A hospitalized patient who was admitted with lung cancer (not on chemo) developed new wheezing and a shortness of breath, but does not have a fever. It has been seven days since their admission. The chest x-ray showed new small pleural effusions but no infiltrates. The urine antigen was positive, and there was no sputum produced. Another urine antigen was done five days later, but it showed up negative.
Could this still be possible hospital acquired Legionella despite the absence of fever (but new clinical respiratory findings) and a repeated urine antibody which was negative? I know the urine antigen can remain positive for weeks, but I did not know about a positive that can become a negative so soon.
The scenario you describe is somewhat unusual, as you indicate. The urine antigen will usually remain positive, even after receiving antibiotic therapy. But, on occasion the second one may be negative for a variety of reasons. One of them is that the pathogen may not be L. pneumophila serogroup 1, but another species or serogroup, and weak cross-reactivity with urinary antigen has occurred.
If you have pleural fluid available, the urinary antigen test may be positive in the pleural fluid. I would also obtain a third urinary antigen for confirmation.
At this point, obtain sera immediately and repeat it as an outpatient. Order antibody titers for Legionella pneumophila serogroups 1-6. The maximal time for seroconversion is three months. So, obtain sera acutely (best time is admission), about 4-6 weeks later, and again 3 months later. This may give you the answer if seroconversion is seen.