Low White Blood Cell Count Distinguishes Lyme Arthritis From Septic Arthritis

Crina Frincu-Mallos, PhD

November 13, 2009

November 13, 2009 (Washington, DC) — The odds that a child living in a Lyme-endemic area of the United States who presents with a joint effusion will be diagnosed as having Lyme arthritis is 29%. The odds are even higher (44%) if the affected joint is the knee. The leukocyte count is useful in distinguishing between septic and Lyme arthritis, researchers announced here.

“There was an increase in the number of cases in the United States by 101% over the past 15 years, possibly due to increased recognition of Lyme disease,” said Aristides I. Cruz Jr., MD, resident in the Department of Orthopedics and Rehabilitation at Yale University in New Haven, Connecticut. During his presentation, he noted that 93% of all Lyme disease cases arise from 10 states, most in the Northeast United States.

“Children are more likely to present with arthritis as initial manifestation of Lyme disease,” Dr. Cruz told the audience.

“Children with Lyme arthritis presenting with a limp and a swollen joint will typically have a lower peripheral white blood cell count,” Dr. Cruz added. "They are less likely to have complete non–weight-bearing on the affected limb, less likely to have a fever, and [arthritis symptoms are] more likely to involve the knee joint compared with children with septic arthritis.”

The findings were reported here at the American Academy of Pediatrics 2009 National Conference & Exhibition.

Basic Diagnostic Tools Help Distinguish Lyme From Septic Arthritis

“In the Northeast, we see a lot of Lyme disease,” said Yi-Meng Yen, MD, PhD, instructor in orthopaedic surgery, Harvard Medical School, Children’s Hospital Boston in Massachusetts. “It is hard to distinguish whether [it] is Lyme disease or whether [it] is septic arthritis,” Dr. Yen agreed.

“Septic arthritis mandates that we take the patient to the operating room and do a surgery, whereas Lyme arthritis theoretically can be treated with antibiotics,” he told Medscape Pediatrics in an interview. For instance, he said, “Our institution has been looking at MRIs [magnetic resonance images] as a way to reliably distinguish between the two, because it takes several days sometimes for the lab tests to come back to definitely tell you whether you have Lyme disease or not. So, in those few days, if you have septic arthritis, that’s a bad thing.”

“To reliably, quickly diagnose what the patient has can help us determine the treatment quickly,” added Dr. Yen, who was not involved in this study.

“If you are clinically susceptible for septic arthritis, it pays to go to the operating room,” Dr. Cruz said in answer to a question from the audience. “In the past, almost all these patients automatically went to the operating room.”

However, if the clinical presentation is consistent with Lyme arthritis, treatment with antibiotics should suffice, he added. “The point of this study was to come up with some clinically useful criteria to arm ourselves with more tools to diagnose the disease.”

Dr. Cruz and his team sought to evaluate clinical parameters that could eventually be used to differentiate Lyme arthritis from septic arthritis in children and help with diagnosis and subsequent treatment.

In this retrospective analysis, the investigators reviewed data from children who underwent lower-extremity joint aspiration at Yale University Medical Center, a tertiary care children’s hospital in a Lyme disease endemic area.

Between August 2002 and August 2008, more than 200 children underwent a total of 212 aspirations for a joint effusion. Cell count, culture, hematologic inflammatory markers, and subsequent surgical intervention were available for 170 of the 212 aspirates.

Dr. Cruz's team compared findings from 50 children with serologically confirmed Lyme disease with data from 21 patients with culture-positive septic arthritis.

They found statistically significant differences between the 2 cohorts. For instance, the peripheral white blood cell count was 9.5 x 1000/μL (range, 3.0 – 14.9 x 1000/μL) in the aspirates from children with Lyme disease vs 12.5 (range, 5.5 – 30.1) in children with septic arthritis (P = .002).

Other parameters, such as joint fluid cell count, erythrocyte sedimentation rate, and C-reactive protein levels, were not significantly different between the 2 groups and could not be used to differentiate between septic and Lyme arthritis.

Interestingly, said Dr. Cruz, of all the children presenting with a joint effusion at their hospital, 29% were likely to be diagnosed as having Lyme arthritis overall compared with 44% if the aspirate was a knee aspirate.

“Is it worthwhile to develop something that’s very reliable? Absolutely!” said Dr. Yen. “Especially in the Northeast centers. It is a growing healthcare problem and a lot more study should be put into it.”

Dr. Cruz and Dr. Yen have disclosed no relevant financial relationships.

American Academy of Pediatrics (AAP) 2009 National Conference & Exhibition (NCE): Abstract 5806. Presented October 17, 2009.

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