Abstract & Commentary
Synopsis: The rapid immunochromatographic urine test Binax NOW was found to be a valuable, sensitive, and rapid test for diagnosing pneumococcal pneumonia even after antibiotic therapy had been started. Source: Smith MD, et al. Rapid diagnosis of bacteremic pneumococcal infections in adults by using the Binax NOW Streptococcus pneumoniae urinary antigen test: A prospective, controlled clinical evaluation. J Clin Microbiol. 2003;41:2810-2813.
A simple, quick, reliable test for diagnosing systemic disease due to Streptococcus pneumoniae would permit appropriate antibiotic therapy to be started and would also allow the efficacy of vaccination to be determined more accurately. The Binax NOW urinary test seems to fit the bill. A swab soaked in urine is placed in the test device, reagent is added, the lid is closed, and the test is read 15 minutes later. A pink/purple color on both the sample and control line indicates a positive test result. (To see how it works, visit http://www.binax.com/images/swab.swf.) The test uses so-called immunochromatographic technology, as well as a rabbit anti-S pneumoniae antibody to detect any C-polysaccharide of the pneumococcus capsule that might be circulating. (This is the same C-polysaccharide to which C-reactive protein binds.)
This study was conducted prospectively and included a group of 107 adult patients with bacteremia due to S pneumoniae of whom 77 (72%) also had pneumonia (67 community-acquired, 10 hospital-acquired), 5 patients had meningitis, 1 had peritonitis, and no focus was identified for the remaining 24 cases. A group of 106 patients with bacteremia due to a pathogen other than S pneumoniae was used as a control group. The results showed a performance within the same range as those reported on the company web site and indicate that the positive result is reliable. Moreover, the performance was better for patients with pneumonia, and antigen could be detected up to 7 days later despite antibiotic therapy. Hence, the Binax test has the makings of a very useful test for the early diagnosis of bacteremic pneumococcal disease.
| Figure |
| Relationship of Prevalence to Post-Test Probability |
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Comment by J. Peter Donnelly, PhD
This test could hardly be simpler. Moisten a swab with urine, place it into the holes in a booklet, place a couple of drops of the reagent over it, seal, wait a quarter of an hour, and that’s it! A positive test is positive and obviates the need for the tedious and lengthy laboratory process usually required. Not only that, the results can be known and therapy started before the patient is moved elsewhere. So why aren’t we all adopting this test now?
Quite apart from logistics, costs, and other management issues there is a more fundamental concern to be addressed. The study presented is analogous to a phase II drug study since the test does work under optimal conditions, namely that the a priori likelihood of the disease among the study populations was 100% and was nil among the control group. What is now needed is what used to be known as a field study. This is analogous to a phase III drug trial in which one group would be subjected to the standard diagnosis and the other to the experimental test. Subjects would be selected by using a clinical case definition. This would necessarily include all patients at risk, as well as all those likely to have the disease, only about one-third of whom are likely to have bacteremia.
A quick glance at the Table shows that for a sensitivity of 82% and a specificity of 97% there would be little effect on the post-test probability when the prevalence is high. However, once it declines below 40%, there would be a marked and rapid loss of performance. So, while the results do indeed look promising, further extensive investigation in different clinical settings is actually still needed if the Binax is to find a proper place in everyday clinical practice.
| Binax Test Performance | |||
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Binax NOW |
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| Pneumococcal disease | Positive | Negative | Total |
| Yes | 88 | 19 | 107 |
| No | 3 | 103 | 106 |
| Total | 91 | 122 | 213 |
| This study | Company web site | ||
| Sensitivity | 82% | 86% | |
| Specificity | 97% | 94% | |
| Accuracy | 90% | 93% | |
Dr. Donnelly is Clinical Microbiologist University Hospital Nijmegen, The Netherlands Section Editor, Microbiology.
The rapid immunochromatographic urine test Binax NOW was found to be a valuable, sensitive, and rapid test for diagnosing pneumococcal pneumonia even after antibiotic therapy had been started.You have reached your article limit for the month. Subscribe now to access this article plus other member-only content.
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