Explanation

Leptospirosis IgM antibodies: what is measured?

Leptospirosis is caused by Leptospira bacteria, which end up in surface water and mud through the urine of rats and mice. People become infected through small wounds or mucous membranes, for example when swimming, fishing, doing a mud run or working in the countryside. This test measures IgM antibodies: the type of antibody the immune system makes first. In leptospirosis, antibodies can usually only be detected from about the fifth to seventh day of illness.

What can it detect?

A recent or acute infection with Leptospira bacteria
The milder form, mud fever, and the severe form, Weil's disease, with jaundice and kidney failure

Result

What does the result mean?

Positive result

A positive IgM result fits an acute or recent leptospirosis, but is not a final verdict. According to the LCI guideline (RIVM), a positive result of a rapid test must always be confirmed with the reference tests of the expertise center, especially if the infection was caught in the Netherlands, because public health measures are also linked to it. Symptoms that fit are sudden fever, severe headache, muscle pain and stiffness; with a severe course, jaundice and kidney failure are added.

Negative result

A negative IgM result does not rule out leptospirosis. With rapid tests the sensitivity is limited, about 60 to 70 percent, and in the first days of illness there are often no antibodies at all yet. If someone has been ill for less than ten days, a PCR on blood is the appropriate test. If the suspicion remains, the guideline also advises confirmatory testing by the expertise center after a negative result, or a second blood sample taken two or more weeks later.

After an abnormal result

With a positive result, the laboratory sends the material on to the expertise center for confirmation; with matching symptoms, the doctor usually starts antibiotics, and with a severe course, hospital admission follows. Leptospirosis is a notifiable group C disease under the Dutch Public Health Act (Wet publieke gezondheid): the doctor or laboratory reports it to the GGD (municipal public health service), which traces the source if needed and involves the water authority.

Reliability

How reliable is the test?

According to the LCI guideline (RIVM) on leptospirosis, specific IgM antibodies in an ELISA point to an acute or recent infection, but paired blood samples are sometimes needed to complete the diagnosis. Of the rapid tests that different laboratories use, it is mainly the sensitivity that is limited, about 60 to 70 percent, so a negative result does not rule out an infection. A positive result always needs confirmation with the reference tests. In people without symptoms and without contact with surface water, mud or rodents, the chance of this rare infection is so small that a positive result is more often false than true.

What can affect the result?

Antibodies can only be detected from about the fifth to seventh day of illness (window period).
The sensitivity of rapid tests is limited, about 60 to 70 percent; a negative result does not rule out the infection.
A positive result must be confirmed with the microscopic agglutination test of the expertise center.
IgM can remain detectable for months after a past infection and then wrongly look like something recent.

Who it is for

Who is the test useful for?

Useful for

Useful with sudden fever with severe headache, muscle pain and stiffness, especially after contact with surface water or mud (swimming, fishing, a mud run, water sports) or after travel to a tropical area. Also in occupations with contact with water, mud or rats, such as farmers, gardeners, sewer workers and pest controllers. Not useful as a general check-up in people without symptoms: most infections are mild or go unnoticed and a positive result without symptoms does not change the management.

How common is it?

In the Netherlands, about 100 to 125 people with leptospirosis are reported each year; in 2023 there were 122, of whom 74 caught the infection in the Netherlands. That was the highest number of cases caught in the Netherlands in decades. It affects men more often than women, with a median age of 44, and most cases occur in summer and autumn. Of the cases caught in the Netherlands in 2023, over 90 in 100 had to be admitted to hospital.

RIVM/LCI guideline on leptospirosis

Drawbacks and risks

An unconfirmed positive result can lead to unnecessary antibiotic use and to measures at a swimming location that turn out not to have been needed.
A negative result can give false reassurance, because the rapid test misses about 3 to 4 out of every 10 infections.

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