Laboratory tests in adults with monoarticular arthritis: can they rule out a septic joint?

It is difficult to differentiate septic arthritis from other causes of monoarticular arthritis solely with a history and physical examination. The clinician must rely on ancillary tests to make a diagnosis, such as the white blood cell count of peripheral blood (WBC), the erythrocyte sedimentation rate (ESR), and the white blood cell count of the joint fluid (jWBC) obtained from arthrocentesis. Although it is known that septic arthritis is associated with abnormalities in these tests, the majority of the data are based on studies in the pediatric population. In addition, although several emergency medicine texts indicate that a jWBC greater than 50,000 cells/mm(3) is "positive," it is known that septic arthritis can occur in patients with low jWBCs.

To determine whether specific ancillary tests have sufficiently high sensitivities to rule out septic arthritis in adults.

This was a retrospective consecutive case series of patients from an urban emergency department (ED). Patients at least 18 years old who had septic arthritis confirmed by positive arthrocentesis culture or operative findings were included in the study. WBC greater than 11,000 cells/mm(3), ESR greater than 30 mm/hr, and jWBC greater than 50,000 cells/mm(3) were considered elevated.

Seventy-three patients met the inclusion criteria. The sensitivities of an elevated WBC, ESR, or jWBC in adults who had septic arthritis were 48%, 96%, and 64%, respectively. There were broad ranges of WBC, ESR, or jWBC among the patients. More than one third of adult patients with septic arthritis had jWBCs less than 50,000 cells/mm(3). All patients had an abnormality in at least one of these tests.

The WBC, ESR, and jWBC are extremely variable in adults with septic arthritis. Laboratory tests do not rule out septic arthritis with accuracy.