Urinary Antigen Test for Pneumococcal Pneumonia

Abstract

To the Editor: We read with interest the editorial by Dr. Pesola1Pesola GR The urinary antigen test for the diagnosis of pneumococcal pneumonia.Chest. 2001; : 9-11Abstract Full Text Full Text PDF Scopus (16) Google Scholar (January 2001) and his comments on our study,2Domínguez J Galí N Blanco S et al.Detection of Streptococcus pneumoniae antigen by a rapid immunochromatographic assay in urine samples.Chest. 2001; 119: 243-249Abstract Full Text Full Text PDF PubMed Scopus (206) Google Scholar which was recently featured in your publication. With a view to enabling better interpretation of the results of our study, we wish to clarify some points that we believe Dr. Pesola has misunderstood. In our study, we evaluated an immunochromatographic membrane test (ICT) to detect Streptococcus pneumoniae in urine samples, in order to assess its utility in the diagnosis of pneumococcal pneumonia. The sensitivity of the test using urine samples from patients with a diagnosis of definite pneumococcal pneumonia (group 1) was 80.4% (41 of 51). In contrast, the sensitivity of the test using urine samples from patients with a diagnosis of presumptive pneumococcal pneumonia (group 2) was 43.7% (7 of 16). The specificity was 97.2%. In order to make the performance of the test clearer, we explained in the “Results” section that results could be differentiated in three groups according to the color intensity reached: weak, medium, and very intense. Since the two false-positive results gave weak-intensity colored lines, which were difficult to interpret, we proposed to consider these results as negative, thus bringing the specificity to 100%, and maintaining the sensitivity at 74.5% in definite pneumococcal pneumonia. Dr. Pesola considers that all weak results are difficult to interpret and, therefore, claims that all these results are negative. Citing the five weak line results in group 2, he concludes that the real sensitivity of the test using group 2 was 12.5%. We don’t agree that all weak results have to be considered negative or that weak results of weak color lines are difficult to interpret. Less colored lines are included in the weak-line group, but only lesser colored lines are very difficult to interpret and can therefore be considered negative. In our study, we obtained a very weak line result from only three patients in group 2. Taking these results as negative, the sensitivity of the test in group 2 would be 25%. Patients from group 2 were diagnosed by results from Gram’s stain, sputum culture, and/or pneumococcal antigen detection by latex in sputum. The low sensitivity and specificity of these techniques3Bartlett JG Breiman RF Mandell LA et al.Community-acquired pneumonia in adult: guidelines for management.Clin Infect Dis. 1998; 26: 811-838Crossref PubMed Scopus (854) Google Scholar is well-known. Problems arise from the difficulty in interpretation due to contamination by the flora of the upper airways, given the possibility that they might contain S pneumoniae. In our opinion, calculating the sensitivity of the ICT with the results from both group 1 and group 2 patients leads to an underestimation. We believe that it is more probable that the real sensitivity of the test is closer to the sensitivity raised in group 1 than in group 2. However, we do agree with Dr. Pesola that this test represents a significant step forward in the definitive diagnosis of pneumococcal pneumonia. Yet, additional studies are needed in order to clarify different issues related to the performance of the test, and to assess its utility in the better management of patients. In our opinion, the ICT test is a rapid and reliable test for detecting S pneumoniae antigen in urine. ICT is a very useful tool for assisting physicians in the diagnosis and focusing of the treatment of patients with pneumococcal pneumonia.