Outcomes in children with provoked venous thrombosis and antiphospholipid antibodies: findings from the Kids-DOTT trial

Low-titer and/or transient antiphospholipid antibodies (aPLs) can be seen in 2% to 28% of healthy children and are particularly common after infection or vaccination.^1,2 These antibodies are thought to be of minimal clinical relevance in patients without venous thromboembolism (VTE).³ Prior work has shown that aPLs are also common in children presenting with first-time or recurrent vascular thrombosis.^4,5 However, the prevalence and potential clinical importance of aPLs in children and young adults with VTE remains unclear and understudied; this is especially true for those with provoked VTE.

Accordingly, we aimed to prospectively determine the incidence of transient/low-titer aPL and persistent aPLs meeting the criteria for antiphospholipid antibody syndrome (APS) in children with acute provoked VTE; and to compare clinical outcomes (ie, development of symptomatic recurrent VTE and/or anticoagulant-associated clinically relevant bleeding [CRB]) between children with transient/low-titer aPLs and APS.

We performed a prespecified analysis of the National Heart, Lung, and Blood Institute–sponsored Multicenter Evaluation of the Duration of Therapy for Thrombosis in Children (Kids-DOTT) randomized clinical trial (RCT; ClinicalTrials.gov identifier: NCT00687882) on the duration of anticoagulant therapy in patients aged <21 years with a first-episode of acute provoked VTE.⁶ The Institutional Review Board at each participating site approved the study. Written informed consent was obtained in all cases.

The trial’s design and eligibility criteria have been previously reported in detail.^6,7 In summary, patients were enrolled within 30 days after diagnostic confirmation of a provoked VTE; managed with anticoagulant agents in accordance with international guidelines^8,9; and followed for 1 year for the development of symptomatic recurrent VTE (defined as radiologically confirmed VTE at a new site or progression at the index site occurring ≥10 days after incident VTE diagnosis) and CRB (defined as major and clinically relevant nonmajor bleeding).¹⁰ aPL testing was obtained in all participants at enrollment and at 6 weeks after VTE diagnosis in those with an initial positive aPL; subsequent testing in patients whose aPLs persisted at 6 weeks after VTE diagnosis was performed at the discretion of the primary hematologist. Patients with negative aPLs at enrollment (or if positive at enrollment, then negative at 6 weeks) were randomized to a 6-week vs 12-week course of anticoagulant therapy. Patients with positive aPLs at 6 weeks were included in a nonrandomized, parallel cohort arm in which a minimum anticoagulant duration of 3 months was prescribed.^6,7 

aPL testing included the following: (1) a lupus anticoagulant (LA) testing method meeting the International Society on Thrombosis and Haemostasis (ISTH) criteria¹¹ (≥1 of LA-sensitive activated partial thromboplastin time, dilute Russell’s viper venom time, or hexagonal phase phospholipid assay [StaCLOT LA]); (2) anti-cardiolipin immunoglobulin M (IgM); and (3) anti–β-2-glycoprotein-1 (aβ2GPI) IgG and IgM. All aPL testing was performed at the local clinical laboratory at participating sites during the study. Testing was not blinded, and clinicians had access to the results to inform clinical decisions.

Descriptive statistics were used to summarize data on patient characteristics and outcomes of interest. Prespecified patient groups were classified as follows: (1) transient or low-titer aPL, which included patients with positive aPL at enrollment but negative on repeat testing at follow-up or patients with persistent (≥2 occasions at least 12 weeks apart) low-level positive aPL titers not meeting revised Sapporo criteria for APS¹²; and (2) APS, which included patients with persistent aPL on ≥2 occasions at least 12 weeks apart meeting revised Sapporo criteria for APS.¹² Continuous variables were summarized as means with standard deviations, and their distributions were compared between groups using independent t test or Mann-Whitney U test, as appropriate. Categorical variables were summarized as counts with percentages and compared between groups via χ² test or Fisher exact test, as appropriate. Associations between aPL status (transient/low titer vs APS) and outcomes were also determined by univariable logistic regression with Firth penalized likelihood approach to obtain odds ratios (ORs) and 95% confidence intervals (CIs). All statistical analyses were performed with SAS version 9.4. All statistical tests were 2-sided, and a P value < .05 was considered statistically significant.

Of the 532 patients in the Kids-DOTT RCT, 524 (99%) underwent aPL testing at enrollment, and 116 (22%) tested positive (Figure 1). Patients with positive aPL at enrollment had a higher frequency of infection as their provoking VTE factor than those with negative aPL (28% vs 39%, respectively; P = .019). They were also older (7.6 vs 9.8 years, respectively; P = .087) and had a lower frequency of catheter-associated VTE (56% vs 47%, respectively; P = .068), although these differences were not statistically significant (Table 1).

On follow-up testing, 70 patients (60%) had a transient (n = 66) or low-titer aPL (n = 4), 11 (10%) met the criteria for APS, and 35 (30%) had no repeat testing (Figure 1). Patients without repeat aPL testing (n = 35) were younger than patients with repeat aPL testing at follow-up (n = 81; 11.5 vs 5.7 years; P = .039) and had a higher proportion of positive aβ2GPI IgG antibodies at baseline (9% vs 26%; P = .014) and a lower proportion of positive dilute Russell’s viper venom time at baseline (88% vs 57%; P = .020). There were no other statistically significant differences between the groups (supplemental Table 1).

Patients meeting the criteria for APS were older than those in the transient or low-titer aPL group (15.8 vs 9.9 years; P = .014). They also had a higher proportion of females and lower rates of catheter-associated VTE, although these differences were not statistically significant (females, 64% vs 43% [P = .41]; catheter-associated VTE, 27% vs 43% [P = .51]; Table 2).

Table 1 shows the aPL characteristics at the time of enrollment. LA was the most common aPL detected. Among patients with positive aβ2GPI IgG and/or IgM (n = 27), 4 (15%) had titers >40 SGU or SMU, whereas none of the patients with positive anti-cardiolipin IgM (n = 17) had titers >40 MPL. Of the 11 children in the APS group, 9 had single-aPL positivity (LA, n = 5; aβ2GPI IgG/IgM, n = 4), 1 was dual-positive, and 1 patient had triple-positive aPL.

Patients in the APS group had a statistically significant higher risk of symptomatic recurrent VTE than those in the transient or low-titer aPL group (18% vs 1%; OR, 12.2; 95% CI, 1.4-108; P = .025) and a statistically nonsignificant but clinically meaningful difference in the risk of anticoagulant-related CRB (9% vs 0%; OR, 20.1; 95% CI, 0.7-558; P = .077).

This observational of the Kids-DOTT trial demonstrates positive aPL acutely in nearly one-quarter of children and young adults presenting with a first episode of provoked VTE, with ∼10% of these patients eventually meeting the criteria for APS. Patients fulfilling ISTH APS criteria appear to have a statistically significant and clinically meaningful increased risk of symptomatic recurrent VTE compared with patients with transient or persistent low-titer aPL. They also exhibited a nonstatistically significant albeit clinically meaningful increased risk of anticoagulant-associated CRB; however, the small sample size likely limited the power to detect a difference. Given that the overall number of events for both outcomes of interest was small, these findings should be interpreted with caution and warrant further investigation. Nevertheless, they have potentially important implications for the field of pediatric provoked VTE treatment, in that they suggest that current approaches to anticoagulation duration and/or intensity in children with acute provoked VTE and persistent aPL positivity may be suboptimal, with a potentially increased risk of symptomatic recurrent VTE in those who eventually meet the criteria for APS.

The incidence of aPL positivity (22%) at the time of an acute provoked VTE episode observed in our trial is similar to that reported in prior smaller retrospective studies.⁷ Most aPLs identified in our cohort were transient, low titer, and resolved within the first 6 to 12 weeks after VTE diagnosis. Despite being present at the time of an acute VTE episode, we suspect that these aPLs are immunologically different from aPLs present in APS, may be expressed as a result of the primary VTE-provoking condition, and are likely nonpathogenic. Our findings suggest specific characteristics that may help in the early identification of those children who will eventually meet APS criteria including older age, female sex, and non–catheter-associated VTE. These findings are consistent with those recently reported in a single-institution retrospective cohort study investigating risk factors for recurrent VTE in children with APS.¹³ 

Although previous studies have shown that pediatric patients with VTE with APS have risks of recurrent and/or progressive thrombotic events as high as 19% to 29%, most of these studies have not distinguished between those with provoked and unprovoked index VTEs; yet, this is an important distinction, because children with unprovoked VTE in general have a higher risk of recurrence than those with provoked VTE.^14,15 Adult data have indicated that the risk of recurrent VTE after a provoked VTE is similar between patients with aPL positivity and those with negative aPL, suggesting no significant clinical impact of aPL positivity.^16,17 We report both a statistically significant and clinically meaningful increase in the risk of symptomatic recurrent VTE among pediatric patients with APS after an episode of provoked VTE, suggesting that (unlike in adults) aPL positivity may contribute to recurrence risk.

The strengths of this study include the prospective follow-up of a well-defined, multinational population of children and young adults with objectively diagnosed VTE and the systematic approach to the diagnosis of VTE outcomes using ISTH guidelines.¹⁰ A limitation of the study is the fact that decision on performing subsequent aPL testing after the 6-week post-VTE time point was left to the discretion of the treating hematologist, such that approximately one-third of patients with persistent aPL positivity at 6 weeks after VTE diagnosis did not undergo further aPL testing. Although this attrition could have potentially affected our findings, this risk is mitigated by our demonstration that the clinical characteristics of these patients were similar to those of the retained cohort. Notwithstanding this potential limitation, this observational analysis demonstrates that aPL positivity is frequent in patients aged <21 years with acute, first-episode, provoked VTE, and those who meet the criteria for APS appear to have an increased risk of symptomatic recurrent VTE with current conventional approaches to anticoagulation. Future collaborative studies should investigate the optimal management of young patients with provoked VTE who have persistent aPL.

The authors thank the patients and families for their generous participation in the Kids-DOTT multicenter trial. The authors thank the members of the Kids-DOTT Executive Steering Committee and the Kids-DOTT Trial Investigators.

The Kids-DOTT trial was supported by a U01 grant (1U01HL130048; Clinical Coordinating Center and Data Coordinating Center) from the National Institutes of Health (NIH), National Heart, Lung, and Blood Institute (NHLBI); and received support from an American Society of Hematology Bridge Grant, an Institutional Research Grant from the Johns Hopkins All Children's Foundation, an NIH NHLBI K23 award (1K23HL084055), and a Hemophilia and Thrombosis Research Society Thrombosis Studies Award.

Contribution: M.B. and N.A.G. designed the study; M.B., C.T., A.V., R.B., T.N., N.E.K., C.N., G.W., L.R.B., and N.A.G. performed patient enrollment and data collection; M.M. and E.A. performed data analyses; M.B. and N.A.G. performed the interpretation of findings and drafted the manuscript; and all authors provided critical revisions and approved the manuscript.

Conflict-of-interest disclosure: The authors declare no competing financial interests.

A complete list of the Kids-DOTT Trial investigators and Kids-DOTT Executive Steering Committee members appears in “supplemental Appendix.”

Correspondence: Marisol Betensky, Johns Hopkins All Children's Hospital, 501 6th Ave S, St. Petersburg, FL 33701; email: marisol.betensky@jhmi.edu.