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Correspondence |

Specificity of a Quantitative Real-Time Polymerase Chain Reaction Assay for the Detection of Invasive Pneumococcal Disease: Identifying Streptococcus pneumoniae Using Quantitative Polymerase Chain Reaction FREE TO VIEW

Cordelia Kee, BSc (Hons); Daniel M. Fatovich, MBBS; Silvano Palladino, MHthMgt; Ian D. Kay, MBSc; Todd M. Pryce, BSc, PostGradDip; James Flexman, MBBS, PhD; Ronan Murray, MBBS; Grant W. Waterer, MBBS, PhD, FCCP
Author and Funding Information

From the School of Medicine and Pharmacology (Ms Kee and Dr Waterer), University of Western Australia; the Centre for Clinical Research in Emergency Medicine (Mr Fatovich), Western Australian Institute for Medical Research; the University of Western Australia Centre for Medical Research and Emergency Medicine (Mr Fatovich), Royal Perth Hospital; and the Department of Microbiology and Infectious Diseases (Messrs Palladino, Kay, and Pryce, Dr Flexman, and Dr Murray) Royal Perth Hospital.

Correspondence to: Cordelia Kee, Department of Microbiology and Infectious Diseases, PathWest Laboratory Medicine, Royal Perth Hospital, Wellington Street, Perth, Western Australia 6000; e-mail: ckee@meddent.uwa.edu.au


Financial/nonfinancial disclosures: The authors have reported to CHEST that no potential conflicts of interest exist with any companies/organizations whose products or services may be discussed in this article.

Reproduction of this article is prohibited without written permission from the American College of Chest Physicians (www.chestjournal.org/site/misc/reprints.xhtml).


© 2010 American College of Chest Physicians


Chest. 2010;137(1):243-244. doi:10.1378/chest.09-2185
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Published online

To the Editor:

Together with colleagues in Spain, we recently reported in CHEST (September 2009)1 on the potential clinical usefulness of pneumococcal bacterial load using a new autolysin (lytA) quantitative polymerase chain reaction (qPCR) assay. At the time of submission one question we could not adequately address was the specificity of the qPCR assay when applied in routine clinical practice over an extended period of time.

We have now completed evaluation of 1,800 samples taken at the same time as blood cultures in our ED from consecutive patients with suspected acute infection. As shown in Table 1, using a positive blood culture for Streptococcus pneumoniae as the gold standard, the qPCR assay has a specificity of at least 99.5% (95% CI, 99.0%-99.8%).

Table Graphic Jump Location
Table 1 —Comparison of Results Obtained From Real-Time Pneumococcal Autolysin Polymerase Chain Reaction and Blood Culture

lytA = autolysin; PCR = polymerase chain reaction.

We further investigated the clinical details of the nine patients who had PCR-positive/blood culture-negative sample results. In eight of these nine cases there was a clinical and radiologic diagnosis of pneumonia with no other pathogen isolated by blood or sputum cultures. We are therefore confident that these eight cases represent true positive results. The quantitative S pneumoniae bacterial load in all of these eight cases ranged from 3.4 copies/mL to 2 million copies/mL with the majority being less than 10,000 copies/mL. As in our previous study, the highest S pneumoniae bacterial loads were seen in patients with severe sepsis.

The final PCR-positive/blood culture-negative sample came from a patient with sepsis of presumed abdominal origin that resolved with ticarcillin/clavulanic acid. No other infecting organism was identified. To further explore the analytical specificity of the qPCR assay, we evaluated 10 separate Enterococcus spp isolates not tested in our original validation study and confirmed that there was no cross reactivity with our lytA PCR assay.2 Although we cannot definitely rule out some other pathogen, from the clinical details it is likely that S pneumoniae was the causative organism.

In summary, we have found our lytA pneumococcal PCR assay to be highly specific, with no definite false-positive result occurring in 1,800 samples taken during routine clinical practice in a busy ED. Unlike many PCR assays introduced into clinical practice and unlike urinary pneumococcal antigen testing, physicians can be confident that a positive lytA result indicates invasive pneumococcal disease.3-5

Rello J, Lisboa T, Lujan M, et al. DNA-Neumococo Study Group Severity of pneumococcal pneumonia associated with genomic bacterial load. Chest. 2009;1363:832-840. [CrossRef] [PubMed]
 
Kee C, Palladino S, Kay I, et al. Feasibility of real-time polymerase chain reaction in whole blood to identify Streptococcus pneumoniae in patients with community-acquired pneumonia. Diagn Microbiol Infect Dis. 2008;611:72-75. [CrossRef] [PubMed]
 
Andreo F, Prat C, Ruiz-Manzano J, et al. Persistence of Streptococcus pneumoniae urinary antigen excretion after pneumococcal pneumonia. Eur J Clin Microbiol Infect Dis. 2009;282:197-201. [CrossRef] [PubMed]
 
Gutiérrez F, Masiá M, Rodríguez JC, et al. Evaluation of the immunochromatographic Binax NOW assay for detection of Streptococcus pneumoniae urinary antigen in a prospective study of community-acquired pneumonia in Spain. Clin Infect Dis. 2003;363:286-292. [CrossRef] [PubMed]
 
Lasocki S, Scanvic A, Le Turdu F, et al. Evaluation of the Binax NOW Streptococcus pneumoniae urinary antigen assay in intensive care patients hospitalized for pneumonia. Intensive Care Med. 2006;3211:1766-1772. [CrossRef] [PubMed]
 

Figures

Tables

Table Graphic Jump Location
Table 1 —Comparison of Results Obtained From Real-Time Pneumococcal Autolysin Polymerase Chain Reaction and Blood Culture

lytA = autolysin; PCR = polymerase chain reaction.

References

Rello J, Lisboa T, Lujan M, et al. DNA-Neumococo Study Group Severity of pneumococcal pneumonia associated with genomic bacterial load. Chest. 2009;1363:832-840. [CrossRef] [PubMed]
 
Kee C, Palladino S, Kay I, et al. Feasibility of real-time polymerase chain reaction in whole blood to identify Streptococcus pneumoniae in patients with community-acquired pneumonia. Diagn Microbiol Infect Dis. 2008;611:72-75. [CrossRef] [PubMed]
 
Andreo F, Prat C, Ruiz-Manzano J, et al. Persistence of Streptococcus pneumoniae urinary antigen excretion after pneumococcal pneumonia. Eur J Clin Microbiol Infect Dis. 2009;282:197-201. [CrossRef] [PubMed]
 
Gutiérrez F, Masiá M, Rodríguez JC, et al. Evaluation of the immunochromatographic Binax NOW assay for detection of Streptococcus pneumoniae urinary antigen in a prospective study of community-acquired pneumonia in Spain. Clin Infect Dis. 2003;363:286-292. [CrossRef] [PubMed]
 
Lasocki S, Scanvic A, Le Turdu F, et al. Evaluation of the Binax NOW Streptococcus pneumoniae urinary antigen assay in intensive care patients hospitalized for pneumonia. Intensive Care Med. 2006;3211:1766-1772. [CrossRef] [PubMed]
 
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