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In diagnosing hospital acquired pneumonia, the physicians may face difficulty because there is no reliable method to diagnose all cases. Initially, the diagnosis is based on clinical grounds that are evident by the finding of a new infiltrate on chest radiograph, fever, purulent sputum, or other signs of clinical deterioration. However, in a series reported by Fagon et al, this clinical method was found to be specific for hospital acquired pneumonia in only 27 of 84 patients. That may be because many other conditions such as congestive heart failure, pulmonary embolism, atelectasis, ARDS, pulmonary hemorrhage, or drug reactions may mimic pneumonia, particularly in critically ill patients. Since, specificity is lacking, it may give rise to the need to have more reliable diagnostic tools so that incidences of the use of antibiotics for noninfectious diseases can be lowered. While there are many different testing modalities that can be used in the diagnosis, there are their own limitations and none of the method is good enough and sufficiently sensitive and specific to be considered as a "gold standard" test.
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Blood cultures are known to have diagnostic and prognostic value; however, their sensitivity is reported to be only 8% to 20%. Therefore, their role is limited. Similarly, examination of expectorated sputum is neither specific nor sensitive. Therefore, this test should not be routinely used. In such scenarios, the most useful noninvasive test is the examination of tracheobronchial aspirates (TBA). This method is highly sensitivity, and it has been demonstrated in a recent study, where the organism causing infection was recovered from tracheal secretions in 29 of 31 patients. Unfortunately, this test also has its drawback. The inability of this test to differentiate between the organism responsible for causing the pneumonia and harmless colonizers, has put the use of TBA in its negative predictive value, its ability to exclude the presence of resistant organisms, and thus to narrow antibiotic coverage.
In invasive bronchoscopic techniques, samples are taken directly from the lower respiratory tract without the contamination from upper airway or oral secretions. This test would seem like an advanced technique in identifying the responsible pathogen. When bronchoscopic methods like the bronchoalveolar lavage (BAL) or the use of protected specimen brushes (PSB) were compared to less invasive methods, surprisingly, they did not appear to differ much in terms of sensitivity, specificity or, more importantly, patient morbidity and mortality. The current lack of consensus on the role of invasive diagnostic testing for hospital acquired pneumonia is the subject of ongoing debate.
One study compared the results of bronchoscopically obtained PSB with those of TBA in 76 mechanically ventilated patients, which were already put on empiric antibiotic therapy. It was found that more patients who received bronchoscopy with PSB changed their antibiotic regimen, but there were no significant change in length of stay, mortality between the two groups or the days requiring mechanical ventilation. This study concluded that the outcome is not influenced by techniques used for microbial investigation.
Another study showed a comparison between the use of invasive testing, such as PSB and BAL, with the use of noninvasive TBA in 413 patients. There was an initial decrease in antibiotic use, mortality, and organ dysfunction at 14 days among patients using invasive techniques. But, at 28-days, analysis of the difference in mortality could not be similarly demonstrated. This study along with other studies has concluded that noninvasive and invasive tools achieve similar diagnostic performance. Therefore, using invasive techniques cannot be justified in every patient with hospital acquired pneumonia. There are many who disagree and suggests that if invasive testing is done within the first 12 hours after diagnosis and before antibiotics are administered, the improvement in diagnostic yield may be sufficient to merit its use.
In a recent review by Ewig and Torres, he stated that invasive and noninvasive techniques do not differ significantly. Both of them are less sensitive than specific. He also states that the false-negative rate for these tests ranges from 30% to 40% and the false-positive rate from 20% to 30%. The review suggested that invasive diagnostic testing should not be performed early in the course of hospital acquired pneumonia and the best way to make adjustments to the empiric antibiotic regimen is by TBA rather than invasive techniques. Further, they stated that due to low degree of sensitivity linked with invasive methods, empiric therapy should not be stopped on the basis of negative diagnostic testing alone. The potential role for invasive diagnostic evaluation, it says, lies in cases of non response to initial treatment.
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