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Diagnosing hospital acquired pneumonia is difficult because there is no method for obtaining a diagnosis that is reliable in all cases.6 The diagnosis is initially made on clinical grounds by the finding of a new infiltrate on chest radiograph, fever, purulent sputum, or other signs of clinical deterioration. Unfortunately, this clinical method was shown to be specific for hospital acquired pneumonia in only 27 of 84 patients in a series reported by Fagon et al 7 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.1 Lack of specificity in the clinical diagnosis gives rise to the need for more reliable diagnostic tools so that fewer patients will be treated with antibiotics for noninfectious causes. While there are many different testing modalities that may be employed to this end, all have their limitations and none is sufficiently sensitive and specific to be considered a "gold standard" test.3
Blood cultures have diagnostic and prognostic value but their reported sensitivity is only 8% to 20%, and their role is therefore limited. Likewise, examination of expectorated sputum is neither sensitive nor specific and should not be routinely used.1 The most useful noninvasive test is the examination of tracheobronchial aspirates (TBA). This method has a high degree of sensitivity, as demonstrated in a recent study where the offending organism was recovered from tracheal secretions in 29 of 31 patients.4 The weakness of this test is its inability to differentiate between the organism responsible for causing the pneumonia and harmless colonizers. Because of this limitation, the use of TBA lies in its negative predictive value, its ability to exclude the presence of resistant organisms, and thus to narrow antibiotic coverage.
Invasive bronchoscopic techniques are able to take samples directly from the lower respiratory tract without contamination from upper airway or oral secretions and would seem to provide an advance in identifying the responsible pathogen.8 Surprisingly, when bronchoscopic techniques such as bronchoalveolar lavage (BAL) or the use of protected specimen brushes (PSB) have been compared with less invasive methods, they do not appear to differ significantly in terms of sensitivity, specificity or, more importantly, patient morbidity and mortality.9 There is currently a lack of consensus on the role of invasive diagnostic testing for hospital acquired pneumonia, and it is the subject of ongoing debate.
One study regarding this issue compared the results of bronchoscopically obtained PSB with those of TBA in 76 mechanically ventilated patients who were already receiving empiric antibiotic therapy.9 In this study, more patients who received bronchoscopy with PSB had a change in their antibiotic regimen, but there were no significant differences in length of stay, days requiring mechanical ventilation, or mortality between the two groups. This study concluded that outcome was "not influenced by techniques used for microbial investigation." 9
Another study compared the use of invasive testing, such as PSB and BAL, with the use of noninvasive TBA among 413 patients. This study showed an initial decrease in mortality, antibiotic use, and organ dysfunction at 14 days among patients in whom invasive techniques were used, but at a 28-day analysis the difference in mortality could not be similarly demonstrated.10 These and other studies have led many to the conclusion that noninvasive and invasive tools achieve similar diagnostic performance and therefore the use of invasive techniques cannot be justified in every patient with hospital acquired pneumonia.8 Others argue 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.
A recent review of this issue by Ewig and Torres8 stated that invasive and noninvasive techniques do not differ significantly, that both are less sensitive than specific, and that the false-negative rate for these tests ranges from 30% to 40% and the false-positive rate from 20% to 30%. The review also stated 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 poor sensitivity associated with invasive methods, empiric coverage should not be stopped on the basis of negative diagnostic testing alone, and that the potential role for invasive diagnostic evaluation lies in cases of nonresponse to initial treatment
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