Septic Joint Difficult to Exclude by Laboratory Tests

Abstract & Commentary

Source: Li SF, et al. Laboratory tests in adults with monoarticular arthritis: Can they rule out a septic joint? Acad Emerg Med 2004; 11:276-280.

The aim of this retrospective case series was to determine the sensitivities of three commonly obtained laboratory tests often used to determine the likelihood that a septic joint is the cause of an acute monoarticular arthritis. The three tests studied were the peripheral white blood cell count (WBC), the erythrocyte sedimentation rate (ESR), and the white blood cell count of the joint fluid (jWBC). The main goal was to determine whether a septic joint could be ruled out by any of these tests.

Adult patients were included in this study if the diagnosis of septic arthritis was confirmed by a positive arthrocentesis culture or operative findings. Demographic, clinical, and laboratory data were obtained from the computerized records of 73 patients. The sensitivities of elevations in WBC, ESR and jWBC were 48% (95% CI 36-60%), 96% (95% CI 88%-100%), and 64% (95% CI 51-76%), respectively. More than one third (36%) of the patients had jWBCs less than 50,000 cells/mm3, and 10% (6 of 61 pts) had counts of less than 10,000 cells/mm3. Staphylococcus aureus was the causative organism in 42 patients (58%), and 36% of patients were either HIV–positive or diabetic.

Commentary by Stephanie Abbuhl, MD, FACEP

It would be wonderful if a low jWBC count could rule out septic arthritis but, unfortunately, this is not the case. In this cohort of patients with septic arthritis, there was a wide range of values for each of the laboratory tests. While the mean jWBC was 127,000 cells/mm3, the range was 168 to 1 million cells/mm3, and 10% had counts of less than 10,000 cells/mm3. Even the ESR, which was found to have a sensitivity of 96%, was low (less than 40mm/hr) in 7% of patients and normal (less than 30 mm/hr) in 3% of patients.

It is clear that a high degree of suspicion for septic arthritis must be maintained in the approach to the patient with monoarticular arthritis, regardless of a low jWBC. In cases where suspicion is high enough, this will mean admission and treatment with antibiotics until culture results return. In cases where suspicion is low, the prudent approach will be very close follow-up for improvement without antibiotics until the culture returns. Finally, if there is only a single drop of fluid obtained from a joint tap, the most important test to obtain is a culture and gram stain, not a cell count.

This study is limited by small numbers with broad confidence intervals. However, even if a large study could push the sensitivities significantly up, the fact that some cases of septic arthritis occur with low jWBCs, ESRs, or WBCs limits our ability to use these tests to make absolute decisions about ruling out septic arthritis.

In addition, as a retrospective chart review of septic arthritis cases, specificities could not be determined, but common practice tells us that all three of these lab tests are nonspecific. Even jWBCs can be remarkably elevated in crystal disease and other inflammatory causes of monoarticular arthritis. This study did not look at the differential of the jWBC, and it would be interesting to know if this would add any sensitivity to the jWBC in ruling out septic arthritis. It also would be helpful to know if the low jWBCs were found more often in patients who were HIV-positive.

Dr. Abbuhl, Medical Director, Department of Emergency Medicine, The Hospital of the University of Pennsylvania; Associate Professor of Emergency Medicine, University of Pennsylvania School of Medicine, Philadelphia, is on the editorial board of Emergency Medicine Alert.