What Clinical Events Count Toward an APS Diagnosis?

When doctors evaluate you for antiphospholipid syndrome (APS), they look for two main things: persistently positive blood tests for antiphospholipid antibodies and at least one qualifying clinical event ^[1]. Historically, the main clinical events have fallen into two categories: objectively confirmed blood clots (thrombosis) and specific patterns of pregnancy complications ^[1][2].

It is important to understand that the strict lists of events—such as the classic Sapporo/Sydney criteria—were designed primarily to classify patients for research studies, not as rigid rules for clinical diagnosis ^[3].

A Note on “Persistently Positive” Blood Tests
An APS diagnosis requires a persistent immune response. This means testing positive for at least one of three specific antibodies: lupus anticoagulant (LA), anticardiolipin (aCL), or anti-beta-2-glycoprotein I (anti-β2GPI) ^[3]. Because infections or acute illness can cause temporary positives, the test must be positive on two separate occasions at least 12 weeks apart ^[4][5]. Additionally, being on blood thinners can interfere with lupus anticoagulant testing, so your doctor will need to interpret these results carefully ^[6]. Having a family history of APS is a risk factor but does not replace the need for positive blood tests or a clinical event ^[7].

Vascular Events (Blood Clots)

To qualify as a classic APS event under the Sapporo/Sydney criteria, a blood clot must meet specific requirements:

• Objectively confirmed: The clot must be proven through imaging (such as an ultrasound showing a deep vein thrombosis or a CT scan showing a pulmonary embolism) or by examining tissue samples under a microscope ^[1].
• No significant inflammation: If diagnosed via tissue sample, the pathology report must show a clot without significant inflammation of the blood vessel wall ^[8][9].
• Location: The clot can occur in veins (like in your leg), arteries (like in a stroke), or small blood vessels (microvascular) in any organ or tissue ^[1].
• Alternative causes: If a clot happens after surgery, during prolonged bed rest, or while taking estrogen, it might still satisfy the definition of a documented clot. However, your doctor will carefully consider whether the clot was truly caused by APS or by the other risk factors ^[10][11]. Having another inherited clotting disorder does not automatically prove or rule out APS.

Pregnancy Complications

Because APS can affect the placenta and its blood flow, specific pregnancy complications also count as qualifying events ^[2]. Under the classic criteria, these include:

• Late fetal loss: One or more unexplained deaths of a morphologically normal (structurally typical) fetus at or beyond the 10th week of pregnancy ^[2].
• Premature delivery: One or more deliveries of a morphologically normal baby before 34 weeks of pregnancy, specifically caused by severe preeclampsia, eclampsia, or placental insufficiency (when the placenta cannot provide enough oxygen and nutrients to the baby) ^[2][12].
• Recurrent early miscarriages: Three or more consecutive, spontaneous miscarriages occurring before the 10th week of pregnancy. For these to count, other explanations—such as maternal anatomical or hormonal issues, and parental chromosomal abnormalities—must be ruled out ^[2][13]. A single or two early miscarriages do not meet this specific threshold.

Classification Criteria vs. Clinical Diagnosis

The criteria described above (the Sapporo/Sydney criteria) rely on a simple checklist. Recently, researchers developed the newer 2023 ACR/EULAR criteria, which use a weighted point system that scores your specific lab results and clinical events (including heart valve disease and low platelets) to determine if you can be classified as having APS for research studies ^[3].

However, neither the classic nor the 2023 criteria are mandatory rules for how your doctor treats you in real life. In clinical practice, your doctor will look at the entire picture ^[14][15]. A doctor may still diagnose and manage your care if your specific experience does not perfectly match the strict research criteria, though a positive antibody test or a symptom alone (without a qualifying event) does not automatically establish an APS diagnosis ^[16]. They will evaluate your complete medical and obstetric history, the timing of events, imaging results, and bleeding risks to determine the best treatment plan for you.