The NOW S. pneumoniae Urinary Antigen Test Positivity Rate 6 Weeks after Pneumonia Onset and Among Patients with COPD

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SIR—The recently published study by Gutiérrez et al. [1] provides additional information on the performance of the NOW Streptococcus pneumoniae urinary antigen test (Binax) and confirms the findings of other studies [2, 3]. For adults with community-acquired pneumonia (CAP), the test has an estimated sensitivity of 75%–85% among bacteremic patients and 50%–80% among nonbacteremic patients. Included among the unresolved issues regarding the performance of the NOW assay are, first, the duration of test positivity after onset of pneumococcal infection and, second, the positivity rate among patients with chronic obstructive pulmonary disease (COPD) who may be colonized by or infected with S. pneumoniae [4]. We present data to help clarify both of these issues. Of the 120 patients with CAP and positive NOW urinary antigen test results in our study [3], 80 had urine samples that were collected at follow-up ∼6 weeks after admission to the hospital (median interval after onset of symptoms, 49 days [range, 33–89]; median interval after collection of first urine sample, 43 days [range, 31–82]). When unconcentrated samples were tested using the NOW assay, 38 (48%) of these samples yielded positive results. The longest duration of positivity of test results for any patient in our study was 89 days after onset of symptoms. Although we do not have more-recent urine samples from these patients, which would allow us to better characterize the duration of antigen positivity, our findings indicate that S. pneumoniae antigen can be detected in urine for several weeks following pneumococcal pneumonia. It is possible that S. pneumoniae antigenuria may be very prolonged in some patients, as is the case with Legionella antigenuria [5]. The practical implications of these findings apply to patients with pneumonia who have recently had a previous episode of pneumonia for whom a positive urinary antigen test result may be due to the first episode. To evaluate the NOW assay among patients with COPD, we tested urine samples obtained from 97 patients with COPD (defined as those with a history of chronic progressive symptoms [cough, wheeze, or breathlessness] with objective evidence of irreversible airway obstruction on spirometry) who had no clinical or radiographic evidence of pneumonia. Forty-nine urine samples were collected from patients during exacerbations of COPD, and the remaining 48 samples were collected from patients with stable COPD during routine outpatient visits. Nasopharyngeal swab specimens and sputum samples (if available) were collected from all patients and cultured on sheep blood agar for S. pneumoniae. Of the 97 urine samples tested, S. pneumoniae antigen was detected in only 3, all of which were obtained from patients with exacerbations of COPD; it was not detected in any samples obtained from patients with stable COPD. Only 4 of the 97 patients (3 with COPD exacerbations and 1 with stable COPD) had nasopharyngeal and/or sputum samples from which S. pneumoniae was cultured, and only 1 of these patients had a positive urinary antigen test result. These preliminary findings indicate a low rate of S. pneumoniae antigenuria among patients with COPD, although it would be interesting to repeat this study with a group of patients with COPD who had a high rate of documented S. pneumoniae colonization and/or infection. The NOW assay is a useful test for the presence of adult pneumonia, with a diagnostic yield that is greater than that of traditional techniques. Preliminary data indicate that specificity may not be reduced in cases of COPD, although positive test results need to be interpreted with caution in cases of recurrent pneumonia. We thank the staff of the Microbiology Unit, Canterbury Health Laboratories, other members of the Christchurch-Waikato Community-Acquired Pneumonia Study research team, and Binax for donating the antigen detection kits.

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