Legionella pneumoniae and Streptococcus pneumoniae urinary antigen assays (UAT)
Rapid tests are both a blessing and a curse. Techs and clinicians need to be aware of these tests' limitations but remember that, within those limitations, they might find some significant information.
A positive UAT gives you a head start on therapy; a negative UAT may not totally rule out suspected infection but, at least, the negative should not be because “I felt crappy and took some pills left over from when my kid had the flu.” Any test that can be run on a serology bench will not take up micro lab space … which is always at a premium!
—Chuck Millstein, MBA, MT(ASCP), CLDir(NCA), Retired
Pneumococcal and Legionella UAT
With the medical lab's “boomers” leaving the work force, years of experience with objective, standardized methodologies is being replaced. I am surprised that so many labs have resisted offering real-time antigen detection methods for L pneumophila and S pneumonia and in the differential diagnosis of community-acquired pneumonias (CAPs).
These tests improve the care of adult patients with CAP and have been the focus of two of the most widely referenced organizations: the Infectious Diseases Society of America (IDSA) and the American Thoracic Society (ATS). Why the resistance?
The low yield and infrequent positive impact on clinical care argue against the diagnostic usefulness of common tests (e.g., blood and sputum culture), although these cultures are important for epidemiologic reasons, including antibiogram data useful for treatment guidelines. The sputum Gram stain is only effective if rigidly screened for acceptability and performed by seasoned microbiologists.
Both UAT are recommended by the IDSA and the ATS for CAP guidelines to augment other diagnostic strategies. The advantages of UATs include improved sensitivity over sputum and blood cultures, the availability of urine in patients unable to produce sputum, and rapid turnaround time. The tests remain valid, even after initiation of antibiotics. The major disadvantage to these tests is the lack of an organism for further microbiologic testing.
Pneumococcal UAT: S pneumoniae is the most common cause of CAP and a major pathogen in healthcare-associated pneumonia . The yield of sputum culture for S pneumoniae rapidly decreases with initiation of antibiotics and after 24 hours fewer than 30% of patients with pneumococcal pneumonia will have positive cultures. Many patients are unable to provide adequate sputum specimens.
The pneumococcal UAT detects S pneumoniae C-polysacharide (a cell wall component) which is present in all serotypes. In an analysis of all currently published trials, the pooled sensitivity was 74% and the specificity was 94% compared to traditional culture. Use of the UAT improves diagnostic yield by 23% to 39%.
A number of reasons exist for the increase in diagnostic yield over traditional microbiology:
- The obvious ease of availability of urine specimens in situations where sputum may not be able to be produced.
- Also, the UAT will typically remain positive for up to three days after antibiotics are initiated.
- The IDSA/ATS CAP guidelines recommend the use of pneumococcal UAT in patients with leucopenia, severe liver disease, chronic alcohol abuse, asplenia, pleural effusions, those who fail outpatient therapy, and patients admitted to the intensive-care unit (ICU).
- The guidelines recommend the use of the pneumococcal UAT in patients who have been initiated on antibiotics. The pneumococcal UAT has decreased specificity in children.
- The pneumococcal UAT is useful in that a positive result allows narrowing of antibiotics from broad spectrum to targeted therapy for pneumococci alone.
Legionella UAT: Culture is the most important technique for the detection of Legionella species; and if infection due to these organisms is suspected, appropriate cultures should be obtained. The Legionella UAT is the most commonly ordered test for the diagnosis of Legionnaires disease and has a sensitivity of around 70% to 80%, a specificity of >99%, and will usually remain positive for days to weeks after effective treatment is initiated.
IDSA/ATS CAP guidelines recommend the use of the Legionella UAT in patients with pleural effusions, recent travel, alcohol abuse, failure of outpatient therapy, and patients admitted to the ICU. A disadvantage of the Legionella UAT is it only is useful in the detection of disease due to serogroup 1. Approximately 80% to 95% of Legionella infections in the U.S. are due to group 1.
Recommendations for UAT use: 1) Utilize in patients with CAP as specified by IDSA/ATS guidelines, and 2) consider use in patients with hospital-acquired pneumonia as a diagnostic adjunctive test to routine sputum and blood cultures.
Putting the cart before the horse: A few times in the beginning of UAT use, Streptococcus pneumoniae was identified and reported out from a blood and/or sputum culture 48 hours after a patient was admitted with pneumonia. It was only then that an order was placed for the urinary antigen test. I guess you could say that it sort of defeats the whole purpose of the rapid testing idea.
—Colleen K. Gannon, MT(AMT) HEW, un-retired,
the “Nancy Grace” for labs
