Pneumococcal antigen persistence in sputum from patients with community-acquired pneumonia.

The purpose of this study was to establish the diagnostic value of pneumococcal capsular antigen by comparing this with the results of Gram stain and culture in representative and nonrepresentative sputa during follow-up in patients with community-acquired pneumonia. Antigen was detected by a latex particle agglutination test. At the time of hospital admission, antigen was detected in 17 representative sputum specimens from 30 patients with pneumococcal pneumonia, which was comparable to the results of Gram stain and culture. In five additional patients, antigen was demonstrated in nonrepresentative specimens. During follow-up under antibiotic treatment, this number increased by six: three patients with representative and three patients with nonrepresentative sputum, respectively. Two of the 22 patients with pneumonia of other known cause had an antigen-positive sputum on admission and in another two patients, sputum antigen was detected during follow-up. Ten of 34 patients with pneumonia of unknown cause had detectable antigen in representative or nonrepresentative sputum on admission. During follow-up, antigen was detected in sputa of an additional seven patients. There was no difference in duration of antigen persistence between patients with pneumococcal pneumonia and pneumonia of unknown cause. It was observed that the first antigen-positive sputum specimen was always detected within the first five days of the hospital stay. We conclude that antigen detection in both representative and nonrepresentative sputum specimens at the time of hospital admission and during follow-up is of additional value for the diagnosis of pneumococcal pneumonia. It markedly increases the number of patients with pneumococcal pneumonia detected, who would otherwise be considered to have pneumonia of unknown cause. However, antigen-positive results should be interpreted carefully, especially in those pneumonia patients with chronic bronchitis, because detectable antigen may be caused by pneumococcal carriership of the lower respiratory tract.