How is antiphospholipid syndrome (APS) diagnosed?
Antiphospholipid syndrome (APS) can only be diagnosed if:
- you have a positive blood test and
- you've suffered either thrombosis or miscarriage.
More and more people who have thrombosis or a miscarriage are routinely tested for APS. If you’ve had either of these problems, especially if they’ve happened more than once, you should discuss with your doctor whether you need these blood tests. Depending on the results, your doctor may want to refer you to a specialist (either a rheumatologist or haematologist).
What tests are there?
There are three main blood tests used to diagnose APS. These are:
• the anticardiolipin test
• the lupus anticoagulant test
• the anti-beta-2-glycoprotein I test.
All three tests detect whether aPL are present in the blood. The result of the lupus anticoagulant test is either positive or negative, while the results of the other two tests are given as numbers. The higher this number is, the more aPL a person has in their blood. Because these tests measure aPL in different ways, around 20% of people with APS will have a negative result in one test or the other. One test alone could therefore miss the diagnosis.
The tests are usually repeated after 12 weeks as levels of aPL vary and can sometimes go up when you have an infection. If you only have one positive test and it quickly becomes negative again then you probably don’t have APS.
Higher levels of antibodies (i.e. higher numbers in the anticardiolipin or anti-beta-2-glycoprotein I tests) suggest you may be at greater risk of blood clots and other symptoms. Being positive in more than one of the three tests also suggests a higher risk.
I repeatedly test positive for aPL. Does this mean I'll definitely get APS?
No, it doesn't mean that you’ll definitely develop APS. Many people who have lupus are tested for these antibodies and about 20–30% will be positive for aPL. People who carry the antibodies but who’ve never had either clots or miscarriages aren't said to have APS but to be aPL-positive.
Doctors have to decide whether people who are aPL-positive are at high or low risk of getting APS in the future. There’s no foolproof way of deciding, but factors that may help include:
- how high the aPL level is
- how many of the three tests are positive
- whether there are other risk factors for thrombosis
- whether you have other typical symptoms (such as migraine).
Am I being tested for lupus?
No, this confusion often arises because one of the blood tests for APS is called the “lupus anticoagulant” test. This is because it was first invented by doctors who were studying patients with lupus. In fact it is a test for APS, NOT a test for lupus. There are other, better blood tests for lupus itself and many people who are positive in the “lupus anticoagulant” test do not have lupus.