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CRP as a trigger for venous thromboembolism
Table 5. Association of C-reactive proteina with risk of venous thromboembolism.
Hazard period (H) compared to individual control periods (C1-C4)
All cases
All cases
H versus C1 βb (95% CI)
0.65 (0.22-1.08) OR (95% CI) 1.92 (1.26-2.95)
H versus C2 βb (95% CI)
0.77 (0.33-1.20) OR (95% CI) 2.15 (1.39-3.33)
H versus C3 βb (95% CI)
0.42 (0.11-0.73) OR (95% CI) 1.52 (1.11-2.08)
H versus C4 βb (95% CI)
0.60 (0.30-0.90) OR (95% CI) 1.82 (1.36-2.45)
OR: odds ratio; CI: confidence interval. aNatural log transformed C-reactive protein. bWhen multiplied by 100, β coefficients can be interpreted as percentage difference com- pared with the reference group.
ically also affect the underlying acute-phase stimulus.31 CRP is commonly used as a marker of inflammation in clinical practice. Since our study was based on clinical data, other markers of inflammation and coagulation were not available as these were only occasionally meas- ured. CRP is therefore well suited to serve as a marker of inflammation, a process linked to coagulation through several pathways as described above. In addition to its role in innate immunity and complement activation,31 CRP has been found to have prothrombotic effects in some studies.32-34 Due to methodological issues, especially the possibility of contamination of CRP preparations with bacterial lipopolysaccharides, controversy regarding a direct role of CRP in thrombosis still exists.35
Our study has both strengths and limitations. The case- crossover design is suitable for studying transient risk fac- tors, as potential fixed confounders are mainly controlled for through the design. Further, the VTE-cases were derived from a large, population-based cohort with high attendance rate, and all VTE events were symptomatic and validated. All hospital care in the region is provided by a single hospital, facilitating the completeness of the VTE registry. However, some VTE cases might have been clinically diagnosed and treated without hospital contact, and some cases of PE presenting as sudden death might have been misclassified. As each subject serves as his or her own control, such potential cases would most likely not affect our results. Our study was limited to informa- tion from hospital records, as we did not have access to data from general practice. In most cases, a high CRP level measured in general practice will increase the likeli-
hood of the patient to be referred to hospital. We cannot find any reason why hospital referral praxis should differ in hazard versus control periods. Our data source was the hospital medical records of VTE-cases, and CRP measure- ments were made according to each clinician’s prefer- ence. As each case serves as his or her own control in the case-crossover design, only those who had a CRP meas- urement in both the hazard period and in one or more control periods were included in the analyses. We can assume that CRP levels were lower in periods where the clinician did not find a reason to have CRP measured than in periods where CRP was measured. As more CRP meas- urements were done in hazard than in control periods, this might have diluted the risk estimates. Sensitivity analyses where missing CRP values were set to 5 mg/L resulted in higher risk estimates than in the main analy- ses. We did not adjust for therapeutic agents that could affect CRP-levels, such as statins, steroids and other immunosuppressive drugs, as we wanted to investigate the impact of inflammation on VTE regardless of the cause of the inflammation. CRP levels measured the last two days before the date of VTE were omitted to avoid reverse causation. However, we cannot be certain that elevated CRP was not caused by the incident VTE more than two days before the date of VTE diagnosis. To address this, we performed sensitivity analyses excluding CRP measurements the last seven days before the date of VTE-diagnosis, and the results were essentially similar.
In conclusion, acute inflammation assessed by CRP was a trigger for VTE in this case-crossover study, also in cases with inflammatory triggers other than infection.
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