Background:
Breast cancer incidence is now the highest among all cancers and accountable for 6.6% of all cancer-related deaths worldwide. Studies of the prognostic utility of plasma C-reactive protein (CRP) measurement in early-stage breast cancer have given discrepant results.

Methods:
We identified 6,942 patients in the Danish Breast Cancer Cooperative Group database with early-stage breast cancer diagnosed between 2002 and 2016 who had a measure of pretreatment plasma CRP. Outcomes were recurrence-free interval and survival for a period up to 10 years. We analyzed associations with plasma CRP using Fine-Gray proportional subdistribution hazards model with recurrence-free interval. Data on plasma CRP were analyzed per doubling of concentration and in relation to CRP levels of <3 mg/L, 3 to 10 mg/L, and >10 mg/L and stratified according to standard clinical parameters in sensitivity analyses.

Results:
A doubling of the plasma CRP concentration was associated with increased risk of recurrence (multivariate adjusted HR, 1.05; 95% CI, 1.01–1.08) and shorter survival (HR, 1.13; 95% CI, 1.09–1.16) in multivariate analyses. Survival was shorter in patients with plasma CRP levels of 3 to 10 and >10 mg/L versus <3 mg/L, with multivariate adjusted HRs of 1.30; 95% CI, 1.17–1.45 and 1.65; 95% CI, 1.39–1.95, respectively.

Conclusions:
Elevated plasma CRP measured before treatment in patients with early-stage breast cancer is an independent biomarker of increased risk of recurrence and early death.

Impact:
CRP measures before treatment might be used to individualize follow-up of patients with early-stage breast cancer.