantial joint effusion even when pain and systemic illness are relatively limited.

Septic arthritis may also cause an acutely swollen knee, but children are often more ill and may have more severe pain, fever, restricted movement, and inability to bear weight.

However, these patterns are not absolute. Children with Lyme arthritis may also have fever, difficulty bearing weight, elevated inflammatory markers, and impressive inflammatory findings in the joint fluid.

Features clinicians may consider include:

• Severity of joint pain
• Ability to bear weight
• History of fever
• Range of motion
• Age of the child
• Geographic exposure to Lyme disease
• C-reactive protein (CRP) and other inflammatory markers
• Lyme disease serology
• Synovial fluid analysis and bacterial culture
• Imaging findings when appropriate

The complete clinical picture is more useful than any single finding.

Can a High Joint Fluid White Blood Cell Count Distinguish the Two?

Not reliably. A high synovial fluid white blood cell count can occur with both Lyme arthritis and bacterial septic arthritis.

This overlap is clinically important because a markedly inflammatory joint aspirate may raise immediate concern for septic arthritis. However, Lyme arthritis can also produce synovial white blood cell counts that would traditionally be considered concerning for bacterial infection.^1,2

Joint aspiration remains important when septic arthritis is suspected because Gram stain and bacterial culture can help identify a bacterial joint infection. But the white blood cell count alone should not be assumed to establish the diagnosis.

What Clinical Findings May Point Toward Septic Arthritis?

Several studies have attempted to identify combinations of findings that increase the likelihood of septic arthritis.

The current concept review discusses a prediction model developed by Baldwin and colleagues for children with knee monoarthritis.¹ Four factors were associated with septic arthritis:

• Pain with a short arc of motion
• History of fever
• CRP greater than 4 mg/L
• Age younger than 2 years

The probability of septic arthritis increased as more of these findings were present. However, prediction models should not replace clinical judgment, particularly because the consequences of missing bacterial septic arthritis can be serious.

Other studies have also found that fever, inability to bear weight, elevated inflammatory markers, and severe limitation of movement may increase concern for septic arthritis. Considerable overlap with Lyme arthritis remains.

Can Lyme Disease Testing Help?

Lyme disease serology is an important part of the evaluation when a child with arthritis has lived in or visited an area where Lyme disease occurs.

Unlike early Lyme disease, when antibody testing may still be negative, patients with Lyme arthritis typically develop a strong antibody response by the time arthritis appears.

The 2023 review discusses research combining Lyme serology with inflammatory markers and other clinical findings to help separate Lyme arthritis from bacterial musculoskeletal infections.¹

A positive Lyme antibody test can substantially change the diagnostic picture, but results still need to be interpreted in the context of the child’s presentation. Meanwhile, concern for septic arthritis may require clinicians to act before all Lyme testing has been completed.

Can MRI Distinguish Lyme Arthritis From Septic Arthritis?

MRI cannot directly diagnose Lyme disease or independently rule out septic arthritis. However, certain MRI patterns may provide additional clues when combined with the history, examination, laboratory results, and Lyme testing.

A study by Yen and colleagues examined 96 children and adolescents presenting with acute knee effusions: 87 had confirmed Lyme arthritis and 9 had septic arthritis.²

The groups had substantial overlap in fever, ability to bear weight, and synovial fluid white blood cell counts.

Several MRI characteristics differed between the groups. Findings that favored Lyme arthritis included:

• Myositis involving multiple muscles, including the vastus lateralis
• Lymphadenopathy near the affected joint
• Absence of subcutaneous edema

In multivariable analysis, the combination of ability to bear weight, CRP below 3 mg/L, absence of subcutaneous edema, myositis involving multiple muscles including the vastus lateralis, and lymphadenopathy predicted Lyme arthritis.²

Could MRI Help Some Children Avoid Unnecessary Surgery?

This is one of the most clinically important reasons for distinguishing Lyme arthritis from septic arthritis.

In the Yen study, 14 of the 87 children with Lyme arthritis—approximately 16%—were initially presumed to have septic arthritis and underwent operative irrigation and debridement.²

The investigators suggested that applying their proposed clinical, laboratory, and MRI criteria might have allowed those children to avoid surgery.

That finding should be interpreted cautiously. The investigation was retrospective, came from a single center, and included only nine patients with septic arthritis. The proposed model therefore requires additional validation before it can be treated as a definitive diagnostic pathway.

Why Avoiding Unnecessary Surgery Matters

When bacterial septic arthritis is reasonably suspected, rapid treatment—including surgery when indicated—is appropriate because delaying treatment can damage the joint.

But surgery is not benign, and children with Lyme arthritis generally do not require operative drainage simply to treat the Lyme infection.

The challenge is therefore not simply avoiding surgery. It is avoiding unnecessary surgery without delaying appropriate treatment in a child who truly has septic arthritis.

This is why research has increasingly focused on combinations of clinical findings, inflammatory markers, Lyme testing, joint fluid results, and imaging rather than relying on one cutoff value.

Septic Arthritis Must Still Be Considered an Emergency

The possibility of Lyme arthritis should never be used to dismiss signs of a potentially serious bacterial joint infection.

A child with acute joint swelling, fever, severe pain, inability to bear weight, markedly restricted movement, or systemic illness requires prompt medical evaluation.

Evaluation may include physical examination, blood tests, Lyme disease serology, blood cultures, arthrocentesis with synovial fluid analysis and bacterial culture, and imaging when appropriate.

If clinicians cannot safely exclude septic arthritis, urgent treatment may be necessary while diagnostic testing is still underway.

When Should Lyme Arthritis Be Considered?

Lyme arthritis should be considered when a child develops otherwise unexplained swelling of a large joint—especially the knee—and has lived in or traveled to an area where Lyme disease occurs.

A remembered tick bite or erythema migrans rash is not required. Arthritis may appear weeks or months after the original infection, when the tick exposure is no longer obvious to the family.

Children with Lyme arthritis may sometimes appear surprisingly well despite a markedly swollen knee. Nevertheless, the presentation varies, and Lyme arthritis cannot be diagnosed simply because pain or fever is mild.

For a broader discussion of childhood presentations, see Pediatric Lyme Disease.

Frequently Asked Questions

Can Lyme arthritis look like septic arthritis?

Yes. Both conditions can cause substantial joint swelling, fever, difficulty bearing weight, elevated inflammatory markers, limited movement, and a high synovial fluid white blood cell count. This overlap can make the initial diagnosis difficult.

How can doctors distinguish Lyme arthritis from septic arthritis?

Doctors consider the child’s symptoms, physical examination, ability to bear weight, fever, inflammatory markers, geographic exposure, Lyme disease serology, joint fluid findings, bacterial cultures, and sometimes MRI. No single finding reliably distinguishes every case.

Does a high joint fluid white blood cell count mean septic arthritis?

No. A high synovial fluid white blood cell count raises concern for infection but can occur in both bacterial septic arthritis and Lyme arthritis. The result must be interpreted with the child’s other clinical and laboratory findings.

Does Lyme arthritis show up on MRI?

MRI does not directly diagnose Lyme disease. However, findings such as lymphadenopathy, myositis involving multiple muscles, and absence of subcutaneous edema may support Lyme arthritis when combined with clinical findings and laboratory testing.

Does Lyme arthritis require surgery?

Lyme arthritis generally does not require surgical drainage and is usually treated with antibiotics. Surgery may be necessary when clinicians cannot safely exclude bacterial septic arthritis or when another indication for surgery exists.

Is septic arthritis an emergency?

Yes. Bacterial septic arthritis can rapidly damage joint cartilage and requires urgent evaluation and treatment. A child with an acutely swollen or painful joint, fever, inability to walk, or systemic illness should receive prompt medical care.

Clinical Takeaway

Lyme arthritis and septic arthritis can be difficult to distinguish in children because symptoms, inflammatory markers, and even synovial fluid findings may overlap considerably.

Recent reviews emphasize combining the history and physical examination with inflammatory markers, Lyme disease testing, joint fluid analysis, bacterial cultures, and imaging when appropriate rather than relying on any single finding.

MRI does not diagnose Lyme disease or independently exclude septic arthritis, but selected MRI findings may improve diagnostic accuracy. In one pediatric knee study, the addition of MRI findings to clinical and laboratory information helped identify features associated with Lyme arthritis.²

The goal is not simply to avoid surgery. It is to recognize Lyme arthritis accurately enough to reduce unnecessary invasive procedures while never delaying urgent treatment for a child with true bacterial septic arthritis.

Related Articles

Preventing Unnecessary Surgery for Children With Lyme Arthritis

Diagnosing Lyme Arthritis of the Hip in Children

High Cost of Treating Lyme Arthritis in Children With Surgery

References

1. Ortiz CD, Barsi J. Differentiating Between Septic Arthritis and Lyme Arthritis in the Pediatric Population: Current Concept Review. Journal of the Pediatric Orthopaedic Society of North America. 2023;5(3). doi:10.55275/JPOSNA-2023-706.
2. Yen YM, Sanborn RM, Donohue K, Miller PE, Milewski MD, Ecklund K. Lyme Arthritis in the Pediatric Knee: Clinical and Magnetic Resonance Imaging Differentiators. JB JS Open Access. 2022;7(4):e22.00067. doi:10.2106/JBJS.OA.22.00067.

This article is for educational purposes and is not a substitute for medical evaluation, diagnosis, or treatment. A child with an acutely swollen joint, severe pain, fever, inability to bear weight, or systemic illness should receive prompt medical evaluation.

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Dr. Daniel Cameron, MD, MPH
Lyme disease clinician with over 30 years of experience and past president of ILADS.

Symptoms • Testing • Coinfections • Recovery • Pediatric • Prevention