Legionnaires' Disease - Diagnosis

What were the events, tests, or exams that led to a diagnosis of Legionnaires' disease?

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How is the diagnosis of Legionnaires' disease made?

Laboratory tests may include a modest increase in white blood cells, mild abnormalities in liver function studies, a low sodium in the blood, and even some decreased function of the kidneys. Nevertheless, these features can also be seen with a variety of different types of pneumonia. Chest X-rays often demonstrate abnormal densities (areas of that lungs that on the X-ray film appear whiter). However, it is difficult to distinguish Legionnaires' disease from other types of pneumonia by symptoms and chest X-ray alone. Additional specific tests are required for diagnosis.

These other tests are specialized and are not routinely performed on people with fever or pneumonia. Several types of tests are available. The most useful test uses a urine sample to detect identifiable proteins of the Legionnaires' bacterium (Legionella antigens). This test has the disadvantage of only testing for Legionella pneumophila serogroup 1 (this is the organism responsible for most of the cases). Additionally, detecting the bacteria in a culture of the sputum may be helpful. Lastly, tests that compare antibody levels of Legionella in two blood samples obtained three to six weeks apart showing a fourfold rise in the antibodies in the blood against the bacterium can be confirmatory after the disease is gone.

Because these tests complement each other, a positive result from each test when Legionnaires' disease is suspected increases the probability of confirming the diagnosis. However, because none of the laboratory tests is 100% sensitive, the diagnosis of legionellosis is not excluded even if one or more of the tests is negative. Of the available tests, the most specific is culture isolation of the bacterium from secretions from the respiratory tract.

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