validate results. Although it has an un- common susceptibility profde, the ATCC 19247 strain of N. asteroides has been well studied and is suggested for use in quality control evaluations (4, 13).

S u m m a r y There is a strong rationale for deter-

mining antimicrobial susceptibility pro- Ides of clinical isolates of Nocardia. Methods for documenting these profiles are seemingly reproducible, if not yet standardized. The NCCLS has recently organized a working group to evaluate and standardize susceptibility test meth- ods for the aerobic act inomycetes , including the Nocardia spp. Once a standard method is recommended , inter- and intralaboratory reproducibil- ity can be assessed and the role of susceptibility profiles as predictors of clinical outcome can be better studied.

Currently, although retrospective re- ports of such studies do exist, most are based on individual vignettes or patient groups too small for appropriate inter- pretation or outcome assessment (1, 2). Frequently, the patients evaluated have underlying conditions too complex to provide a simplistic evaluation of bug/drug outcomes (1, 2). Inadequate length of follow-up for relapse to occur may also provide erroneous correlation between in vitro and in vivo results.

Until standardization and clinical evaluations have been achieved, care must be exercised in susceptibility test- ing of the Nocardia spp. and interpreta- tion of results (1, 5, 17). Individual laboratories should weigh their own ex- perience (and volume of tests) in the performance of such testing before pro- viding the service; more appropriately, isolates should be submitted to refer- ence laboratories having greater experi-

I

Editorial

ence (5). Results of in vitro studies should be interpreted conservatively and should be reported with the under- standing and comment that they are "not standardized.'"

References

1. Land, G. et al. 1991. Aerobic patho- genic Actinomycetales, pp. 340-359. In A. Balows et al. (ed.), Manual of clinical microbiology, 5th ed., American Society for Microbiology, Washington, D.C.

2. Rolfe, M. W., R. M. Stricter, and J. P. Lynch, 11I. 1992. Nocardiosis. Semin. Respir. Med. 13:216-233.

3. McNeil, M. M. et al. 1990. Comparison of species distribution and antimicrobial susceptibility of aerobic actinomycetes from clinical specimens. Rev. Infect. Dis. 12:778-783.

4. Wallace, R. J., Jr., and L. C. Steele. 1988. Susceptibility testing of Nocardia species for the clinical laboratory. Di- agn. Microbiol. Infect. Dis. 9:155-166.

5. Neumann, M. A. et al. 1991. Cmnitech 6A, new developments in anthnicrobial agent susceptibility testing: a practical guide. Coordinating ed., J.E. McGowan, Jr. American Society for Microbiology, Washington, D.C.

6. Dewsnup D. H., and D. N. Wright. 1984. In vitro susceptibility of No- cardia asteroides to 25 antimicrobial agents. Antimicrob. Agents Chemother. 25:165-167.

7. Gombert, M. E. et at. 1987. Susceptibil- ity of Nocardia asteroides to new qui- nolones and [3-1actmns. Anthnicrob. Agents Chemother. 31:2013-2014.

8. Berkey, P., D. Moore, and K. Rolston. 1988. In vitro susceptibilities of No- cardia species to newer antimicrobial agents. Anthnicrob. Agents Chemother. 32:1078--1079.

9. Gutmann, L. et al. 1983. Susceptibility of Nocardia asteroides to 46 antibiotics, in- cluding 22 ~lactams. Antimicrob. Agents

I

i

Chemother. 23:248--251

10. Fried, J., et M~ 1988. Cure of brain abscess caused by Nocardia asteroides resistant to multiple antibiotics. South. Med. J. 81:412--413,

11. Wallace, R..L .It. el "at. 1983. Differ- ences among N~:ardia spp. m suscepti- bility to aminoglycosides and 13-1actam antibiotics and their potential use in tax- enemy. Antimicrob. Agents Chemother. 23:19--21,

12. Boiron, P. and P. Provost. 1988 In vitro susceptibility testing ofNocardia spp. and its taxonomic implication. J. An- timicrob. Chcmother. 22:623-629.

13. Wallace, R. J. Jr. et at. 1988. Antimicro- bial susceptibility patterns of Nocardia ao'teroides. Antimicrob. Agents Chemother. 32:1776-1779.

14. W',dlace, R. L Jr. et al. 1990. Cefo- taxime-resist~alt Nocardia asteroides strains are isolates of the controversial species Nocardiafarcinica. J. Clin. Mi- crobiol. 28:272(~-2732.

15. Wallace, R. J. J,, eI al. 1977. Disk diffu- sion susceptibility testing of Nocardia species. J. Infect. Dis. 135:568-576.

16. Carroll, G. F:,, J. M. Brown, and L. D. Haley. 1977. A method for detennina- lion of in-vitro drug susceptibilities of some nocardiae and actinomadurae: re- suits of 17 aniimicrobial agents. Am. J. Clin. Pathol. 68:279-283.

17. Brown, B. A~ and R. J. Wallace, Jr. 1992. Broth microdilution MIC test for Nocardia spp. Procedure 5.12, In H.D. Isenberg (ed. in chief), Clinical microbiology procedures handbook. American Society for Microbiology, Washington. D.C

18. National Coimnittee for Clinical Labo- ratory Standards. Methods for dilution anthnicrobial susceptibility tests for bac- teria that grow aerobically. Approved standard M7-A2, vol 10. Villanova, Pa: National Committee for Clinical Labo- ratory Standards, 1990.

III

Quantitative Cultures for Diagnosing Ventilator-Associated Pneumonia

C. Glen Mayhall, M.D. Division of Infectious Diseases Department of Medicine College of Medicine The University of Tennessee, Memphis and The Hospital Epidemioiogy Unit The Regional Medical Center at Memphis Memphis, TN38163

Nosocomial pneumonia is one of the most common hospital-acquired infec- tions and is the nosocomial infection as- sociated with the highest mortality rate. Like other nosocomial infections, noso- comial pneumonia is highly associated with instrumentation. In the acute-care

setting, most cases of nosocomial pneu- monia occur in patients who have en- dotracheal tubes and are on mechanical ventilation.

The study of ventilator-associated pneumonia has been confounded by the difficulty in accurately diagnosing this

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infection. Studies of ventilator-associ- ated pneumonia over the past two dec- ades have relied on clinical and roentgenographic criteria and culture of tracheal secretions for diagnosis. More recent data from autopsy studies and studies using bronchoscopic techniques to obtain lower respiratory tract secre- tions for quantitative culture indicate that misclassification is common when clinical and radiographic criteria are used for the diagnosis of ventilator-asso- ciated pneumonia (1-5). Further, bacte- ria present in a patient's trachea are often not present in the patient's lung tissue obtained by open lung biopsy (6). When fiacteria are present in the lower respiratory tract at the site of a pulmo- nary infiltrate on a chest X ray, they are frequently of a different species than many of the microorganisms obtained from simultaneous culture of tracheal aspirates (7). Pneumonia is frequently underdiagnosed in patients with the Adult Respiratory Distress Syndrome and overdiagnosed in patients who have respiratory failure caused by other pul- monary diseases (8).

Owing to the substantial degree of misclassification in studies of ventilator- associated pneumonia published to date, the rates of nosocomial pneumo- nia in patients on mechanical ventila- tion have not been accurately determined and risk factors for ventila- tor-associated pneumonia have not been precisely defined. Though no epidemiologic studies using definitive bronchoscopic techniques for diagnosis have yet been published, a number of studies have made use of fiberoptic bronchoscopic techniques to diagnose nosocomial pneumonia in patients on mechanical ventilatory support and to identify the exact microbial causes of these infections (4, 5, 7, 9-16). These studies have shown that pneumonia that complicates mechanical ventilation is caused largely by nonenteric microor- ganisms. Unlike earlier studies that used cultures of tracheal secretions, most studies using cultures of lower respiratory tract secretions obtained by fiberoptic bronchoscopy have found that enteric gram-negative bacilli are the causative agents of pneumonia in fewer than 20% of cases.

The finding that enteric gram-nega- L !l I l l I [ I I I I I I

Clinical Microbiology Newsletter 15:22,1993

rive bacilli are uncommon causes of ventilator-associated pneumonia has cast some doubt on the role of over- growth of bacteria in the stomachs of patients on mechanical ventilation in the pathogenesis of ventilator-associ- ated pneumonia. These patients are fre- quently given Ha-receptor blockers and/or antacids as prophylaxis to pre- vent stress ulcer bleeding. The conse- quent rise in gastric pH permits overgrowth of bacteria in the stomach, and microorganisms may reach concen- trations as high as 108 colony-forming units (CFU) per ml (17). There are stud- ies suggesting that bacteria that reach high concentrations in the stomach may be regurgitated and aspirated into the lower respiratory tract, where they cause pneumonia (17-19). However, most of the microorganisms cultured from the stomachs and tracheal secre- tions of patients on mechanical ventila- tion have been enteric gram-negative bacilli (17, 18, 20, 21), whereas there is a paucity of these microorganisms in lower respiratory tract secretions in pa- tients with ventilator-associated pneu- monia. The possible role of the stomach in the pathogenesis of pneumonia in these patients clearly needs further study using definitive techniques for the diagnosis and identification of the specific microbial causes of the pneu- monia that complicates mechanical ven- tilation.

Short of an open-lung biopsy or per- cutaneous aspiration of a lung abscess, the most definitive technique for the di- agnosis of nosocomiai pneumonia is quantitative culture of secretions ob- tained from the lower respiratory tract with a fiberoptic bronchoscope. Secre- tions may be obtained with a protected specimen brush (PSB) or by bron- choalveolar lavage (BAL). BAL speci- mens are the specimens obtained by fiberoptic bronchoscopy that are most likely to be contaminated with upper respiratory tract flora, but the microor- ganisms causing pneumonia can be dif- ferentiated from upper airway contaminants by quantitative culture of the specimen. The causative microor- ganisms of pneumonia obtained from the lower respiratory tract are present in higher concentrations than are contami- nants from the upper respiratory tract.

© 1993 Elsevier Science Publishing Co., Inc,

Two studies have found that a concen- tration of >105 CFU per ml in a BAL specimen with <1% squamous epi- thelial cells is highly associated with the presence of pneumonia (10, 11). More refined approaches to diagnosis of nosocomial pneumonia using fiberop- tic bronchoscopy include use of pro- tected techniques to minimize contamination of lower respiratory tract secretions by upper airway flora fol- lowed by quantitative culture of the specimens. Use of the PSB described by Wimberley etal. (22) and protected bronchoalveolar lavage (PBAL) using a balloon-tipped catheter developed by Meduri et at. (5) protect specimens from contamination during collection and further increase the specificity of di- agnosis of pneumonia by fiberoptic bronchoscopy (3, 5, 23). The diagnostic threshold for pneumonia when the PSB is used is >10 3 CFU per brush and when PBAL is used the diagnostic threshold is > 104 CFU per ml.

While the diagnostic accuracy of specimens obtained by the PSB may be adversely affected by antimicrobial ther- apy at the time of sampling (7, 9), the diagnostic accuracy of PBAL is unaf- fected by antimicrobial therapy (5). Even though antimicrobial therapy may decrease the diagnostic accuracy of the PSB, this protected bronchoscopic tech- nique is still more accurate in the diag- nosis of pneumonia than clinical and radiographic criteria combined with cul- ture of tracheal secretions.

Unfortunately, many pulmonary and critical-care physicians have viewed bronchoscopic techniques for the diag- nosis of nosocomial pneumonia as too invasive, too time-consuming, and too costly. Laboratorians have often balked at requests for quantitative culture of respiratory secretions, considering them too time-consuming and of question- able value. However, since the majority of patients on mechanical ventilation who are suspected of having pneumo- nia do not, in fact, have pneumonia, an- timicrobial therapy can be avoided in the majority of patients with suspected pneumonia at a considerable cost sav- ings (3).

Use of protected bronchoscopic tech- niques to obtain lower respiratory tract secretions for quantitative culture for

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the definitive diagnosis o f nosocomial pneumonia is crucial for future studies of the etiology and epidemiology of nosocomial pneumonia that compli- cates mechanical ventilation. Only when the problems with misclassifica- tion that have plagued previous studies of nosocomial pneumonia are avoided by using definitive techniques for diag- nosis can the pathogenesis, etiology, and epidemiology of ventilator-associ- ated pneumonia be better understood.

One of the major problems that has tended to limit the use o f fiberoptic bronchoscopy and quantitative cultures of respiratory secretions has been the uncertainty as to how these specimens should be cultured in clinical laborato- ries. Fortunately, standards for processing and interpreting lalxaatory specimens from patients with suspected ventilator- associat~ pneumonia were recently pro- posed at the First International Consensus Conference on the Clinical Investigation of Ventilator-Associated Pneumonia (24, 25). This conference o f interna- tional experts on ventilator-associated pneumonia was held in May 1992 in Memphis. The conference was cospon- sored by the Society for Hospital Epidemiology of America and the American College o f Chest Physicians. The proceedings o f the conference were published simultaneously in the official journals o f the two organizations in No- vember 1992 (24, 25).

References 1. Andrews, C. P. et at. 1981. Diagnosis

of nosocomial bacterial pneumonia in acute, diffuse lung injury. Chest 80:254-258.

2. Bell, R. C. et at. 1983. Multiple organ system failure and infection in adult res- piratory distress syndrome. Ann. Intern. Med. 99:293-298.

3. Fagon, J. et al. 1988. Detection ofnoso- comial lung infection in ventilated pa- tients. Use of a protected specimen brush and quantitative culture tech- niques in 147 patients. Am. Rev. Respir. Dis. 138:110-116.

4. De Castro, F. R. et at. 1991. Reliability of the bronchoscopic protected catheter brush in the diagnosis of pneumonia in

mechanically ventilated patients. Crit. Care Med. 19:171-175.

5. Meduri, G. U. et al. 1991. Protected bronchoalveolax lavage. A new bron- choscopic technique to retrieve uncon- taminated distal airway secretions..tun. Rev. Respir. Dis. 143:855-864.

6. Hill, J. D. et al. 1976. Puhnonary pathol- ogy in acute respiratory insufficiency: lung biopsy as a diagnostic tool. J. Thorac. Cardiovasc. Surg. 71:64-71.

7. Torres, A. et al. 1989. Diagnostic value of quantitative cultures of bronchoalveo- lax lavage and telescoping plugged catheters in mechanically ventilated pa- tients with bacterial pneumonia. ,nan. Rev. Respir. Dis. 140: 306-310.

8. Meduri, G. U. 1990. Ventilator-associ- ated pneumonia in patients with respira- tory failure. A diagnostic approach. Chest 97:1208-1219.

9. ChasU'e, J. et al. 1984. Prospective evaluation of the protected spechnen brush for the diagnosis of puhnonary in- fections in ventilated patients. Am. Rev. Respir. Dis. 130:924-929.

10. Kahn, F. W. and J. M. Jones. 1987. Di- agnosing bacterial respiratory infection by bronchoalveolar lavage. J. Infect. Dis. 155:862-869.

11. Thorpe, J. E. et at. 1987. Bronchoalveo- lar lavage for diagnosing acute bacterial pneumonia. J. Infect. Dis. 155:855- 861.

12. Torres, A. et al. 1988. Diagnostic value of telescoping plugged catheters in me- chanically ventilated patients with bac- terial pneumonia using the Metras catheter. Am. Rev. Respir. Dis. 138:117-20.

13. Chastxe, J. et at. 1988. Diagnosis of noso- comial bacterial pnemnonia in intubated patients undergoing ventilation: omnpari- son of the usefulness of bronchoalveolax lavage and the protected spechnen brush. Am. J. Med. 85:499-506.

14. Jimrnez, P. et at. 1989. Incidence and etiology of pneumonia acquired during mechanical ventilation. Crit. Care Med. 17:882-885.

15. Fagon, J. et al. 1989. Nosocomial pneu- monia in patients receiving continuous mechanical ventilation. Prospective analysis of 52 episodes with use of a protected specimen brush and quantita- tive culture techniques. Am. Rev. Respir. Dis. 139:877-884.

16. Dreyfuss, D et at. 1991. Prospective study of noso:omial pneumoxfia and of patient and cisrcuit colonization during me- chanical ventilation with circuit changes every 48 hours versus no change. Am Rev. Respir. Dis, 143:738-743.

17. du Moulin, t L C, et al. 1982. Aspiration of gastric bacteria in antacid-treated pa- tients: a frequent cause of lX~stoperative colonization of the airway. Lancet i:242-245

18. Atherton, S. T. and D. J. White. 1978, Stomach as source of bacteria coloniz- ing respiratory tract during artificial ventilation. Lancet ii:968--969.

19. Dasctmer, Fo et al. 1988. Stress ulcer prophylaxis and ventilation pneumonia: prevention by antibacterial cytoprotec- tive agents? hlfect. Control Hosp. Epidemiol. 9:59--65

20. Donowitz. LG. et al. 1986. Alteration of normal gastric flora in critical care patients receiving antacid and cimeti- dine therapy. Infect. Control 7:23--26.

21. Kappstcin, 1, ~:t al. 1991. Incidence of pneumonia in mechanically ventilated patients treated with sucralfate or chnetidh~e as prophylaxis for stress bleeding: bacterial colonization of the stomach. Am° J. Med. 91 (suppl 2A): 125S-131S

22. Wimberly, N., L. F. Fating, and J.G. Bartlett. 1979~ A fiberoptic broncho- scopy techifique to obtain uncontami- nated lower airway secretions for bacterial culture. Am. Rev. Respir. Dis. 119:337-M3

23. Kirkpatrick, M. B. and J. B. Bass, Jr. 1989. Quantitative bacterial cultures of bronchoalveolar lavage fluids and pro- tected brush catheter specimens from nonnal subjects. Am. Rev. Respir. Dis. 139:546--54S

24. Baselski, V. S, el at. 1992. The stand- ardization of criteria for processing and interpreting laboratory specimens in pa- tients with SUSlXX:ted ventilator-associ- ated pneumonia. Infect. Control Hosp. Epideaniot. 13:657-666.

25. Baselski, V S. et al. 1992. The stand- ardization of criteria for processing and interpreting laboratory specimens in patients with suspected ventilator- associated pneumonia. Chest 102:571S-579S.

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