Explanation

Cardiolipin IgM antibodies: what is measured?

Cardiolipin is a fat-like substance (phospholipid) in cell membranes. This test measures only the IgM class of the antibodies against cardiolipin. IgM is the antibody the body makes first and that often disappears again; IgG stays in the blood longer. A raised IgM against cardiolipin is therefore fairly often temporary, for example during or shortly after an infection.

What can it detect?

Support when antiphospholipid syndrome is suspected, a condition in which the blood clots more easily
Possible explanation for thrombosis without a clear trigger or for repeated miscarriages, usually together with the other tests

Result

What does the result mean?

Positive result

A positive result means that IgM antibodies against cardiolipin were found. Of the two classes, this is the least meaningful one: a raised IgM is often temporary and disappears again after an infection. According to the Dutch Society for Clinical Chemistry (NVKC), the test is therefore repeated after about 12 weeks. Only a persistent and clearly raised IgM counts, and even then only together with thrombosis or a pregnancy problem. Without such events, a positive result usually means nothing.

Negative result

A negative result means that no IgM antibodies against cardiolipin were found. This does not rule out antiphospholipid syndrome: someone can have only IgG antibodies, or only lupus anticoagulant or antibodies against beta-2 glycoprotein I. So after thrombosis without a clear cause, further testing is sometimes still needed.

After an abnormal result

After a positive result, the test is repeated after about 12 weeks, according to the Dutch Society for Clinical Chemistry (NVKC), to see whether the antibodies persist. If they do and there has also been thrombosis or a pregnancy problem, the GP refers you to an internist (specialist in internal medicine); there, lupus anticoagulant and antibodies against beta-2 glycoprotein I are also measured. According to the Dutch medical specialists' guideline (FMS), a positive result without thrombosis or a pregnancy problem is no reason to start blood thinners.

Reliability

How reliable is the test?

The result is mainly meaningful after thrombosis or repeated miscarriages. Of the three tests for antiphospholipid syndrome, an isolated raised IgM against cardiolipin is the weakest signal; it is often temporary and also occurs without disease. In people without thrombosis or pregnancy problems, a positive result is therefore much more often a false positive than a true one. According to the Dutch medical specialists' guideline (FMS), the result only counts if it is positive again after about 12 weeks and clearly raised.

What can affect the result?

A temporary positive result is common during or shortly after an infection, with some medicines and at older age; the Dutch Society for Clinical Chemistry (NVKC) therefore advises repeating the test after about 12 weeks.
Only a persistently positive result with a clearly raised amount counts; a slightly raised value measured once says little.
Cardiolipin antibodies are only one of the three tests for antiphospholipid syndrome; the other two are lupus anticoagulant and antibodies against beta-2 glycoprotein I.
Laboratories use different methods and cut-off values; results from different labs cannot simply be compared.

Who it is for

Who is the test useful for?

Useful for

Useful when a doctor has a specific question: thrombosis at a young age or in an unusual place, thrombosis that comes back without a trigger, repeated miscarriages or other pregnancy problems, or a known autoimmune disease such as lupus. In people without such events, the predictive value is low: a positive result then usually does not mean that you have antiphospholipid syndrome, and no treatment follows.

How common is it?

Antiphospholipid syndrome is rare. Cardiolipin antibodies are found much more often than the syndrome occurs: also in healthy people, temporarily after infections and more often at older age. Thrombosis itself is fairly common, but usually has another cause, such as surgery, sitting still for a long time, pregnancy or the pill.

NVKC and the FMS guideline on antithrombotic policy; no Dutch prevalence figure for APS has been verified

Drawbacks and risks

A positive result without thrombosis and without pregnancy problems is common and usually means nothing. This can lead to unnecessary worry about thrombosis and to repeated tests without anything being found.
A negative result does not rule out antiphospholipid syndrome, because lupus anticoagulant and antibodies against beta-2 glycoprotein I are measured separately.

Get tested