The NOW S. pneumoniae
urinary antigen test is easy to perform, provides results within a few minutes, and detects an antigen common to all S. pneumoniae
). The manufacturer's own study confirmed that the test was able to detect 44 different strains of S. pneumoniae
, representing the 23 serotypes responsible for at least 90% of pneumococcal infections (product instructions, NOW S. pneumoniae
urinary antigen test; Binax, Inc.). Our findings indicate that the sensitivity of the test is 80% when positive blood cultures are used as the “gold standard.” The absence of S. pneumoniae
antigen in the urine from controls suggests that the specificity is high.
These findings are similar to those of other investigators who have used the NOW S. pneumoniae
urinary antigen test. Domínguez et al. (9
) tested urine samples from patients with pneumococcal pneumonia, and detected S. pneumoniae
antigen in 23 of 28 bacteremic patients (82%) and in 18 of 23 nonbacteremic patients (78%). The specificity was 97%, based on two positive results among 71 patients with documented infections caused by microorganisms other than S. pneumoniae
. Yu et al. detected S. pneumoniae
antigen in the urine from 86% of bacteremic patients (V. L. Yu, J. A. Kellog, J. F. Plouff, J. A. Coladonato, J. Manzella, W. Alves Dos Santos, R. B. Kohler, A. Torres, T. M. File, and J. D. Rihs, Abstr. 38th IDSA Annu. Meet., abstr. 262, 2000). In another study, S. pneumoniae
antigen was detected in urine from 24 of 45 patients with CAP and the diagnostic yield for pneumococcal pneumonia was increased by 60% (15
In contrast, the performance of other methods for detection of S. pneumoniae
antigen in urine has been inconsistent, with sensitivities ranging from 0 to >80%, usually <50% (2
). The improved sensitivity of the NOW S. pneumoniae
urinary antigen test may be due, in part, to the detection of the cell wall C polysaccharide common to all S. pneumoniae
strains, rather than type-specific capsular polysaccharides. This has been demonstrated in at least one comparative study using latex agglutination assays, where the C polysaccharide was detected in the urine of 23 of 33 patients with pneumococcal bacteremia, while type-specific polysaccharides were detected in only 17 of these patients (3
The NOW S. pneumoniae
urinary antigen test may be less useful for detecting pneumococcal pneumonia in children because of the high false-positive rate due to nasopharyngeal colonization with S. pneumoniae
. In one series, the test was no more likely to be positive among 88 children with pneumonia than among 198 control subjects but was significantly more likely to be positive among children who were nasopharyngeal carriers of S. pneumoniae
). While we did not test for nasopharyngeal carriage in the present study, the absence of positive results in our control group suggests this may not be an issue in adults. These findings may reflect the lower rates of pneumococcal colonization in adults than in children.
It is unclear why S. pneumoniae antigen was not detected in urine from four patients with pneumococcal bacteremia. Although two of these patients had urine collected many days after the onset of symptoms, S. pneumoniae was isolated from blood cultures within 2 days of urine collection in both cases, suggesting recent or concurrent antigenemia. Three of the 4 bacteremic patients with negative urinary antigen tests were taking antibiotics at the time of urine collection, compared with 6 of the 16 with positive tests. However, the numbers are too small to determine whether this trend is a true association. It is possible that the relative dilutions of the urine samples may affect test results and that pneumococcal antigen was present in very low concentrations in the four negative samples. Unfortunately, we were unable to test concentrated urine from two of the four patients because of insufficient volume. The relatively low urinary antigen positivity rate among patients with sputum cultures positive for S. pneumoniae may partly reflect the inherent problems of interpreting sputum cultures. It is difficult to be certain that S. pneumoniae isolated from sputum represents infection rather than colonization, and, although we cytologically screened all samples, it is likely that some of our positive samples were due to contamination with oropharyngeal flora.
Concentrating urine samples before antigen testing is widely practiced, but few studies have determined the increase in yield by testing samples both before and after concentration. Twentyfold concentration of urine resulted in a 1.6-fold increase in yield of positive S. pneumoniae
antigen results (from 14 to 22%) using counterimmunoelectrophoresis (13
). Using the NOW S. pneumoniae
urinary antigen test, Marcos et al. noted a 1.4-fold increase in yield following concentration of urine (from 38 to 53%) (15
), which is at variance with the small increase in sensitivity that we documented (from 29 to 30%). They did not record the method they used for concentrating urine, but it is unlikely that using a concentrator with a lower-molecular-mass cutoff would increase sensitivity. The molecular mass of the C polysaccharide is 20 to 30 kDa (22
), whereas the Minicon-B15 concentrator has a molecular mass cutoff of 15 kDa (product instructions, Minicon and Miniplus clinical sample concentrators; Millipore). While the identification of these additional cases may be important, the small increase in yield in our study would not justify routine concentration of samples by diagnostic laboratories, especially given the substantial increase in costs that this would entail.
Although new tests for diagnosing pneumococcal disease have traditionally been compared with blood cultures, bacteremia was documented in only 20 to 30% of patients with pneumococcal pneumonia (4
). In the present study, of the pneumonia patients with positive urinary antigen tests who had blood cultures collected, only 18% were bacteremic. The NOW S. pneumoniae
urinary antigen test will be especially useful for identifying the large number of patients with nonbacteremic pneumococcal pneumonia and for rapidly identifying a group of patients in whom narrow-spectrum antibiotics may be used.
We conclude that the NOW S. pneumoniae urinary antigen test is a useful adjunct to culture for determining the etiology of CAP in adults. Further research should focus on the time course of urinary antigen positivity and the use of this test in settings other than adult pneumonia.