The antiphospholipid syndrome (APS) was first described in 1983 [1], and the recent Consensus Conference held in Sydney in 2006 quite clearly defines laboratory (antiphospholipid [aPL] antibodies—assays and cut off values) and clinical criteria for the diagnosis of APS syndrome [2]. Defining these criteria is more closely related to research purposes than to clinical practice, thus avoiding enrollment of patients with aspecific types of positivity into clinical trials or laboratory studies.

aPL antibodies have been widely confirmed as a strong risk factor for both thrombotic events (venous and arterial) and for serious pregnancy complications. Recent experimental data show that aPL positivity (first hit) is a necessary but not sufficient condition to trigger a thrombotic event, and that a “second hit” is necessary to induce a clinical manifestation of the syndrome.

The definition of “syndrome” requires an “a posteriori” laboratory diagnosis, i.e., after a clinical event has already been diagnosed and objectively documented. In this setting, there is considerably good agreement concerning therapeutic strategies for secondary prevention, even though many open questions still remain.

An unsolved issue is whether or not to treat asymptomatic patients with aPL confirmed positivity (primary prevention). This aPL positivity is (sometimes) a coincidental finding that is found in these patients in the absence of clinical events.

Two well-documented and evidence-based papers, respectively by Gerosa et al. [3] and Finazzi [4], were published in Internal and Emergency Medicine. The former supports the effectiveness (in some cases) of primary aspirin thromboprophylaxis in asymptomatic, aPL-positive patients, while the latter argues for a lack of effectiveness.

The prevalence of thromboembolic events in asymptomatic aPL-positive patients seems to be relatively low, ranging from 0 to 3.8%/year [5–7]. Few studies are available in this clinical setting, and they usually include small cohorts of patients.

Primary prevention strategy has to take into consideration that the efficacy of aspirin must outweigh the negative, aspirin-related side effects (mainly bleeding) in order to obtain a clear benefit. Finazzi’s paper emphasizes that the risk of aspirin-related bleeding in patients with a thrombotic event rate of 1%/year or less (as is the case for asymptomatic aPL patients, according to some reports) would not outweigh the clinical benefits (reduction of thrombotic events). This benefit would become appreciable in a population with an event rate of 3%/year or more.

Different event rates have been reported in two prospective, cohort studies on asymptomatic aPL-positive patients. In the first study [5], an event rate of 0%/year is found in patients who have no associated autoimmune diseases; however, these patients were all treated with specific prophylaxis during high risk periods such as surgery. An event rate of 3.8%/year is found in a second study [7] enrolling asymptomatic aPL patients with systemic lupus erythematosus (SLE).

Event rates taken from the Italian Registry on aPL are comparable.

Ninety-eight patients were enrolled in the APLASA study (anti-phospholipid antibody acetylsalicylic acid) [8] and randomized to receive placebo or low doses of aspirin (81 mg/day). No differences were observed in the two groups with regards to the event rate.

Unfortunately, the above-mentioned studies have two major limits, the first is the small sample size, the second is that they do not take into consideration associated risk factors for thrombosis, if present. The relevance of associated risk factors in asymptomatic aPL patients has clearly been emphasized in the paper by Meroni et al. These additional risk factors cannot be ignored in clinical practice, and are listed below:

  1. 1.

    Associated autoimmune diseases: both SLE and rheumatoid arthritis must be considered conditions that significantly increase thrombotic risk. In particular, SLE itself is an independent risk factor for thrombosis.

  2. 2.

    Associated inherited pro-thrombotic conditions: factor V Leiden mutation and G20210 prothrombin mutation have a high prevalence (about 2%) among the Italian population, and must be considered an additional risk factor for deep venous thrombosis.

  3. 3.

    “Conventional” risk factors: older age, hypertension, diabetes, hypercholesterolemia, cigarette smoking, and obesity must be taken into account as additional risk factors.

  4. 4.

    Another unsolved issue mentioned by Meroni et al., concerns the most appropriate prophylactic strategy for aPL-positive women who do not fulfill the Sydney criteria since they have “only” had one or two miscarriages. In this setting, ASA proves to be effective at significantly reducing non-pregnancy-related vascular thrombosis [9].

Moreover, the laboratory pattern of aPL positivity itself must be evaluated. Category I positivity (the presence of any combination of more than one laboratory criteria) shows a stronger correlation with thrombotic events than other categories of positivity (LA or ACA or anti beta 2 GP1 positivity alone) [2].

In conclusion, each asymptomatic patient with confirmed aPL positivity has a different risk profile, and this profile must be carefully evaluated. Prospective studies (which are difficult to carry out because of the relatively small number of patients) including thrombotic risk stratification are needed. Available data do not recommend treating all asymptomatic, confirmed aPL-positive patients with aspirin. Risk profiles must be drawn up for each patient, and the patient subset having a “higher risk” profile (likely with a thrombotic event rate above 3%/year), must be taken into consideration for aspirin prophylaxis.