Tracheal aspirate Gram stain has limited sensitivity and specificity for detecting Staphylococcus aureus

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Sodienye Tetenta1, Mark L Metersky

  • 1Mountain Falls Pulmonary Clinic, Cheyenne Regional Medical Center, Cheyenne, Wyoming Division of Pulmonary and Critical Care Medicine, University of Connecticut School of Medicine, Farmington, Connecticut, USA.

Respirology (Carlton, Vic.)
|October 6, 2010
PubMed
Abstract

Insights

Gram stains of tracheal aspirates do not reliably predict Staphylococcus aureus (S. aureus) growth. This diagnostic method showed low sensitivity and specificity, impacting treatment decisions for infections caused by this pathogen.

Area of Science:

  • Microbiology
  • Infectious Diseases
  • Clinical Diagnostics

Background:

  • Rising incidence of methicillin-resistant Staphylococcus aureus (MRSA) infections necessitates effective diagnostic tools.
  • Empirical antibiotic use is increasing for respiratory infections potentially caused by S. aureus.
  • Limited data exist on the accuracy of Gram stain for predicting S. aureus in respiratory secretions.

Purpose of the Study:

  • To evaluate the accuracy of Gram stain in predicting the growth of Staphylococcus aureus (S. aureus) in tracheal aspirates.
  • To determine the sensitivity, specificity, and predictive values of Gram stain for S. aureus detection.
  • To assess the clinical utility of Gram stain in guiding S. aureus-targeted therapy.

Main Methods:

  • Retrospective review of 136 tracheal aspirate Gram stains and corresponding culture results.
  • Microbiologists blinded to culture results assessed Gram stains for S. aureus morphology.
  • Correlation of Gram stain findings with semiquantitative culture results.

Main Results:

  • Staphylococcus aureus (S. aureus) was cultured from 37% of tracheal aspirates (50/136).
  • Gram stain demonstrated 68% sensitivity and 72% specificity for S. aureus detection.
  • Positive and negative predictive values were 59% and 80%, respectively.

Conclusions:

  • Tracheal aspirate Gram stain, even when interpreted by experienced clinicians, is not sufficiently accurate to reliably predict S. aureus growth.
  • The diagnostic performance is limited, particularly for low-yield cultures.
  • Relying solely on Gram stain may lead to suboptimal treatment decisions for S. aureus respiratory infections.

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