Head of Department
María V Torres
Department of Clinical Microbiology and Infectious Diseases
Hospital General Universitario Gregorio Marañón
Ventilator-associated-pneumonia (VAP) is the most frequent intensive care unit (ICU)-acquired infection, with mortality often ranging from 20% to 50% (reaching 70% in some situations). Early microbiological diagnosis and the administration of initial appropriate antibiotic therapy have proven to be associated with decreased mortality.
Conventional microbiological reports based on full bacterial identification and standard antimicrobial susceptibility tests rarely reach physicians attending VAP patients within 48–72 hours of lower respiratory tract (LRT) sampling.
The E-test is a well-known antimicrobial susceptibility test which uses antimicrobial-inoculated strips and is inoculum-size-independent. It can be used on isolated bacteria or placed directly on clinical samples.(1) This method has been shown to be reliable for predicting the susceptibility of microorganisms directly from positive blood cultures.
Use of the E-test in ICU patients with VAP
VAP, one of the most common infections acquired by ICU patients, occurs in between 9% and 27% of all intubated patients. It is associated with high morbidity and mortality and considerable economic loss.(2) Mortality related to VAP – attributable mortality – has been estimated to be between 33% and 50% in several case-matching studies of VAP.
The most frequently isolated bacteria depend on time of onset of pneumonia:
- Early onset of VAP is defined as occurring within the first four to five days of hospitalisation, usually have a better prognosis, and are more likely to be caused by antibiotic-sensitive bacteria. Potential microorganisms are methicillin-sensitive Staphylococcus aureus, anaerobes, Haemophilus influenzae, Streptococcus pneumoniae and Enterobacteriaceae (Escherichia coli, Klebsiella pneumoniae, Enterobacter spp, Proteus spp and Serratia marcescens).
- Late-onset VAP (five days or more) is more likely to be caused by multidrug-resistant (MDR) pathogens, such as Pseudomonas aeruginosa, Acinetobacter spp, Citrobacter spp, Stenotrophomonas maltophilia or methicillin-resistant S aureus (MRSA) and is associated with increased patient mortality and morbidity. However, patients with early-onset VAP who have received antibiotics or been hospitalised within the previous 90 days are at greater risk of colonisation and infection with MDR pathogens and should be treated in the same way as patients with late-onset VAP.
Early optimal antimicrobial therapy is an essential part of the successful management of VAP because inadequate initial therapy is consistently associated with increased mortality.
Conventional microbiological reports based on full bacterial identification and standard antimicrobial susceptibility tests rarely reach physicians attending VAP patients within 48–72 hours of lower respiratory tract (LRT) sampling. Standard microbiology procedures include quantitative culture in blood agar, chocolate agar and McConkey agar. After 18–24 hours of incubation, the colonies are counted. Colony counts of ≥104 CFU/ml are considered significant, whereas counts <104 CFU/ml indicate contamination with oronasal microbiota and are discarded as negative. Identification of the microorganisms and antimicrobial susceptibility testing are performed using an automatic system (MicroScan Dade Behring; Sacramento, CA, USA), and breakpoints are determined following the Clinical and Laboratory Standards Institute (CLSI) guidelines.(3)
Meanwhile, the patients must be given empiric broad-spectrum antibiotics, and de-escalation of treatment is performed only when bacterial identity and antimicrobial susceptibility is known. Delays in de-escalation and unnecessary administration of broad-spectrum antibiotics may have immediate and long-term consequences, contributing to the emergence of multiresistant pathogens, increasing the risk of severe superinfections and increasing morbidity, mortality and costs.(4,5)
The E-test uses a strip impregnated with an increasing concentration of antibiotic, which is usually placed on purified bacterial cultures, thus allowing the minimum inhibitory concentration (MIC) for different antimicrobial agents to be determined. The fact that the E-test does not depend on inoculum size makes it ideal for working on direct clinical secretions in which no standard inoculum is used.(6,7) Taking a direct antibiogram straight from the sample, without waiting for bacterial isolation, may provide preliminary information which has a very good correlation with standard procedures.
A preliminary report for our institution also showed a very good correlation between direct E-test susceptibility testing on respiratory samples compared with a standard microbroth dilution method after quantitative cultures (total agreement of 96.1%).(1) The rapid test consisted of an antibiogram performed by directly placing E-test® strips (AB BiodisK; Solina, Sweden) on 150 mm of Mueller–Hinton agar plates seeded with lower respiratory tract secretions. The antibiotic strips tested were: oxacillin, piperacillin/tazobactam, cefepime, imipenem, ciprofloxacin and amikacin. Plates were incubated at 35–37ºC for reading the following day with transmitted light, and the results were evaluated as resistant or susceptible following the CLSI guidelines.(3)
The preliminary information on antimicrobial susceptibility using the E-test on a direct sample can be available within 24 hours after receiving the sample, whereas conventional microbiological reports based on full bacterial identification and standard antimicrobial susceptibility tests rarely reach physicians attending VAP patients within 48–72 hours of lower respiratory tract sampling. In our expertise, preliminary information on antimicrobial susceptibility using the E-test on a direct sample was delivered to the patient’s chart within a mean of 1.4 days (±0.75) (minimum one day, maximum four days). Susceptibility results using direct samples were available in 75.4% of cases of VAP within 24 hours of receiving the sample, and definitive reports were issued a mean of 4.2 days after samples were obtained.(8)
Our study tried to assess the clinical impact of sending the E-test information to our ICU clinicians with no further intervention. Patients managed with an E-test preliminary antimicrobial susceptibility test had fewer days of fever per episode, fewer days of antibiotic administration to resolve the VAP episode, lower antibiotic consumption, less C difficile-associated diarrhoea, lower costs of antimicrobial agents and fewer days on mechanical ventilation from VAP diagnosis. We were able to demonstrate a considerable decrease in antimicrobial misuse and important cost savings. The estimated cost for the test is around €25, personnel cost included. As we saved €318 in antibiotics per episode, we think that the test is clearly economically viable.
The E-test is very simple and within the reach of any microbiology laboratory. It is clear that direct E-test in respiratory secretions helps to improve the adequacy of antimicrobial therapy in a disease that is so difficult to treat. Rapid but imprecise information from the microbiology laboratory may be of greater value than delayed but more precise information.
- Cercenado E, et al. Rapid antimicrobial susceptibility testing in patients with ventilator-associated pneumonia: direct E-test on respiratory samples. Interscience Conference on Antimicrobial Agents and Chemotherapy (ICAAC); San Diego (USA); 27–30 September 2002:132.
- Bouza E, et al. Crit Care Med 2003;31(7):1964-70.
- Clinical and Laboratory Standards Institute CLSI. Performance standards for antimicrobial susceptibility testing: Fifteenth Informational Supplement. In: M100-SIS. Wayne (PA): Clinical and Laboratory Standards Institute; 2005.
- Torres A, et al. Am Rev Respir Dis 1990;142(3):523-8.
- Rello J. Crit Care 2005;9(3):259-65.
- Chomarat M, et al. Pathol Biol (Paris) 2002;50(10):595-8.
- Ozakin C, et al. Scand J Infect Dis 2003;35(10):700-3.
- Bouza E, et al. Clin Infect Dis 2007;44(3):382-7.