Cultures and ventilator-associated pneumonia: not how, but how many

Evaluation of semi-quantitative scoring of Gram staining or semi-quantitative culture for the diagnosis of ventilator-associated pneumonia: a retrospective comparison with quantitative culture

Background

Semi-quantitative Gram stain and culture methods are still commonly used for diagnosing ventilator-associated pneumonia (VAP), due to its convenience. Only a few studies, however, have assessed the reliability of these methods when compared with quantitative cultures, a current standard for the diagnosis of VAP. The objective of this study was to assess the utility of semi-quantitative scores obtained using Gram stains and cultures of endotracheal aspirates when compared with quantitative cultures in the diagnosis of VAP.

Methods

A retrospective chart review of mechanically ventilated patients with clinically suspected VAP in a single intensive care unit was performed. Semi-quantitative scores of Gram stains or culture results were compared with quantitative culture results of endotracheal aspirate for the diagnosis of VAP in 136 samples for 51 patients.

Results

The semi-quantitative scores of Gram stains and the semi-quantitative culture results significantly correlated with the log value of the quantitative culture results (r_s = 0.64 and 0.75). When using a log count ≥6 of quantitative cultures as the reference standard for the diagnosis of VAP, the sensitivity and specificity was 95% and 61% for Gram stain score of ≥1+, and was 42% and 96% for Gram stain score ≥3+, respectively. The sensitivity and specificity was 96% and 40% for the semi-quantitative culture score of ≥2+, and was 59% and 86% for the semi-quantitative culture score of ≥3+, respectively.

Conclusions

Absence of bacteria in semi-quantitative Gram stain and poor growth (≤1+) in semi-quantitative culture method could be utilized to exclude the possibility of VAP, whereas detection of abundant (≥3+) bacteria in semi-quantitative Gram stain could be utilized to strongly suspect VAP.

Ventilator-associated pneumonia: a review

Ventilator-associated pneumonia (VAP) is the most frequent intensive-care-unit (ICU)-acquired infection, with an incidence ranging from 6 to 52% [1,2,3,4]. Several studies have shown that critically ill patients are at high risk for getting such nosocomial infections [3,4]. VAP continues to be a major cause of morbidity, mortality and increased financial burden in ICUs [5,6,7,8]. Over the years there has been a significant advance in our understanding of ventilator associated pneumonia. This article reviews the various aspects of VAP such as definition, risk factors, etiological agents, diagnosis, treatment and prevention with emphasis on the recent advances.

Community-acquired bacteria frequently detected by means of quantitative polymerase chain reaction in nosocomial early-onset ventilator-associated pneumonia*

OBJECTIVE

To test whether real-time polymerase chain reaction allows for rapid quantitative detection of Streptococcus pneumoniae, Chlamydia pneumoniae, Mycoplasma pneumoniae, and Legionella pneumophila in bronchoalveolar lavage fluids and to determine the prevalence of these pathogens in nosocomial ventilator-associated pneumonia.

DESIGN

Prospective epidemiologic study applying a new molecular biology-based diagnostic tool during a 27-month period.

SETTING

Three medical intensive care units of a tertiary care university hospital.

PATIENTS

One hundred patients suffering from nosocomial ventilator-associated pneumonia, hospitalized for > or =14 days, intubated for reasons other than pneumonia, and mechanically ventilated for >48 hrs.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

S. pneumoniae, M. pneumoniae, and C. pneumoniae were detected in bronchoalveolar lavage fluids of 100 patients in 20 (20%), three (3%), and two (2%) cases, respectively. There of 17 (71%) revealed no growth or no significant growth by conventional culture. In one patient, S. pneumoniae and M. pneumoniae were detected simultaneously. Corresponding colony-forming units/mL were partly up to 10 CFU/mL with Gram stainings showing signs of acute inflammation in 80%. A significant temporary correlation between the number of days on ventilator, development of nosocomial pneumonia, and the frequency of detection of these pathogens was found for day 4.

CONCLUSIONS

S. pneumoniae, M. pneumoniae, and C. pneumoniae should be considered as causative agents in critically ill patients who develop early-onset nosocomial ventilator-associated pneumonia. Thus, empirical antimicrobial regimens should cover S. pneumoniae, Chlamydia, and Mycoplasma alike. Quantitative polymerase chain reaction is a fast diagnostic tool allowing for detection of these bacteria within 3 hrs in pretreated patients.

[Difficulties on diagnosis of ventilator associated pneumonia]

Ventilator associated pneumonia is associated with high morbidity and mortality. It is important a correct diagnosis in way to guide the antibiotic therapy in the most appropriate way. However, its diagnosis is difficult, because clinical and radiologic features are not specific and approaches to standard diagnosis, that allow its confirmation, are very invasive or not very frequent. Protected techniques and quantitative cultures have been trying to outline the problem of the contamination of the samples obtained by routine methods and to allow the distinction between colonization and infection. The author makes a revision on the different methods of diagnosis of this clinical entity.