Prognostic value of fibrinogen in patients with coronary artery disease and prediabetes or diabetes following percutaneous coronary intervention: 5-year findings from a large cohort study

Background Fibrinogen (FIB) is an independent risk factor for mortality and cardiovascular events in the general population. However, the relationship between FIB and long-term mortality among CAD patients undergoing PCI remains unclear, especially in individuals complicated with diabetes mellitus (DM) or prediabetes (Pre-DM). Methods 6,140 patients with CAD undergoing PCI were included in the study and subsequently divided into three groups according to FIB levels (FIB-L, FIB-M, FIB-H). These patients were further grouped by glycemic status [normoglycemia (NG), Pre-DM, DM]. The primary endpoint was all-cause mortality. The secondary endpoint was cardiac mortality. Results FIB was positively associated with hemoglobin A1c (HbA1c) and fasting blood glucose (FBG) in CAD patients with and without DM (P < 0.001). During a median follow-up of 5.1 years (interquartile range 5.0–5.2 years), elevated FIB was significantly associated with long-term all-cause mortality (adjusted HR: 1.86; 95% CI 1.28–2.69; P = 0.001) and cardiac mortality (adjusted HR: 1.82; 95% CI 1.15–2.89; P = 0.011). Similarly, patients with DM, but not Pre-DM, had increased risk of all-cause and cardiac mortality compared with NG group (all P < 0.05). When grouped by both FIB levels and glycemic status, diabetic patients with medium and high FIB levels had higher risk of mortality [(adjusted HR: 2.57; 95% CI 1.12–5.89), (adjusted HR: 3.04; 95% CI 1.35–6.82), all P < 0.05]. Notably, prediabetic patients with high FIB also had higher mortality risk (adjusted HR: 2.27; 95% CI 1.01–5.12). Conclusions FIB was independently associated with long-term all-cause and cardiac mortality among CAD patients undergoing PCI, especially in those with DM and Pre-DM. FIB test may help to identify high-risk individuals in this specific population. Supplementary Information The online version contains supplementary material available at 10.1186/s12933-021-01335-1.

Regarded as a pivotal component of coagulation as well as a biomarker of inflammation, fibrinogen (FIB) plays a crucial role in the pathophysiological process of thrombosis and atherosclerosis2-5. Previous evidences suggested that FIB was an independent risk factor of CAD development and cardiovascular events in the general population6, 7. Similar results on the prognostic value of FIB were also observed in patients with CAD8-10.
Glycemic metabolism abnormality, including DM and prediabetes (pre-DM), is increasingly prevalent on a global scale. By 2045, over 600 million individuals are projected to develop DM, with about the same number developing pre-DM11. The cardiovascular disease (CVD) risk, disability and mortality brought by glycemic metabolism abnormality is undisputedly a serious public health concern. Interestingly, FIB level was found to be higher in diabetic and prediabetic patients, and was involved in glycemic metabolism abnormality and insulin resistance12, 13. Moreover, recent studies reported that FIB was positively related with the glycemic metabolism (hemoglobin A1c [HbA1c] and fasting blood glucose [FBG]) in patients with acute coronary syndrome (ACS) or stable CAD9, 10. However, few data are available examining the correlation between FIB and glycemic metabolism in CAD patients undergoing percutaneous coronary intervention (PCI). Furthermore, the association of FIB with long-term outcomes in this population was far less investigated, particularly in those with impaired glycemic metabolism.
In light of the above, we aimed to evaluate the relationship between FIB and glycemic metabolism, and further determine the combined effect of FIB and impaired glycemic metabolism on long-term all-cause and cardiac mortality in CAD patients undergoing PCI.

Study population
This study was based on a prospective, observational, single-center cohort. From January 2013 to December 2013, 10,724 CAD patients were consecutively enrolled undergoing PCI at Fuwai Hospital, Chinese Academy of Medical Sciences (Beijing, China) (Fig. 1). Patients with missing FIB (n = 4431) and Low-density lipoprotein cholesterol (n = 153) values were excluded. A total of 6,140 patients were ultimately included in the analysis. The study protocol was approved by the Institutional Review Board of Fuwai Hospital and complied with the Declaration of Helsinki. All patients provided written informed consent before the intervention. Regular follow-up assessment of patients was performed at five time points (1-month, 6-month, 12-month, 2-year, and 5-year after the discharge). Follow-up data were collected through medical records and telephone interview. The primary endpoint was all-cause mortality. The secondary outcome was cardiac mortality. Mortality that could not be attributed to a noncardiac etiology was considered cardiac mortality. All endpoints were adjudicated centrally by 2 independent cardiologists, and disagreement was resolved by consensus.

Procedure and medications
Before the procedure, patients receiving selective PCI were treated with aspirin (300 mg) and ticagrelor (loading dose, 180 mg) or clopidogrel (loading dose, 300 mg), except for patients already on dual antiplatelet therapy; for patients with ACS receiving emergency PCI, the same dose of aspirin and ticagrelor or clopidogrel (loading dose, 300-600 mg) were administered as soon as possible. All patients were administered with unfractionated heparin (100 U/kg), and interventional cardiologist decided whether to use glycoprotein IIb/IIIa antagonist according to the clinical conditions and coronary lesions during the procedure. After the procedure, the dual antiplatelet therapy including aspirin (100 mg daily), ticagrelor (90 mg, twice daily) or clopidogrel (75 mg, daily) were recommended for at least 1 year. The choice of equipment and techniques during PCI was at the discretion of the physicians.

Definition of clinical status
Glycemic categories were based on the guideline recommendations of American Diabetes Association14. Diabetes mellitus (DM) was defined by HbA1c levels ≥ 6.5%, or fasting blood glucose (FBG) ≥ 7.0 mmol/L, or 2-h blood glucose levels of oral glucose tolerance test ≥ 11.1 mmol/L), or current use of hypoglycemic medications. Prediabetes (Pre-DM) was defined as nondiabetic patients with FBG ranging from 5.6 to 6.9 mmol/L, HbA1c levels ranging from 5.7% to 6.4%. Patients without Pre-DM or DM were defined as normoglycemia (NG). Hypertension was defined as systolic blood pressure ≥ 140 mmHg and/or diastolic blood pressure ≥ 90 mmHg and/or current use of antihypertensive drugs. Low-density lipoprotein cholesterol ≥ 3.4 mmol/L, fasting total cholesterol ≥ 5.2 mmol/L, triglyceride ≥ 1.7 mmol/L, high-density lipoprotein cholesterol < 1.0 mmol/L and/or chronic use of lipid-lowering drugs were considered criteria for dyslipidemia. Left main disease was defined as stenosis of ≥ 50% in left main coronary artery, and three-vessel disease was defined as stenosis of ≥ 50% in all three main coronary arteries (right coronary artery, left circumflex artery and left anterior descending artery) confirmed by coronary angiography.

Laboratory analysis
Fasting blood samples were drawn from all patients within 24 h after admission. Enzymatic hexokinase method was used to measure the concentrations of blood glucose. Tosoh Automated Glycohemoglobin Analyzer (HLC-723G8) was used to measure the HbA1c levels. Stago autoanalyzer with the STA fibrinogen kit (Diagnostica Stago, Taverny, France) was used to measure the concentrations of FIB. All other laboratory measurements were conducted at the biochemistry center of Fu Wai Hospital by standard biochemical techniques.

Statistical analysis
Continuous variables were presented as mean ± standard deviation, while categorical variables were presented as frequency and percentage. Differences of continuous and categorical variables were analyzed by analysis of variance or Kruskal-Wallis test and χ 2 test or Fisher's exact test, as appropriate. Pearson correlation and linear regression analysis were performed to evaluate the correlation between FIB and glycemic metabolism (HbA1c and FBG). In survival analysis, the association between FIB and clinical endpoint was initially examined using restricted cubic splines. The FIB was subsequently analyzed as both a continuous and a categorical variable. For categorical analysis, patients were grouped according to tertiles of the distribution [FIB-L(< 2.98 mg/dL), FIB-M(2.98-3.59 mg/dL), FIB-H(< 3.59 mg/dL)]. Survival distributions were presented by Kaplan-Meier curves and compared by log-rank test. Cox regression analyses were performed to calculate the hazard ratios (HRs) and 95% confidence interval (CI). Proportional hazards assumption was verified by Schoenfeld residuals. In multivariate Cox analyses, Fig. 1 Study flowchart. FIB, fibrinogen, LDL-C, low-density lipoprotein cholesterol, PCI, percutaneous coronary intervention covariates including age, sex, body mass index (BMI), hypertension, family history of CAD, prior PCI/CABG, LVEF, LDL-C, creatine, DES implantation, clopidogrel, ACEI/ARB were adjusted because of their statistical significance in univariate analysis or clinical importance. The prognostic impact of glycemic metabolism status (NG, Pre-DM and DM) for all-cause mortality was also assessed by using the model mentioned above. Patients were further divided into 9 groups by both FIB levels and glycemic metabolism status to calculate HRs for all-cause mortality using FIB-L plus NG as reference. Statistical analyses were conducted with SPSS version 25.0 (IBM Corp., Armonk, N.Y., USA), R Programming Language version 4.0.0 (R Core Team, 2014), and GraphPad Prism version 7.0.0 for windows (Graph-Pad Software, San Diego, California USA). A two-tailed P value < 0.05 was considered statistically significant.

Baseline characteristics of patients with different FIB levels
Among the 6,140 patients, the mean age was 58.4 ± 10.4 years, and 4771(77.7%) were male. The baseline characteristics of patients according to the tertiles of FIB are summarized in Table 1. Patients with higher FIB levels were older and less likely to be male (all P < 0.05). In addition, they had higher proportion of diabetes, hypertension, prior stroke, ACS, and left main or three-vessel disease (all P < 0.05). Moreover, patients with elevated FIB levels had higher FBG, HbA1c, hs-CRP, D-dimer, TC, LDL cholesterol, creatinine, lesion vessels, SYNTAX score, but lower LVEF and lower rate of complete revascularization (all P < 0.05). No significant differences were noted regarding dyslipidemia, family history of CAD, smoking, HDL cholesterol, number of stents and DES implantation among these groups (all P > 0.05).

Comparison of clinical data among groups with different glycemic metabolism status
In Table 2, Patients were divided into three subgroups based on different glycemic metabolism status. In general, the DM and pre-DM group had a less favorable cardiovascular risk profile. Patients with DM or pre-DM tended to be older and female, with a larger burden of concomitant diseases, such as hypertension, dyslipidemia and prior stroke compared with those in NG group (all P < 0.05). Additionally, the prevalence of prior PCI/ CABG and left main or three-vessel disease was higher in the DM and pre-DM group (all P < 0.05). Meanwhile, there were also higher BMI, FBG, HbA1c, TG, number of diseased vessels, SYNTAX score, and lower LVEF, HDL cholesterol, complete revascularization, DES implantation in the DM group (all P < 0.05). Notably, FIB levels were significantly elevated from NG to DM group (P < 0.001).

Predictive value of FIB on all-cause mortality and cardiac mortality
The median follow-up time was 5.1 years (interquartile range 5.0-5.2 years), and the response rate was 91.2% ( Fig. 1). During follow-up, 214 (3.5%) patients died, with 127 (59.3%) of whom from cardiac causes. Myocardial infarction (37.8%) was the most frequently reported cause of cardiac mortality, while malignancy (13.1%) was the most common cause of non-cardiac mortality (Additional file 1: Table S1).
The incidence of all-cause mortality in FIB-L, FIB-M and FIB-H group was 2.1%, 3.7% and 4.7%, respectively. Restricted cubic splines visualized a positive relation between FIB on a continuous scale with long-term risk of all-cause mortality and cardiac mortality (all P for non-linearity > 0.05) (Additional file 1: Figure S1). The Kaplan-Meier survival curve revealed that patients with higher FIB levels had significantly increased risk of all-cause mortality and cardiac mortality (all logrank P < 0.001) (Fig. 3a, Additional file 1: Figure S2a). The univariate Cox analysis showed a strong relation between continuous FIB with all-cause mortality (HR: 1.36; 95% CI 1.20-1.55 per 1 unit increase in FIB; P < 0.001) and cardiac mortality (HR: 1.46; 95% CI 1.25-1.71 per 1 unit increase in FIB; P < 0.001). On multivariate analysis, the relationship between continuous as  remained statistically significant after adjustment for potential confounders (Table 4).

Glycemic metabolism, FIB levels and occurrence of all-cause mortality
The prevalence of all-cause mortality in NG, Pre-DM and DM group was 2.4%, 3.3% and 4.7%, respectively. The Kaplan-Meier curve demonstrated that patients with DM had significantly increased risk of all-cause mortality and cardiac mortality among the three groups (all log-rank P < 0.05) (Fig. 3b, Additional file 1: Figure S2b). Univariate Cox analysis revealed that DM group had 1.91-fold higher risk of all-cause mortality (HR: 1.91; 95% CI 1.28-2.84; P = 0.001) and 2.18-fold higher risk of cardiac mortality (HR: 2.18; 95% CI 1.28-3.37; P = 0.004) when compared with NG group. And this significant association remained unchanged after adjustment for other covariates. However, Pre-DM group did not increase the risk of all-cause mortality and cardiac mortality compared with NG group (Fig. 4, Additional file 1: Figure S3). When patients were evaluated according to both glycemic metabolism and FIB levels, the Kaplan-Meier curve showed that those with DM and FIB-H levels had significantly highest risk of all-cause mortality compared with the reference group (NG plus FIB-L group, log rank P < 0.001). Furthermore, NG plus FIB-H, Pre-DM plus FIB-M, Pre-DM plus FIB-H and DM plus FIB-M groups also had significantly increased risk of all-cause mortality than the reference group (NG plus FIB-L group, all log rank P < 0.05) (Fig. 3c)

Discussion
Using a large, real-world, prospective cohort sample, we found that FIB positively correlated with glycemic metabolism in CAD patients undergoing PCI. Moreover, higher FIB levels, analyzed as continuous or categorical variables, were strongly associated with increased risk of long-term all-cause and cardiac mortality. Furthermore, poorer long-term outcomes were also found in diabetic patients, but not in prediabetic patients. Interestingly, when patients were categorized into 9 groups according to both FIB levels and glycemic metabolism status, patients with pre-DM plus high FIB levels, DM plus medium FIB levels and DM plus high FIB levels had increased risk of all-cause mortality than those with NG and low FIB levels. For the first time, our study demonstrated that FIB might affect the long-term prognosis in CAD patients with pre-DM undergoing PCI, and indicated a joint prognostic value of FIB levels and impaired glycemic metabolism on mortality in CAD patients undergoing PCI.
FIB is a crucial glycoprotein consisting of three different polypeptides, which is mainly synthesized in the liver15. Upon action of thrombin, FIB is transformed into fibrin monomer which then crosslinks platelets, increases blood viscosity and ultimately leads to clot formation3. Besides, FIB levels are elevated in response to various chronic inflammatory conditions, including DM, obesity and atherosclerosis7, 12, 16. It is also directly involved in the pathogenesis of atherosclerosis through multiple mechanisms, such as inducing endothelial dysfunction, stimulating smooth muscle cell proliferation and migration, facilitating monocyte or macrophage adhesion and infiltration of atherosclerotic lesions, which will jointly potentiate plaque evolution17.
To date, studies have been conducted on the prognostic value of FIB in different clinical settings. Aside from the positive association with all-cause and CVD mortality in general individuals6, 18, 19, FIB was reported to be an independent risk factor of the occurrence and severity of CAD20. Further, both small sample and large epidemiological studies showed that FIB was associated with worse clinical outcomes in patients with stable CAD10, 21, 22. A recent prospective study from China indicated elevated FIB was also strongly associated with MACE risk in ACS patients, especially when complicated with DM9. Similarly, the present study found FIB had an independent association with long-term all-cause and cardiac  mortality in CAD patients undergoing PCI. Instead, the ADVANCE study including 3,865 diabetic patients showed FIB was not an independent predictor of 5-year mortality. It is worth noting that only limited number of patients in the ADVANCE study underwent coronary revascularization, while in our study all patients were treated with PCI [23]. The PRIME study including 926 men aged 50 to 59 without CAD found that FIB was not a risk marker of MI-coronary death. The differences in endpoints and sample size might contribute to the difference in results between this study and ours24. The EPIC-Norfolk study including 16,850 participants who were free of cancer, MI and stroke at baseline found that FIB was not a predictor of all-cause and cardiovascular mortality. However, the data used in the EPIC-Norfolk cohort are rather old and the serum used for measuring FIB and other biomarkers was stored frozen for more than ten years, which might limit the reliability of the results 25. Notably, none of these studies focused only on CAD population. This may be another possible explanation for the controversy between these studies and our findings. In spite of the conflicting findings mentioned above, data from the latest clinical trials confirmed the benefit of anti-inflammatory effect both in patients with chronic coronary disease and acute MI26, 27. Considering the role of FIB as an inflammatory biomarker, additional Currently, glycemic metabolism abnormality including DM and pre-DM is prevalent in clinical practice, especially in patients with established CAD11. It has been previously demonstrated that DM independently increased the risk of adverse CVD events in CAD patients28. Notably, CAD patients with pre-DM seemed to share similar clinical outcomes with those with nor-moglycemia10, 29. However, when combined with other disorders, such as dyslipidemia or hypertension, prediabetic patients with CAD were demonstrated to have significantly less favorable prognosis30-32. Interestingly, a large-sample observational study recently reported that elevated FIB increased the MACE risk in patients with stable CAD only in the presence of DM and pre-DM, indicating FIB to be valuable for prognostic assessment in prediabetic patients with stable CAD10. However, the combined value of FIB and impaired glycemic metabolism on prediction of mortality in CAD patients undergoing PCI is still unclear. In this study, we observed that among CAD patients undergoing PCI, diabetic individuals with medium or high FIB levels had 2.57-fold and 3.04-fold higher risk of mortality respectively during a median follow-up of 5.1 years. Furthermore, prediabetic patients also had higher mortality risk in the subgroup of high FIB levels, indicating that FIB may be useful for further risk stratification in CAD patients with mild impaired glycemic metabolism after PCI.
Patients with DM were confirmed to have higher levels of plasma FIB12. Chronic mild inflammation is a recognized pathological mechanism of DM33. Elevated FIB existing in diabetic patients aggravates the inflammatory process and the burden of atherosclerosis4, 34. FIB also involves in insulin resistance and impair the normal glycemic regulation13. Moreover, elevated FIB could weaken platelet inhibition with clopidogrel in the presence of DM35. And this effect is mediated through its direct interaction with the GP IIb/IIIa receptor, which is independent from inflammation. Indeed, our study found that the average levels of FIB elevated from NG, pre-DM to DM. Moreover, FIB was also positively associated with glycemic metabolism (HbA1c and FBG) both in CAD patients with or without DM, which was basically consistent with the prior studies9, 10. Collectively, although without established causality, the present study revealed a significant association between FIB and glycemic metabolism, as well as the long-term mortality in CAD patients undergoing PCI. Given the relatively simple and cost-effective test of FIB, these findings encourage its potential value as a biomarker in this specific population to identify high-risk patients, especially in those with DM and pre-DM. Meanwhile, the importance of routine screening for impaired glycemic metabolism also cannot be neglected.
Another issue to be discussed is the potential significance of lowering FIB levels in this specific population. Previous evidence showed the contribution of some lifestyle factors such as smoking, sedentary behavior and unhealthy diet to the elevation of plasma FIB lev-els36-38. On the contrary, exercise training significantly reduced FIB levels and improved cardiorespiratory fit-ness39. Therefore, lifestyle modification for patients with high FIB levels to achieve clinically favorable outcomes may be a reasonable exploration. In addition to plasmin, several medications such as fibrates are known to reduce FIB levels as an additional effect40. However, to date, no medication can specifically reduce FIB levels in the long run. Future studies are warranted to investigate whether patients could benefit from pharmacologic intervention on the premise that specific drug targeting FIB on a longterm basis is discovered.
This study has some limitations. First, FIB data at baseline was not available in about 40% patients, which might impair the strength of our study. Second, glycemic evaluation to identify new-onset diabetes and prediabetes was not routinely performed during followup. Third, information on other coagulation parameters such as prothrombin time (PT), activated partial thromboplastin time (APTT) and thrombin time (TT)