What Are the Different Legionella Tests?

M.C. Huguelet

Legionella is a type of bacteria which can cause two illnesses: the fairly minor Pontiac fever and the potentially life-threatening Legionnaire’s disease. While legionella can usually be treated with antibiotics, early detection of these bacteria is critical to a successful recovery. Common legionella tests include urine antigen testing, sputum sampling, and multi-stage blood testing.

A multi-stage blood test can be used to test for legionella.
A multi-stage blood test can be used to test for legionella.

Typically, legionella bacteria thrive in warm water, such as that found in ornamental fountains and hot tubs, and usually enter the body through inhalation. Two distinct illnesses, Pontiac fever and Legionnaire’s disease, result from infection by these bacteria. Pontiac fever is a mild condition which causes flu-like symptoms that last for a week or less, and which generally does not require treatment. Conversely, Legionnaire’s disease is a potentially life-threatening condition which occurs when legionella bacteria infect the lungs. If the disease is not detected and treated early, it can cause failure of the lungs, kidneys, and heart.

Legionella tests include the collection of a urine sample.
Legionella tests include the collection of a urine sample.

Luckily, there are three commonly used legionella tests that can help physicians identify these bacteria. Urinary antigen screening is often the first test used to screen for legionella, partly because it produces rapid results. An antigen is a material contained in bacteria or other foreign bodies which, when detected by an infected person’s immune system, encourages the creation of infection-fighting antibodies. In a urinary antigen screening, a urine sample is taken from an individual suspected of infection by legionella. The sample is then examined for evidence of legionella antigens.

If a urinary antigen screening shows evidence of legionella antigens, other legionella tests will usually be performed before a diagnosis is confirmed. Sputum sampling is a common method for confirming legionella infection. To perform this test, the infected person provides a sample of sputum, also known as phlegm. This sample is sent to a lab, where it is examined to determine whether legionella bacteria are growing within it. While sputum sampling can be among the most accurate legionella tests, it takes longer to produce results than urinary antigen screening.

Multi-stage blood testing is also sometimes used to confirm a legionella infection. This form of testing involves taking a blood sample as soon as possible following suspected infection by these bacteria, and then taking a second sample several weeks later. The level of legionella-fighting antibodies in these samples is then compared. A notably higher level of antibodies in the first blood sample may verify a suspected legionella infection. As with sputum sampling, while this type of blood testing can be accurate, it takes time to produce results.

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