Introduction
Approximately 19 million fatalities worldwide in 2020 were linked to cardiovascular disease (CVD), representing a rise of 18.7% compared to the numbers recorded in 2010 [
1]. As a result, preventing CVD is crucial for maintaining public health, as CVD substantially affects nations with high medical costs and economic burdens [
2,
3]. Atherosclerosis is caused by lipid buildup in large arteries. Inflammatory pathways are activated as a result of endothelial dysfunction, leading to plaque enlargement, necrotic core formation, and plaque calcification [
4] The dysfunctional endothelial cells promote lipid infiltration and leukocyte adhesion, further exacerbating the inflammation and contributing to the progression of the condition [
5‐
9]. CRP is an inflammation-associated protein [
10] and is primarily synthesized in hepatocytes. It exhibits a brief lifespan of approximately 18–20 h [
11,
12]. In non-inflammatory states, CRP release rate is slow but secreted rapidly after an increase in inflammatory cytokines level, most notably Interleukin 6 (IL-6), Interleukin 1 (IL-1), and Tumor necrosis factor- α (TNF-α) [
13,
14]. The American Heart Association has recommended the inclusion of high-sensitivity C-reactive protein (hs-CRP) in the overall assessment of cardiovascular risk [
15]. Over time, this concept has evolved, and in 2010, the American College of Cardiology Foundation advised that evaluating CRP levels is a sensible approach for individuals at intermediate risk of cardiovascular events [
16]. Coronary artery calcification (CAC) is an advanced feature and indicator of atherosclerosis [
17]. It performs better than any cardiovascular risk factor and adds predictive value to traditional equations [
18]. CAC can happen via the active or passive route; inflammation is critical for both courses [
19]. CAC score is usually calculated by Agatston’s method, and it has been widely used to evaluate the risk of an acute coronary event [
20,
21] It is generally known that inflammation causes atherosclerotic plaque instability and promotes the expression of osteogenic factors, which leads to the differentiation of local cells into osteoblast-like cells and calcification. Subsequently, anti-inflammatory factors reduce the expression of osteogenic factors [
22‐
24]. Although inflammation and calcification may seem to work together, the truth is more complicated. Lee H et al. showed that hs-CRP is significantly associated with CAC progression among clinical parameters. Still, the association disappears after adjusting for traditional risk factors [
3]. Oh et al. discovered a significant difference in hs-CRP levels among high-risk subjects with CAC scores > 300 [
25]. On the other hand, Zeb et al. reported an insignificant association between hs-CRP concentrations and the CAC progression [
26]. This study intends to explore the clinical evidence concerning the prognostic significance of CRP as an inflammatory index in assessing the likelihood of CAC development.
Methods
Registration and protocol
The present systematic review and meta-analysis followed the meta-analysis of observational studies in epidemiology (MOOSE) guidelines (Supplementary Appendix
1). The protocol was registered in the International Prospective Register of Systematic Reviews PROSPERO (CRD42021242397).
Databases and search strategy
Until October 2023, four databases, including Pubmed, Scopus, Embase, and Web of Science, were systematically explored to find relevant studies. The population, exposure, comparison, and outcome (PECO) protocol was followed, with participants with symptomatic or asymptomatic CVD as the target population. CRP as the exposure, and CAC as the primary or secondary outcome. Our search was conducted with the MeSH terms of Coronary Artery Calcification, C-Reactive Protein, Coronary Artery Disease, Acute Coronary Syndrome, Ischemic heart disease, ST-elevation Myocardial Infraction, Non-ST Elevation myocardial infraction, Stable Angina, Non-Stable Angina, and Myocardial Infraction. The retrieved studies’ reference lists were examined to find any missing pieces.
Eligibility criteria and study selection
The two investigators assessed the titles and abstracts of the retrieved records before searching the full text of documents for those intended to fulfill our inclusion criteria. In addition, we requested papers by email twice where the full text was not available, and in case we did not receive an answer, the article was excluded. A third author was enlisted to create certain conclusions to settle disagreements. We included observational studies, including case-control, cohort, and cross-sectional studies, without considering language or publication date. Human research was assessed to determine the relationship between CRP and CAC scores in patients with CVD. The following were excluded: technical reports, conference papers, case reports, animal research, and review articles. Studies involving participants with diseases other than CVD were also excluded.
Data abstraction and quality assessment
Two reviewers extracted data independently and sorted by first author, year, country, age, population, sample size, effect size (OR, RR, HR) with confidence intervals (CI), study outcome, and results. NOS was used to assess the study’s methodological quality (Supplementary Appendix
2) [
27]. A maximum of 9 points were awarded based on selection (4 points), comparability (2 points), and outcome/exposure (3 points) assessment. Scores under 4 indicate low quality, 4–6 moderate, and more than 6 indicate good study quality. All included studies were assessed by two investigators and verified by a third member. Discrepancies in score allocation were resolved by consensus.
Statistical analysis
We examined the relationship between CRP and CAC using odds ratio (OR) to estimate the effect size. We conducted a random-effects meta-analysis employing the Der-Simonian and Laird method to calculate the combined OR and 95% CIs. Using a random-effects meta-analysis allowed us to consider conceptual and clinical heterogeneity among the studies. We used a forest plot to illustrate the ORs and their respective 95% confidence intervals. To assess the heterogeneity across the studies, we used the I2 statistics, where an I2 value of 50% or higher suggests significant heterogeneity. Additionally, we employed Cochran’s Q statistic with a significance threshold of P < 0.10 to indicate heterogeneity among the studies.
To test the consistency of the results and robustness of the pooled estimates, we conducted sensitivity analysis systematically, removing specific studies or groups of studies (Supplementary Appendix
3). Furthermore, we performed a subgroup meta-analysis to examine the relationship between CAC and CRP based on how CRP was measured, whether in milligrams per deciliter (mg/dl) or Logarithm of milligrams per Liter (log mg/L) and gender. To assess the publication bias, we visually examined funnel plots where we plotted log odds ratios (log ORs) against their standard errors to measure study precision. Also, we conducted Egger’s regression asymmetry test and Begg’s adjusted rank correlation test to evaluate publication bias statistically. We used two-tailed statistical tests, and the significance level was considered less than 0.10. All statistical analyses were conducted using Stata version 14 software developed by STATA Corp. in College Station, TX, USA.
Discussion
Our systematic review and meta-analysis examined 15 observational studies, including 12 cross-sectional and three cohort studies, to describe the relationship between CRP and CAC. The majority of the studies have stated no association between CRP and CAC. Nevertheless, several studies indicated a statistically significant correlation between CRP and CAC, whether in univariate or multivariable analysis. Moreover, our results showed no significant role of CRP in risk stratification for CAC score, supporting a different pathophysiology approach of CRP and CAC development. Therefore, additional prospective cohort studies must be conducted, including well-designed methodologies and recruiting healthy individuals. To validate the association, these studies should examine the relationship between CRP and CAC at baseline and follow-up stages.
Measuring CAC can be costly and involve radiation exposure. Recent studies showed the potential role of novel biomarkers as a prognostic value in predicting CAC score [
75‐
79], suggesting novel pharmacological targets in reducing CAC burden.
The recent guidelines from the American Heart Association emphasize the importance of performing CAC testing for predicting cardiovascular events and stratifying CVD risk. This underscores the significance of CAC as a diagnostic marker in CVD [
80‐
84].
CRP is a useful marker in assessing inflammation because it rapidly increases in concentration following a stimulus, reaching its peak around two days later, and takes several days to return to baseline levels, indicating the duration and intensity of the inflammatory response [
85‐
90].
Local vascular inflammation is linked to calcification. It’s widely recognized that inflammation within atherosclerotic lesions plays a significant role in triggering the rupture of these plaques [
23].
Local factors, including inflammatory cytokines stimulate the transformation of vascular smooth muscle cells (VSMCs) into osteogenic cells. On the other hand, unsaturated fatty acids like eicosapentaenoic acid can counteract this process by reducing the expression of factors related to bone formation [
24]. Clinical evidence has established a direct connection between vascular inflammation and the development of calcification. Interestingly, positron emission tomography (PET) reveals that vascular inflammation tends to occur before calcification becomes visible through standard tomography [
91]. Furthermore, research findings suggest that inflammation in the vascular system can fluctuate over time, and each episode of acute inflammation might contribute to the progression of calcification. Importantly, from a clinical standpoint, it’s unclear whether calcification can trigger inflammation [
19].
Our nine included cohort studies found no significant association between CAC and CRP except for one, Lee H et al. In a retrospective observational cohort study, they investigated 1015 Korean subjects who underwent CAC scoring by computed tomography. During 39 months of follow-up, they found a significant association between CAC progression and CRP in men and not women. However, the association was not adjusted for additional risk factors. Also, male subjects were the predominant study population (80.6%), a big difference from the general composition of society [
3].
In thirty-two of our included cross-sectional studies, researchers found no association between CRP and CAC. In the remaining ten studies, however, Oh et al. investigated 456 participants from South Korea; subjects with a CAC score of ≥ 300 agatston units, in univariate regression analysis, found log hs-CRP to be significantly associated with the high CAC group [OR: 2.812 (1.600,4.942)]. Nevertheless, the study with cross-sectional design and no adjustment for conventional CVD risk factors, failed to verify the association between CAC and CRP [
25]. In another study, Fu et al. found an association between CAC and hs-CRP in multiple linear regression analysis. However, they only included subjects with complaints of chest pain in a cross-sectional manner, making it challenging to establish a clear causal link [
42]. In the research involving individuals without any evident cardiovascular issues, Wang et al. observed a connection between CRP levels and the presence of CAC in both male and female participants. This association remained significant even after considering age, individual conventional risk factors, and Framingham risk score. Nonetheelss, CRP was measured 4 to 8 years before conducting electron beam computed tomography; it’s possible that the relationship between CRP and the presence of CAC was influenced by the progression of atherosclerosis in individuals who had elevated CRP levels [
44]. The remaining cross-sectional studies found an association between CAC and CRP, but the association can not be generalized to the community. In this regard, Qasim et al. found a significant link between CAC and CRP levels in type 2 diabetic women. Conversely, there was no evident relationship between CRP and CAC in diabetic or nondiabetic men [
47]. Sung et al. found CAC score > 0 to be significantly associated with high CRP concentration levels in individuals with low HDL-C [
45]. Additionally, Quagli et al. discovered a distinct and independent relationship between CRP levels and the presence of CAC in older adults (aged 80 years or older) without underlying health issues [
46].
Based on the pooled estimation in our study, sub-group analysis by gender and CAC threshold revealed no significant relationship between CAC and CRP. To confidently address this issue, however, more research is needed, specifically focusing on gender-specific analysis.
Research conducted in animal models has yielded mixed findings. In a study by Paul et al., they observed a correlation between the presence of CRP in atherosclerotic plaques and enlargement in their size [
92]. On the other hand, Tennent et al., who worked with mice expressing transgenic human CRP, found no indications of heightened atherosclerotic buildup, increased complexity of these lesions, or occurrences of spontaneous thrombosis and plaque-related fatalities [
93].
The current systematic review comes with certain limitations. CRP concentrations were measured by traditional and highly sensitive methods, which can detect low levels of chronic inflammation suited for detecting CVD. This might lead to the heterogenicity in results.
The variability in recorded CAC cutoff values has led to heterogenicity in findings among different studies.
The studies exhibited differences in several aspects, including the ratio of males to females, the existence of other cardiovascular risk factors among participants, the presence of documented CVD, the duration of the follow-up period, and the average age of the subjects.
Therefore, conducting additional primary studies, particularly prospective cohort studies with larger sample sizes, while considering the shortcomings of previous research could help confirm the link between CRP and CAC score.
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