Tuesday, September 28, 2010

Urine test can help pulmonologists too!

This is a urine test done to find the presence of the pneumococcal polysaccharide capsule(the antigen) for diagnosing Community acquired pneumonia caused by Streptococcus pneumoniae. The advantage is that the test can be done in 15 min, and so you have a result  quickly.This may help narrow antibiotics quickly. The next question is ..how far is it reliable. Lets see...

In a study done in Newzealand(but urine antigen was tested in Duke Univ, NC!!) urine antigen was checked in pts with pneumonia along with blood and sputum samples.Subjects without pneumonia served as controls. The most important finding was that antigen testing was negative in all the controls. This gives the test....a very high specificity (so if it is positive..then pt is more likely to have pneomococcal pneumonia). It can also be positive in patients with strep throat, urine having strep mitis.

A study from Spain, in 2001, showed that the positivity of the urine antigen test does not differ much between bacteremic and non-bacteremic patients with Strep Pneumo pneumonia. Have to note...that the test was not reliable in pts who had only a presumptive diagnosis of pneumonia.again specificity was 98%.
The next study from Germany compared the urine test in pts with infiltrate Vs those without infiltrate (what we call as exacerbation of COPD!). The detection rate in pts without an infiltrate (i.e COPD exacerbation) was very low..suggesting its not useful in those group of patients. This has been incorporated in the latest ATS guidelines on CAP. (Just peek at page S40 regarding where its useful).
This recent study from Arch of Int Med also showed the usefulness of urine antigen testing. To be more creative and quick..you can even do the rapid antigen test in a blood culture bottle sample with good detection rates.(Clin Micro 2005)

Whats the downside! - Well..not many. We are trying to diagnose the etiology of only a 3rd of CAP. And even in those found to be positive...there is a possibility of another infection(especially in ICU admits).
 The test can be positive upto 5 days after a pneumococcal vaccine.
 Also ...once had a pneumococcal pneumonia...the urine test will be positive for weeks.(so..not useful if pt is readmitted within 3-5 weeks).
Watch out for false positives with Strep mitis (a cause of endocarditis).