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was 71 years, and 53.6% were women. Moreover, 416 (58.8%) VTEs were not related to recent hospitalization, 135 (19.1%) occurred during hospitalization, and 156 (22.1%) were diagnosed within 30 days after hospitaliza- tion (Table 1). Infection was the most common risk factor in the hazard period, recorded in 267 (37.8%) of the peri- ods, followed by immobilization (31.4%), and cancer (24.3%). The distribution of VTE risk factors and triggers in the hazard- and control periods are shown in Table 2. Prophylactic treatment with low-molecular weight heparin was prescribed in 138 (19.5%) of the 707 hazard periods, and in 78 (2.8%) of the 2828 control periods.
missing CRP values were set to 5 mg/L, the estimated VTE risk per unit increase in ln-CRP was 2.4-fold increased (OR 2.36, 95% CI 2.14-2.61) (Online Supplementary Table S2).
When comparing CRP levels in the hazard period to each control period (C1-4) separately, there was no trend for change in CRP level according to time between con- trol and hazard period (the CRP level was increased by 65%, 77%, 42% and 60%, respectively) (Table 5). The estimated risk of VTE by ln-CRP was 1.9-fold increased when comparing the hazard period with C1, 2.2-fold increased when compared with C2, 1.5-fold increased when compared with C3, and 1.8-fold increased when compared with C4.
Discussion
In this case-crossover study including 707 incident VTEs, we found that acute inflammation, assessed by increase in CRP, was a trigger for VTE. The association remained after adjustment for immobilization and for infection. In stratified analyses, inflammation assessed by CRP was associated with increased risk of VTE also in cases without infection. The strength of the estimated risk of VTE by CRP remained similar when separately compared to the different control periods.
Clinically, DVT often presents with the cardinal signs of inflammation; i.e., redness, swelling, heat, pain and disturbance of function. In a case-control study investi- gating inflammatory markers, patients with DVT had sig- nificantly higher levels of inflammatory markers, includ- ing CRP, than controls.20 Based on these results the authors suggested that inflammation was a consequence rather than a cause of VTE. However, the fact that inflammation is a consequence of VTE does not exclude the possibility that inflammation can be a cause of the disease. Previous prospective studies on the association between inflammation and VTE have shown conflicting results. In a cohort of healthy men followed for more than 8 years, baseline hs-CRP was associated with increased risk of arterial but not venous thrombosis.9 Similarly, two long-term population-based cohorts reported no association between inflammatory markers and VTE.7,8 In contrast, studies with shorter follow-up time have shown an association between CRP and inci- dent VTE. In a population-based case-cohort study with 515 VTE-cases and 1505 controls, an association between
Table 2. Triggers and risk factors for venous thromboembolism.
In total, after exclusion of hospital contacts the last two days before VTE-diagnosis, 1283 hospital contacts were registered during the hazard period and control periods (Table 3). The number of hospital contacts was higher in the periods closest to the VTE, increasing from 165, 172, 187 and 199, respectively, in the control periods, to 560 contacts in the hazard period. CRP was measured in 298 cases during the hazard period, and in 75, 72, 86 and 96 cases during the four control periods. Median CRP was highest in the hazard period (CRP 107 mg/L), and ranged from 7 mg/L to 16 mg/L in the control periods (Table 3).
Based on β coefficients for ln-CRP obtained from logis- tic regression analyses, the mean CRP level was 58% (95% CI 39-77%) higher in hazard than in control peri- ods. After adjustment for immobilization, the CRP level was 51% (95% CI 31-70%) higher, and after adjustment for infection the CRP level was 40% (95% CI 19-61%) higher in the hazard than in the control periods (Table 4). This corresponded to a 1.8-fold increased estimated VTE risk per unit increase in ln-CRP (OR 1.79, 95% CI 1.48- 2.16), which were only slightly attenuated after adjust- ment for immobilization (OR 1.66, 95% CI 1.37-2.02) and for infection (OR 1.50, 95% CI 1.21-1.85).
In analyses stratified for infection, the mean CRP level was 57% (95% CI 20-94%) higher in the hazard versus control periods in those without infection and 44% (95% CI 1-87%) higher in those with infection. Estimated increase in VTE risk according to a one-unit increase in ln- CRP was 1.8-fold (OR 1.77, 95% CI 1.22-2.57) in those without infection and 1.6-fold in those with infection (OR 1.55, 95% CI 1.01-2.38). Adjustment for immobiliza- tion revealed similar results (Table 4).
Sensitivity analyses restricted to CRP measurements that were conducted more than 7 days before the date of VTE-diagnosis yielded essentially similar results (Online Supplementary Table S1). In sensitivity analyses where
Table 1. Characteristics of study participants at the time of venous thromboembolism (VTE) diagnosis.
Triggers/risk factors
Infection; n (%) Immobilizationb; n (%)
Cancer; n (%)
Surgery; n (%)
Trauma; n (%)
Central venous catheter; n (%)
Hazard period (n=707)
267 (37.8) 222 (31.4) 172 (24.3) 118 (16.7) 71 (10.0) 56 (7.9)
Control periods (n=2828)a
107 (3.8) 57 (2.0) 375 (13.2)c 88 (3.1) 25 (0.9) 17 (0.6)
CRP as a trigger for venous thromboembolism
Median age; years ± SD Female sex; n (%)
DVT only; n (%) PE+/-DVT; n (%)
VTE during hospitalization; n (%)
VTE within 30 days after hospitalization; n (%)
No hospitalization the last 30 days before VTE; n (%) DVT:deep vein thrombosis,PE:pulmonary embolism.
71 ±14 379 (53.6) 408 (57.7) 299 (42.3) 135 (19.1) 156 (22.1) 416 (58.8)
a707 cases, four control periods for each case; bbedrest ≥3 days, ECOG 4, other immo- bilizing factor (e.g.,wheelchair use); cbased on 106 unique individuals with cancer in one or more of the control periods.
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