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Updated: Oct 12, 2025

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Gram stain microscopy in septic arthritis

Karam Al-Tawil, Frederick Quiney, Louis Pirkis

    Acta Orthopaedica Belgica
    |November 22, 2021
    PubMed

    Insights

    Gram stain is a rapid diagnostic test for septic arthritis but has low sensitivity (22%). Clinicians should rely more on clinical findings and biochemical markers due to potential false negatives in diagnosing joint infections.

    Area of Science:

    • Orthopedics
    • Infectious Diseases
    • Microbiology
    • Diagnostic Pathology

    Background:

    • Septic arthritis is a severe joint infection requiring prompt diagnosis and treatment to prevent rapid joint destruction.
    • Annual incidence is 10 per 100,000, rising to 70 per 100,000 in at-risk populations (e.g., rheumatoid arthritis, immunocompromised patients).
    • Gram stain is a common initial diagnostic test for suspected septic arthritis.

    Purpose of the Study:

    • To evaluate the clinical utility of Gram stain for diagnosing septic arthritis.
    • To determine the sensitivity and specificity of Gram stain in identifying septic arthritis.
    • To assess the impact of Gram stain results on clinical decision-making for joint infections.

    Main Methods:

    • Retrospective case series analysis of joint aspirates from May 2015 to April 2018 at a single center.
    • Inclusion of 830 samples from both native (701 cases) and prosthetic (129 cases) joints.
    • Comparison of Gram stain results with final culture results for all analyzed samples.

    Main Results:

    • A discrepancy between Gram stain and culture results was observed in 74 cases (9%).
    • The sensitivity of Gram stain for detecting septic arthritis was found to be 22%.
    • The specificity of Gram stain was high, recorded at 99.6%.

    Conclusions:

    • Gram stain demonstrates low sensitivity in diagnosing septic arthritis, indicating a high rate of false negatives.
    • Despite low sensitivity, Gram stain exhibits high specificity, meaning positive results are generally reliable.
    • Clinicians should maintain a low threshold for considering septic arthritis even with negative Gram stain results, emphasizing clinical presentation and biochemical markers.

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