Gram stain and culture: Recommendations for confirming pneumonia – Clinical Consultation
Thomas M. Jr. File
What is the current role for sputum cultures in the diagnosis of community-acquired pneumonia (CAP)?
* Although culture of sputum has been a traditional basis for the bacteriologic diagnosis of pneumonia, its role is controversial. However, when a good-quality sputum specimen is available and is interpreted by trained personnel, I believe it can be useful in the management of CAP–particularly when hospitalization is required. Moreover, information obtained from such cultures is important in determining the pattern of predominant pathogens and antimicrobial resistance within local communities.
The use of sputum culture is limited in that many patients cannot produce a good specimen, and many specimens provide inconclusive results. Arguments against the use of such tests include their limitations in sensitivity and specificity, the low yield in many reports, the possibility of contamination by oral flora, the negative effect previous antimicrobial use has on the results, and the lack of documentation of benefits in terms of cost and outcome.
Nonetheless, the sputum Gram stain and culture can be useful when performed appropriately and when interpreted with expertise. This approach has proved useful when sputum is readily available because the procedure entails no risk to the patient. Also, sputum assessment is inexpensive and, in some patients, can provide invaluable information.
Three recently published guidelines for the management of CAP in adults variably recommend the use of sputum culture (Table). (1-3) These statements agree that there is limited value in relation to clinical outcome in obtaining sputum culture from patients who have mild illness and are treated as outpatients. The Infectious Disease Society of America (IDSA) statement lists the sputum Gram stain, with or without culture, as desirable but optional.
I believe it is reasonable to send sputum to a local laboratory for culture, even in this setting, if the patient presents with new production of purulent sputum. This information may be valuable for deciding on more effective therapy if the illness is not responding to initial empiric therapy–for example, when the patient is infected with a pathogen subsequently determined to be resistant to the initially prescribed therapy.
Although the guidelines differ somewhat in the emphasis placed on sputum culture for patients who require admission to the hospital, they generally agree that this test can be useful, particularly in identifying resistant pathogens. Moreover, if the specific etiologic pathogen is identified, there are a number of advantages, including the ability to select the optimal drug; to reduce antibiotic abuse in terms of cost, resistance, and adverse drug reactions; and to identify organisms that have potential epidemiologic significance (Legionella species, drug-resistant Streptococcus pneumoniae, or Mycobacterium tuberculosis). The IDSA statement acknowledges the limitations of culture of expectorated sputum but recommends this “relatively simple, inexpensive procedure, provided that a deep-cough specimen is obtained before antimicrobial therapy and is properly processed in the laboratory.”
It must be stressed, however, that since early antimicrobial therapy is associated with a better clinical outcome, therapy should not be delayed in an attempt to obtain specimens from acutely ill patients. In my experience, if patients are able to produce a good sputum sample, they are readily able to expectorate such specimens. If patients are unable to produce expectorated sputum in an expeditious manner, appropriate empiric antimicrobial therapy should be initiated without delay. The value of “induced sputum” for detecting pulmonary pathogens other than Pneumocystis carinii or M tuberculosis is poorly established, and I generally do not recommend the time and effort required to obtain such samples.
Specific criteria have been developed to improve the clinical relevance of sputum cultures; for example, preliminary Gram stain evaluation of specimens can be used to determine the extent of oral pharyngeal contamination. Approximately 25% to 50% of sputum samples prove to be of good quality by present criteria. The degree of contamination can be estimated from the proportion of leukocytes and squamous epithelial cells in the Gram stain of the specimen.
I acknowledge that only a minority of patients have specimens that are good enough for interpretation. But even if the test is useful in only a minority of patients, it should not be neglected. Why ignore a potentially valuable piece of information? Until more accurate and more cost-effective tests are available (such as DNA amplification and polymerase chain reaction), the Gram stain and culture of respiratory secretions should be used, within their limits, whenever the patient can ultimately benefit.
THOMAS M. FILE, JR, MD
Professor of internal medicine, Northeastern Ohio Universities College of Medicine, and chief of the infectious disease section, Summa Health System, Akron.
(1.) Bartlett JG, Dowell SF, Mandell LA, et al. Practice guidelines for the management of community-acquired pneumonia in adults. Guidelines from the Infectious Diseases society of America. Clin Infect Dis. 2000;31:347-382.
(2.) Mandell LA, Marrie TJ, Grossman RF, et al. canadian guidelines for the initial management of community-acquired pneumonia: an evidence-based update by the Canadian Infectious Diseases Society and the Canadian Thoracic Society. The Canadian Community-Acquired Pneumonia Working Group. Clin Infect Dis. 2000;31:383-421.
(3.) Niederman MS. Mandell LA, Anzueto A, et al. Guidelines for the management of adults with community-acquired pneumonia. Diagnosis, assessment of severity, antimicrobial therapy, and prevention. Am J Respir Crit Care Med. 2001:163:1730-1754.
Table — Use of sputum cultures in recent CAP guidelines
Patient group IDSA (1)
Outpatients Sputum Gram stain and
culture for conventional
bacteria are optional.
Inpatients Recommended. Should be
deep-cough specimen ob-
tained before antibiotic
therapy. Gram stain should
be interpreted by trained
personnel and culture
performed only if specimen
is adequate by cytologic
criteria, except for
Legionella and mycobacteria.
Inability to obtain a
specimen should not delay
Patient group CIDS/CTS (2)
Outpatients Not recommended for majority.
Exceptions: to rule out tubercu-
losis, Pneumocystis carinii
pneumonia, Legionella infection,
Inpatients Recommended. Gram staining
and culture should be performed
before antibiotic treatment,
if there is an adequate
specimen and interpretation
is done by properly trained
staff. Therapy should not be
delayed if there is difficulty
in obtaining an adequate
Patient group ATS (3)
Outpatients Not routinely recommended unless
drug-resistant bacteria, not
covered by the usual empiric
therapy options, are suspected.
Inpatients Sputum cultures are recommended
if a drug-resistant pathogen or
an organism not covered by
usual empiric therapy is
suspected. If a sputum culture
is obtained, it should be before
antibiotic administration. It
should be correlated with the
predominant organism identified
on Gram stain. Avoid delays
in the administration of
CAP, community-acquired pneumonia; IDSA, Infectious Disease Society of
America; CIDS/CTS, Canadian Infectious Diseases Society/ Canadian
Thoracic Society. (Canadian Consensus Group); ATS, American Thoracic
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