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Abstract & Commentary
Molecular Diagnosis for Pediatric Parapneumonic Empyema
By Hal B. Jenson, MD, FAAP, Dean, Western Michigan University School of Medicine, Kalamazoo, MI, is Associate Editor for Infectious Disease Alert.
Dr. Jenson reports no financial relationship to this field of study.
Synopsis: PCR testing of pleural fluid significantly increased bacterial identification compared to culture of blood and pleural fluid (84% vs. 35%; P < 0.001). PCR was particularly more sensitive for detection of Streptococcus pneumoniae (71% vs. 24%; P < 0.001). Dual infections were identified by PCR in 10% of cases.
Source: Blaschke AJ, et al. Molecular analysis improves pathogen identification and epidemiologic study of pediatric parapneumonic empyema. Pediatr Infect Dis J 2011;30:289-294.
A study compared culture- and PCR-based bacterial identification methods among 85 children hospitalized at a single medical center with parapneumonic effusion in 2009. Pleural fluid specimens for multi-species PCR testing were available from 63 of 85 (74%) hospitalized children. The median length of illness prior to presentation was 6 days (range, 1-16 days) and 86% had been treated with antibiotics prior to pleural fluid sampling.
By culture, pathogens were isolated from the blood and/or pleural fluid of 22 of the 63 children (35%). Streptococcus pneumoniae was isolated in 15 of 63 (24%), Streptococcus pyogenes in three (5%), and Staphylococcus aureus in four (6%; all MRSA). Pleural fluid culture identified a pathogen in 15 of 63 (24%) cases. Blood cultures were the only positive specimen in seven of 22 children (32%) in whom a pathogen was identified by culture.
By PCR, 59 pathogens were identified in the pleural fluid of 53 of the 63 children (84%). S. pneumoniae was isolated in 45 of 63 (71%), S. pyogenes in seven (11%), S. aureus in five (8%; two MSSA and three MRSA), Haemophilus influenzae in one (2%), and Mycoplasma pneumoniae in one (2%). Pleural fluid PCR identified a pathogen in 53 of the 63 cases (84%). PCR was positive in all cases in which a pathogen was identified by pleural fluid culture. Two bacterial pathogens were identified by PCR in six pleural fluid samples (10%).
Of the 41 of 63 patients (65%) with no pathogen identified by culture of either pleural fluid or blood, 32 (78%) had a pathogen identified by pleural fluid PCR.
Of the 45 samples positive for S. pneumoniae by culture and/or PCR, serotyping using molecular methods could be performed in 78% of cases, which showed only four serotypes: 1, 3, 7F, and 19A. Serotype 7F was the most frequently detected serotype by both culture and PCR, representing eight of 15 (53%) S. pneumoniae culture isolates and 21 of 45 (47%) S. pneumoniae isolates identified by culture and/or PCR.
Microbiologic diagnosis and effective management of parapneumonic effusion in children is hampered greatly by the high rate of sterile bacterial cultures of pleural fluid and blood. In this series, a pathogen was identified by culture in only 35% of cases, which is typical. One major factor contributing to the high rate of culture-negative cases is the frequency of prior antibiotic treatment before pleural fluid sampling (86% in this study).
Using PCR, pathogens were identified in 84% of cases. S. pneumoniae was the most frequent pathogen identified by PCR in culture-negative cases. This study demonstrates the usefulness of molecular diagnostic testing and evaluation for parapneumonic effusions and other serious infections. It also shows that dual infections appear to be more frequent (10%) in parapneumonic effusions than commonly appreciated. In this single-center study, the rate of MRSA as a cause of parapneumonic effusion was very low. It is possible that this may vary regionally.
The study was conducted just prior to the introduction of PCV-13 vaccine. The four S. pneumoniae serotypes found in the study are contained in the PCV-13 vaccine. One concern to monitor in the future is the likelihood of replacement disease caused by non-vaccine S. pneumoniae serotypes.