What is the first choice of diagnostic investigation for Legionnaires' disease?

The first choice of diagnostic investigation for Legionnaires' disease is urinary antigens. This test targets specific antigens from the Legionella bacteria that can be detected in the urine of affected patients. One of the primary reasons for preferring this method is its high sensitivity and specificity for detecting Legionella pneumophila, the most common causative agent of Legionnaires' disease. The urinary antigen test is also simple and non-invasive, allowing for rapid diagnosis which can guide prompt treatment.

In addition, the test can be performed quickly, providing results in a matter of hours, which is crucial for initiating appropriate antibiotic therapy in cases of suspected pneumonia due to Legionnaires' disease. While chest X-rays and blood cultures can be performed for pneumonia diagnosis and bacterial infections, they are not as specific or reliable for Legionella as the urinary antigen test. Sputum culture could also be used; however, it is less frequently performed due to the difficulty in obtaining adequate samples and the less reliable nature of sputum cultures in identifying the organism compared to urine tests.