BacteriologyEvaluation of a rapid bacterial ATP assay for screening BAL samples from ICU patients submitted for quantitative bacterial cultures
Introduction
Bacterial pneumonia commonly complicates the course of patients treated in the intensive care unit (ICU), and has been associated with increased length of stay, cost, and mortality Canadian Critical Care Trials Group et al., 1998, Fagon et al., 1989, Fagon et al., 1996, Vincent et al., 1995. The diagnosis of ventilator-acquired pneumonia (VAP) in critically ill patients is difficult, because of the high rates of non-specific clinical signs, non-infectious pulmonary infiltrates, and colonization of the upper respiratory tract with potentially pathogenic bacteria in this population Chastre et al., 1998, Croce, 2000, George et al., 1998, Meduri et al., 1994. Bronchoscopy with quantitative culture of bronchoalveolar lavage (BAL) fluid has emerged as the diagnostic standard for VAP in many centers Pittet and Bonten, 2000, Rello et al., 2001. Although there are problems with defining a gold standard, bacterial cultures of BAL fluid at ≥104 colony forming units (cfu) per mL are sensitive and specific for the diagnosis of infection of the lower respiratory tract when obtained in a compatible clinical setting Balthazar et al., 2001, Butler et al., 1999, Light, 1999, Torres and el-Ebiary, 2000. Management strategies using bronchoalveolar lavage (BAL) as compared to endotracheal aspirate culture directed therapy have been associated with both reduced antibiotic usage and mortality Fagon et al., 2000, Heyland et al., 1999.
Quantitative culturing techniques are hampered by the delay in processing and reporting of results. This delay and the lack of sensitivity of microscopic examination leads to many patients suspected of having VAP being treated with broad-spectrum antibiotics until cultures are reported negative Meduri et al., 1998, Papazian et al., 1997, Sole-Violan et al., 1994. A rapid assay to detect significant bacterial pathogens in BAL specimens would be valuable in managing critically ill patients suspected of having VAP and would potentially reduce antibiotic use in the ICU. Endotoxin assays on BAL fluid have been proposed for this purpose but they are expensive and do not detect the presence of Gram-positive organisms Flanagan et al., 2000, Flanagan et al., 2001, Kollef et al., 1996, Nys et al., 2000.
Detection of bacterial ATP has been used as a screening method for detecting significant amounts of bacteria in a variety of samples including food (Ukuku et al., 2001), water (Lee et al., 2001), environmental surfaces de Boer and Beumer, 1999, Marena et al., 2002, Worsfold and Griffith, 2001, blood (Nilsson et al., 1989), and urine Koenig et al., 1992a, Milagro et al., 1999a, Pezzlo et al., 1989b, Rippin et al., 1995a, Velasco et al., 2002a. The UTIscreen™ (Coral Biotechnology, San Diego, CA) is a bacterial adenosine triphosphate (ATP) detection system that has been developed and validated for screening for significant bacteriuria in urine specimens Koenig et al., 1992a, Milagro et al., 1999a, Pezzlo et al., 1989b, Rippin et al., 1995a, Velasco et al., 2002a. This assay uses the luciferin-luciferase reaction, which in the presence of ATP releases light. A photo-multiplier tube detector measures the light produced by this reaction and the signal is converted to relative light units (RLU). The RLU produced is proportional to the amount of bacterial ATP present in the sample.
We applied this bacterial ATP assay technology to BAL specimens from critically ill patients and assessed its diagnostic utility in comparison to parallel quantitative cultures.
Section snippets
Patient poulation
The Calgary Health Region (CHR) administers medical services to the residents of the cities of Calgary and Airdrie and approximately 20 nearby small towns, villages, and hamlets (total population 950,000). It also serves as the main tertiary care referral center for southern Alberta and parts of southeastern British Columbia, Canada. In the CHR, all routine bacterial specimens are sent to Calgary Laboratory Services (CLS), a region-based diagnostic laboratory (Church and Hall, 1999).
Results
A total of 128 samples from 94 patients were evaluated. All specimens had adequate sample volume to include parallel bacterial ATP testing. Twenty-three percent (30/128) of samples cultured ≥104 CFU/mL bacteria and their etiology is shown in Table 1.
Using a cut-off value for a positive bacterial ATP test as any calibrated value of ≥1%, 62 samples were classified as negative and 66 were positive. Four samples that were negative by the bacterial ATP assay had ≥104 organisms/mL of BAL fluid. One
Discussion
In this study, a rapid bacterial ATP assay was used to screen for significant bacteria in BAL specimens from critically ill patients. The bacterial ATP assay independently performed fairly well as a diagnostic test for significant bacteria in BAL specimens and was more sensitive than Gram's stain. It is not clear why the assay failed to identify significant bacterial counts in four of the samples. The manufacturer has noted that antibiotics may inhibit the ATP assay and this may have
Acknowledgements
Dr. Laupland was supported by the 2000 Bayer Healthcare/Canadian Institutes of Health Research/Canadian Infectious Diseases Society Research Fellowship and a clinical fellowship award from the Alberta Heritage Foundation for Medical Research. This work was supported by an award from the Calgary Laboratory Services 2002 Research Competition.
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