Septic Arthritis in Children Ruled Out by New Test

Janis C. Kelly

February 19, 2013

Septic arthritis is unlikely in children from Lyme disease–endemic areas who have knee monoarthritis, and have an absolute neutrophil count (ANC) of 10 × 103 cells/mm3 or lower, and an erythrocyte sedimentation rate (ESR) of 40 mm/hour or lower, researchers report in an article published online February 18 in Pediatrics.

"Laboratory criteria can be used to identify children with knee monoarthritis at low risk for septic arthritis who may not require diagnostic arthrocentesis," Julia K. Deanehan, MD, from the Division of Emergency Medicine, Boston Children’s Hospital and Harvard Medical School, Boston, Massachusetts, and colleagues conclude.

This approach was developed using patients from areas with endemic Lyme disease and should not be generalized to patients from other areas, according to the authors and outside experts.

Septic Arthritis Rare

Overall, 673 patients with knee monoarthritis took part in the study, with 474 children included in the derivation population and 199 children in the validation population. Of those, 19 (3%) had septic arthritis, 341 (51%) had Lyme arthritis, and 313 (46%) had other inflammatory arthritis.

"We were surprised by the low incidence of knee septic arthritis," senior author Lise E. Nigrovic, MD, MPH, told Medscape Medical News. "At the 2 study institutions with 21 study years (between the 2 institutions), we identified only 19 kids with knee septic arthritis."

Clinicians can safely bypass arthrocentesis, parenteral antibiotics, and surgical irrigation for children in Lyme-endemic areas who meet the ANC and ESR criteria, said Dr. Nigrovic, an attending physician in the Division of Emergency Medicine and assistant professor of Pediatrics at Boston Children's Hospital.

"As you can see from the number of cases [in this study], Lyme vastly outweighs septic arthritis," said Karen Onel, MD, division chief for Pediatric Rheumatology and Associate Professor of Pediatrics at the University of Chicago, Illinois, who was not involved in the work. "It is unclear how the criteria would hold up if the relative frequency of disease were more equal; that is, where both were rare diseases. And in that situation, if a few children with Lyme arthritis underwent arthrocentesis, it would have less significance than if all the children with Lyme arthritis in an endemic area needed the procedure."

The researchers developed the clinical prediction rule using data from children who presented with knee monoarthritis to the emergency department of Boston Children's Hospital between January 1, 2000, and January 31, 2012. Inclusion criteria included physician-documented knee monoarthritis and either evaluation for Lyme disease or the presence of septic knee arthritis. Exclusion criteria included recent knee injury or knee surgery, previous arthritis, history of rheumatologic disease or immunocompromised state, knee cellulitis, or multiple joint involvement.

The investigators validated the prediction rule using data from children who presented with knee monoarthritis to the emergency department of Yale–New Haven Hospital, all of whom presented with joint effusion and had a knee aspiration performed. Both hospitals are located in Lyme disease–endemic areas.

The main outcome measure was septic arthritis, defined as synovial fluid culture positive for a bacterial pathogen or by synovial fluid pleocytosis with a positive blood culture for a bacterial pathogen.

Test Not Useful Everywhere

All of the 341 children with Lyme arthritis had positive serology for Lyme disease, and none of the 257 who had arthrocentesis had septic arthritis. Clinical and laboratory features were generally similar in septic arthritis and Lyme arthritis, but children with septic arthritis had higher ANC counts, higher ESR levels, and higher C-reactive protein levels.

"Of the 19 children with septic arthritis, all had a positive synovial fluid culture result and 2 patients also had positive blood culture results for the same bacterial pathogen that grew from the synovial fluid," the authors write. Causative organisms included methicillin-sensitive Staphylococcus aureus, group A streptococcus, Streptococcus pneumoniae, and Enterobacter agglomerans.

"Data collected by these authors demonstrate that the use of ANC and ESR can be useful in identifying children with high or low risk of septic arthritis, and consequently in deciding whether arthrocentesis is mandatory or can be avoided," said Susanna Esposito, MD, a pediatric infectious disease specialist at the University of Milan, Italy.

Similar to other experts interviewed, Dr. Esposito, who recently analyzed management of pediatric Lyme disease in endemic and nonendemic areas, using data from the Italian Society for Pediatric Infectious Diseases Lyme Disease Registry, emphasized that this prediction model should not be used everywhere. "This prediction model can be used only in geographic areas where Lyme disease is endemic and in limited resources settings, but not when it is an uncommon illness," she said.

Moreover, Dr. Onel warned that the exclusion criteria (such as previous knee surgery or cellulitis) may further limit applicability of the criteria.

The authors, Dr. Onel, and Dr. Esposito have disclosed no relevant financial relationships.

Pediatrics. Published online February 18, 2013. Abstract