Letter to the Editor
Clinical Relevance of Pneumococcal Antigen Detection in Urine Pneumococcal antigen detection has proven to be of additive diagnostic value in community-acquired pneumonia . Antigen detected in body fluids strongly indicates that Streptococcus pneumoniae has been involved in the infection. Detection of antigen is therefore especially valuable in those cases which would otherwise be classified as unknown aetiology. Reliable sputum specimens are often difficult to obtain from patients with pneumonia, and therefore some previous studies have focused on testing for pneumococcal capsular antigen in urine. However, there are great discrepancies in the results of these studies [2-5]. Using latex agglutination (LA), which is easy to perform and gives rapid results, we evaluated the utility of pneumococcal antigen detection in 539 urine samples from patients diagnosed with pneumonia or lower respiratory tract infection. Pneumonia was defined as an acute, febrile illness accompanied by infiltrate(s) on the chest x-ray. Patients with community-acquired pneumonia were subsequently classified into the following three groups. Pneumococcal pneumonia (n = 31) was diagnosed if blood culture yielded S. pneumoniae or Gram stain of washed representative sputum  showed pneumococci; results of Gram stain were confirmed by a culture. Pneumonia of other known aetiology (n = 21) was diagnosed if a non-pneumococcal pathogen was predominant in Gram stain or culture of washed representative sputum; if a positive blood culture was obtained; or if a >- fourfold rise/fall in antibody titre was detected by any of the serological tests performed. Patients in whom no positive microbiological or serological results could be demonstrated (n = 34) were diagnosed as having pneumonia of unknown aetiolog3,. Lower respiratory tract infection (n=23) was diagnosed in patients with chronic obstructive pulmonary disease who were admitted with an increase in symptoms of dyspnoea, sputum
manufacturer's instructions. Whenever possible, urine was concentrated ten4old or more by freeze-drying. A subgroup of urine samples was tested in parallel with another LA test (Slidex Pneumo-kit, Bio-Mrrieux, France). Antigen could be demonstrated in urine in only one patient with pneumococcal pneumonia, yielding a sensitivity of 3% (Table 1). The specificity for pneumococcal antigen detection in urine, determined in the group of patients with pneumonia of other known aetiology, was 100%. In none of the patients with pneumonia of unknown aetioIogy or lower respiratory tract infection could antigen be detected in urine. Concentration of urine specimens, maximally 300-fold, did not lead to more antigen-positive results. The 22 urine specimens that were concurrently tested with the other LA test all yielded negative results as well. The two antigen-positive urine specimens originated from a patient with pneumococcal pneumonia whose urine cultures yielded Candida albicans in a concentration of > 108 CFU/ml. This strain reacted with pneumococcal Omniserum in pure culture, indicating a possible cross-reaction. Antigen was also detected in sputum and pleural fluid in the same patient. The failure to detect pneumococcal antigen in urine by LA may be due to the following reasons: 1. The type-specific polysaccharide that is excreted in the urine differs from the polysaccharide found in other body fluids in its immunologic properties and smaller molecular size . Because of the changed molecular structure of the capsular polysaccharide, bridging of the latex particles may be more difficult, which may explain the difference in sensitivity found between LA and counterimmunoelectrophoresis. The minimal number of pneumococci required for antigen detection by both techniques is comparable (approximately 106 CFU/ml). 2. The unfavourable microenvironment due to pH, salt content
Table 1: Pneumococcal antigen detection in urine specimens from patients diagnosed with pneumonia or lower respiratory tract infection.
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