Justifying the use of blood cultures when diagnosing community-acquired pneumonia

Can we predict which patients with community-acquired pneumonia are likely to have positive blood cultures?

BACKGROUND

Blood cultures (BC) are commonly ordered during the initial assessment of patients with community-acquired pneumonia (CAP), yet their yield remains low. Selective use of BC would allow the opportunity to save healthcare resources and avoid patient discomfort. The study was to determine what demographic and clinical factors predict a greater likelihood of a positive blood culture result in patients diagnosed with CAP.

METHODS

A structured retrospective systematic chart audit was performed to compare relevant demographic and clinical details of patients admitted with CAP, in whom blood culture results were positive, with those of age, sex, and date-matched control patients in whom blood culture results were negative.

RESULTS

On univariate analysis, eight variables were associated with a positive BC result. After logistic regression analysis, however, the only variables statistically significantly associated with a positive BC were WBC less than 4.5 × 10(9)/L [likelihood ratio (LR): 7.75, 95% CI=2.89-30.39], creatinine >106 μmol/L (LR: 3.15, 95%CI=1.71-5.80), serum glucose<6.1 mmol/L (LR: 2.46, 95%CI=1.14-5.32), and temperature > 38 °C (LR: 2.25, 95% CI =1.21-4.20). A patient with all of these variables had a LR of having a positive BC of 135.53 (95% CI=25.28-726.8) compared to patients with none of these variables.

CONCLUSIONS

Certain clinical variables in patients with CAP admitted to hospitals do appear to be associated with a higher probability of a positive yield of BC, with combinations of these variables increasing this likelihood. We have identified a subgroup of CAP patients in whom blood cultures are more likely to be useful.

Predicting Positive Blood Cultures in Patients presenting with Pneumonia at an Emergency Department in Singapore

Introduction: Routine blood cultures have been recommended for all patients in treatment guidelines for community-acquired pneumonia (CAP). This practice has become a major area of resource utilisation, despite the lack of evidence in its clinical utility. Calls for abandoning the practice is balanced by the occasions of uncovering an unexpected pathogen or an unusual antimicrobial resistance pattern. The aim of this study is to identify factors that predict positive blood cultures among patients hospitalised for pneumonia upon presentation at the Emergency Department (ED). Materials and Methods: A case control study was carried out on patients treated for pneumonia in the ED who had routine blood cultures performed as part of their management. The pneumonia severity index (PSI) was used to categorise patients into low- and high-risk for 30-day mortality. Logistic regression was carried out to determine factors significantly associated with positive blood cultures, from which a predictive probability equation was used to identify patients whose blood cultures were negative at a pre-determined cut-off, with minimum number of culture positive misclassification. A scoring system was devised, with scores predicting which patients would be likely to have a positive or negative blood culture. Results: A total of 1407 patients with pneumonia were treated at ED from May to December 2006, from whom 1800 blood cultures were performed. Of these, 140 cultures (7.8%) grew organisms, comprising 96 (5.3%) true positive cultures and 44 (2.4%) contaminated cultures. Logistic regression analysis identified ill patients with higher PSI classes, smokers and Malay patients to be more likely to have positive blood cultures. Patients who had prior treatment with antibiotics, chronic obstructive pulmonary disease and cough were less likely to have positive blood cultures. An index to predict a negative blood culture resulted in the accurate classification of all but 4 positive patients while still correctly classifying 27.8% of blood culture negative patients. The area under the ROC curve was 0.71 (95% CI, 0.65-0.76). A simplified scoring system was devised based on the predictive model had a sensitivity of 82% and specificity of 38.2% for a positive blood culture. Conclusion: Routine blood cultures yielded negative results in 94% of patients presenting with pneumonia. The development of the clinical scoring system is a first step towards selecting patients for whom blood cultures is performed and improve cost-effectiveness. Introduction: Routine blood cultures have been recommended for all patients in treatment guidelines for community-acquired pneumonia (CAP). This practice has become a major area of resource utilisation, despite the lack of evidence in its clinical utility. Calls for abandoning the practice is balanced by the occasions of uncovering an unexpected pathogen or an unusual antimicrobial resistance pattern. The aim of this study is to identify factors that predict positive blood cultures among patients hospitalised for pneumonia upon presentation at the Emergency Department (ED). Materials and Methods: A case control study was carried out on patients treated for pneumonia in the ED who had routine blood cultures performed as part of their management. The pneumonia severity index (PSI) was used to categorise patients into low- and high-risk for 30-day mortality. Logistic regression was carried out to determine factors significantly associated with positive blood cultures, from which a predictive probability equation was used to identify patients whose blood cultures were negative at a pre-determined cut-off, with minimum number of culture positive misclassification. A scoring system was devised, with scores predicting which patients would be likely to have a positive or negative blood culture. Results: A total of 1407 patients with pneumonia were treated at ED from May to December 2006, from whom 1800 blood cultures were performed. Of these, 140 cultures (7.8%) grew organisms, comprising 96 (5.3%) true positive cultures and 44 (2.4%) contaminated cultures. Logistic regression analysis identified ill patients with higher PSI classes, smokers and Malay patients to be more likely to have positive blood cultures. Patients who had prior treatment with antibiotics, chronic obstructive pulmonary disease and cough were less likely to have positive blood cultures. An index to predict a negative blood culture resulted in the accurate classification of all but 4 positive patients while still correctly classifying 27.8% of blood culture negative patients. The area under the ROC curve was 0.71 (95% CI, 0.65-0.76). A simplified scoring system was devised based on the predictive model had a sensitivity of 82% and specificity of 38.2% for a positive blood culture. Conclusion: Routine blood cultures yielded negative results in 94% of patients presenting with pneumonia. The development of the clinical scoring system is a first step towards selecting patients for whom blood cultures is performed and improve cost-effectiveness.

The contribution of blood cultures to the clinical management of adult patients admitted to the hospital with community-acquired pneumonia: a prospective observational study

STUDY OBJECTIVE

To assess the clinical usefulness of blood cultures (BCs) in the management of patients hospitalized with community-acquired pneumonia (CAP).

DESIGN

A prospective, observational study to investigate the contribution of BCs to the management and outcomes of adult patients presenting with CAP.

SETTING

Nineteen Canadian hospitals.

PATIENTS

Adults admitted to the hospital with CAP between January 1, 1998, and July 31, 1998.

INTERVENTIONS

The courses of therapy in patients for whom BC results yielded organisms considered to be clinically significant were analyzed to determine whether the BCs had contributed to management or outcome.

MEASUREMENTS AND RESULTS

Forty-three of 760 patients had significantly positive BC results. Patients with CAP who had BCs performed had a 1.97% chance (15 of 760 patients) of having a change of therapy directed by BC results. Patients in whom BCs yielded positive results had a 34.8% chance (15 of 43 patients) of having a change in therapy determined by BC results, and had a 58.1% chance (25 of 43 patients) of having a course of therapy contraindicated by BC results. Severity of illness, as measured by the pneumonia severity index, correlated poorly with the yield of BCs. BC results were positive in 8.0% of patients in risk classes I and II, 6.2% of patients in risk class III, 4.6% of patients in risk class IV, and 5.2% of patients in risk class V.

CONCLUSION

BCs have limited usefulness in the routine management of patients admitted to the hospital with uncomplicated CAP.

Laboratory diagnosis of community-acquired lower respiratory tract infection

This article has focused on the evaluation of outpatients with lower respiratory illness. In large part, the need for microbiological work-up is host-dependent. Healthy patients usually do well, and laboratory data are often unnecessary. The abnormal host requires a different approach and, in general, the more compromised the host, the more aggressive the laboratory evaluation. A renal transplant patient with respiratory symptoms often follows the dictum that "common things happen commonly;" however, the clinician needs that extra level of assurance in this case. Some transplant patients may have respiratory illness caused by strongyloidiasis. Cystic fibrosis is another example of the need for a more comprehensive laboratory evaluation. Specialized selective media and additional susceptibility studies may be needed to evaluate isolates associated with exacerbation of symptoms in these patients. The clinical laboratory should be forewarned of any materials coming from invasive diagnostic techniques, so they can prepare and offer useful advice regarding specimens, transport, and follow-up. Microbiological laboratories are often most knowledgeable regarding what type of testing is appropriate. Direct communication with the laboratory is essential to assure the best patient care.

Clinical policy for the management and risk stratification of community-acquired pneumonia in adults in the emergency department

This clinical policy represents an approach that emphasizes key clinical information to determine the severity of CAP. By using this approach, a determination of whether the patient can be treated as an outpatient or inpatient may be made. Recommendations about the utility of ancillary studies and the use of antibiotics are also given. As more of the questions are answered through controlled studies, an evidence-based approach to this problem will become increasingly important in improving the outcome of patients with CAP.

Value of the polymerase chain reaction assay in noninvasive respiratory samples for diagnosis of community-acquired pneumonia

We studied the causes of community-acquired pneumonia (CAP) in 184 patients. Microbiologic evaluation included sputum examination, blood culture, assessment of acute and convalescent antibody titers for Legionella pneumophila, Mycoplasma pneumoniae, Chlamydia pneumoniae, Coxiella psitacci, Coxiella burnetii and respiratory viruses, polymerase chain reaction (PCR) assays for M. pneumoniae and C. pneumoniae in throat swab, and PCR assay based on the amplification of pneumolysin gene fragment in sera. The causative pathogen was identified in 78 patients (Streptococcus pneumoniae, 44; M. pneumoniae, 26; C. pneumoniae, 1; others, 7). S. pneumoniae was detected in serum by the PCR assay in 41 patients, five of whom also had a positive blood culture. PCR assay was negative in two patients with positive blood culture for S. pneumoniae. C. pneumoniae was detected by PCR in nine patients, but only one showed seroconversion. M. pneumoniae was detected by PCR in only three patients (two without seroconversion). The diagnosis of pneumonia caused by S. pneumoniae was five times greater using PCR in serum than with blood culture. Detection of C. pneumoniae by PCR without fulfilling criteria for acute infection may be considered a prior infection. The PCR assay for the diagnosis of M. pneumoniae has a lower sensitivity than serologic methods.