# Joint Aspiration for the Diagnosis of Suspected Native Septic Arthritis

## TL;DR

This study evaluates the accuracy of joint aspirations in diagnosing septic arthritis and identifies clinical factors that help in making a quick diagnosis.

## Contribution

The study provides evidence on the diagnostic accuracy of joint aspirations and identifies key clinical indicators for septic arthritis.

## Key findings

- 13% of joint aspirations resulted in a diagnosis of septic arthritis, with 9.9% culture-positive and 3.1% culture-negative.
- Gram stain, high synovial white cell count, fever, previous septic arthritis, and intravenous drug use were associated with septic arthritis.
- A history of crystal arthropathy reduced the likelihood of septic arthritis diagnosis.

## Abstract

Introduction: The diagnostic accuracy of these joint aspirations in suspected native septic arthritis (SA) is debated as it is unclear whether negative culture results should preclude the use of antibiotics or the need for urgent surgical intervention. This study aimed to determine the diagnostic accuracy of native joint aspirations for SA in adult patients, and to determine clinical factors associated with a positive SA diagnosis.

Methods: All adult patients (≥16 years) undergoing aspiration of a major native joint at our centre over a two-year period between September 2019 and August 2021 were identified through a retrospective search of electronic laboratory records. Data relating to patient demographics, clinical and biochemical data, joint aspiration details, post-aspiration diagnosis, management, and patient outcomes were collected and analysed. Culture-negative SA was defined using Newman’s criteria. Patients with joint prostheses or any existing orthopaedic metalwork in the affected joint were excluded.

Results: There were 161 native joint aspirations included in the study. The rate of positive diagnosis of SA was 21/161 = 13.0%, of which 16/161 = 9.9% were culture-positive SA and 5/161 = 3.1% were culture-negative SA. The presence of organisms on Gram stain analysis, a high synovial white cell count (WCC), being pyrexial, a previous diagnosis of SA, and intravenous drug use (IVDU) were associated with a diagnosis of SA (all p < 0.05). A history of crystal arthropathy made the diagnosis of SA less likely (Odds Ratio (OR) = 0.195, 95% Confidence Interval (CI) 0.044 - 0.874, p = 0.019). The diagnostic value of serum WCC and C-reactive protein (CRP) were not statistically different in the SA and non-SA groups (both p > 0.05).

Conclusion: Synovial Gram stain and culture is still the preferred diagnostic test when it comes to accurately diagnosing native joint SA. However, while culture results are pending, a high index of suspicion for SA can be based on a combination of patient’s clinical presentation, such as history of fever, previous SA, being IVDU, and a high synovial WCC. These diagnostic aids can help in guiding immediate and targeted antimicrobial therapy, and informing clinical decisions on the need for emergency surgical washout of the joint in the acute setting.