Fibrinogen and D-dimer are by product following the process of fibrin clot breakdown. Both fibrinogen and D-dimer represent the activation of coagulation process[11]. Studies have looked into the relationship between the coagulation process and inflammation process. Coagulation biomarkers have pro-inflammation effect: Fibrin mediates inflammation process and D-dimer promotes neutrophil & monocyte activation. Persistent inflammation process also contributes to hyper coagulable state[12, 13].
Based on the AUC of each biomarkers, Fbg(0.89), CRP(0.90) and ESR(0.88) exhibited similar diagnostic value, while D-dimer(0.79) exhibited poor diagnostic value comparing to other biomarkers. The sensitivity and specificity of D-dimer were much lower than that of Fbg, CRP and ESR. Previous researches have reported conflicting results towards D-dimer and its role in the diagnosis of PJI. One assumption is that serum and plasma D-dimer exhibit different diagnostic performance. In serum sample, D-dimer is measured after standardized coagulation. Cross-linked fibrin degradation products are in the blood before standardized coagulation and are mostly included in the sample after coagulation. The fibrin degradation product remained in the serum sample would add to part of serum D-dimer measured, which may result in higher level of serum D-dimer comparing to plasma D-dimer. However, The effect of remained fibrin degradation product in serum sample could not be quantified[14–16]. Paniccia et al[14] found serum D-dimer was significantly higher than plasma D-dimer in the majority of pregnant women. They found no correlation between serum D-dimer and plasma D-dimer. To our knowledge, there is no study reporting the difference between serum and plasma D-dimer in arthroplasty populations. Current studies reported limited diagnostic value of plasma D-dimer and controversial results regarding to serum D-dimer. 2 studies[8, 17] reported limited diagnostic value of plasma D-dimer comparing to CRP and ESR. Shahi et al[3] reported promising results of serum D-dimer, while Pannu et al[18] and Huang et al[7] reported limited diagnostic value of serum D-dimer.
Racial difference is considered one of the reason for different diagnostic value of D-dimer in different studies. However, based on the data from previous researches, studies published in USA reported controversial outcome on the diagnostic value of D-dimer[3, 18], so did studies published in China[7, 8, 17, 19].
Plasma Fbg showed comparable diagnostic performance comparing to CRP and ESR. Our result is in consistent with the results from Li et al[8]. Fibrinogen has been found to play several key roles in antimicrobial host defense. Fibrinogen can limit growth and dissemination of bacteria within infected tissue and can support activation of host immune cells.[20]. In peritoneal infection, fibrinogen can contain Staphylococcus aureus and other pathogens[21].
Fbg and D-dimer did not exhibit better diagnostic performance than CRP and ESR in chronic PJI. There were 21 cases with chronic PJI in our series. Among them, Fbg was positive in 12 cases. D-dimer was positive in 13 cases, CRP was positive in 11 cases, ESR was positive in 13 cases. There were 5 PJI cases with negative culture results. Fbg, D-dimer and CRP were positive in 2 cases, ESR was positive in 4 cases. Coagulas-negative Staphyloccus were identified in 9 cases, Fbg and CRP were positive in all cases, D-dimer was positive in 7 cases, ESR was positive in 8 cases. Unlike our series, Shahi et al[3] reported that D-dimer was positive in 17 of 19 PJI cases with negative culture result, performing better than CRP and ESR.
Patients’ comorbidities and demographic characteristics could influence the results of each biomarkers. In our study, patients with comorbidities were not excluded. Each PJI case were matched to 2 Aseptic cases based on comorbidities and demographic characteristics using propensity score matching. No significant difference was found in sex, age, height, BMI and comorbidities between the two groups. Propensity score matching was utilized to eliminate bias caused by confounding factors. Another benefit of matching is to gain more efficiency in studies with a small sample size.
Our study has several limitations: 1. due to the rare incidence of PJI, we did not acquire enough cases to further analyze the diagnostic performance of biomarkers in different subtypes of PJI. 2. 4 cases were excluded because no suitable matches were found, which further limited the sample size of our study 3. there were significantly more knee cases in the PJI group than in the Aseptic group, which may lead to potential bias.