2.1 Diagnostic results
A total of 67 patients with CBDS were included in 80 patients with occulted CBDS, and 13 patients without CBDS after operation. Fifty-eight patients with CBDS and 22 patients without CBDS were diagnosed via ERCP alone, with a diagnosis rate of 88.75%. Fifty-seven patients with CBDS and 14 without CBDS were diagnosed via ERCP combined with CRP, CEA and IL-6 in serum, with a diagnosis rate of 98.75%. The diagnosis rate of a combined way was clearly elevated versus ERCP alone (P < 0.05), as shown in Figure 1, 2.
2.2 Elevated CRP, CEA and IL-6 exist in serum of CBDS patients
Versus patients without CBDS, CRP, CEA and IL-6 in serum in CBDS patients were apparently up-regulated (P < 0.05) (Table 1, Figure 3). CRP, CEA and IL-6 in serum might be involved in the pathological process of CBDS.
2.3 ROC curve analysis
ROC curve analysis manifested that there was a high value of CRP, CEA and IL-6 levels in serum in the diagnosis of CBDS in patients with occult CBDS (P < 0.05) (Figure 4). CRP, CEA and IL-6 levels in serum could be used as a reference for the diagnosis of CBDS.
CBDS is one of the most general clinical diseases of the digestive system and is also one of the most principal gallstone diseases. It is statistical that CBDS accounts for more than 40% of gallstone diseases [17,18]. There are the triad of abdominal pain, chills, high fever and jaundice in most CBDS patients, but no symptoms are taken place in 30% ~ 35% CBDS patients, clinically calling occult CBDS, thus increasing the difficulty of diagnosis and treatment of patients . It is manifested in a clinical study that regardless of symptoms, patients with CBDS should take timely diagnosis and treatment for prevention of further deterioration of the disease and occurrence of complications such as biliary tract infection, which may endanger the health and even lives of patients . Nowadays, clinically preoperative hematologic examination with ultrasound, MRCP, EUS, CT and other non-invasive examinations are mainly employed for occult CBDS, and the results of surgical examination are taken as the gold standard. Low sensitivity and specificity are manifested in preoperative examination, which reveals certain differences with the results of postoperative gold standard examination.
The contrast agent is injected into the common bile duct through dodecoscopy in ERCP technology, and X-ray is applied for photography to finally reveal the pancreatic bile duct, and the technology has been widely applied in the diagnosis of a variety of pancreatic bile duct diseases and minimally invasive surgical treatment due to its characteristics such as less trauma, time period and fewer complications, manifesting a high clinical application value [21-23]. Sethi Set al.  found that ERCP diagnostic technology is applied to the diagnosis of suspected choledocholithiasis, achieving a good diagnostic effect, helping to define the criteria of low, medium and high-risk choledocholithiasis. Whereas the study of Borgosz J et al.  suggested that ERCP technology in the diagnosis of CBDS may be affected by bile duct deposits, resulting in misdiagnosis. However, few clinical studies have discussed the diagnostic value of ERCP technology in occult CBDS, therefore, ERCP checking with CRP, CEA and IL-6 test in serum was combined in this study to observe its diagnostic value in patients with occult CBDS, thus finding a more reliable diagnostic method for patients with occult CBDS.
Inflammatory activity is one of the crucial reactions involved in occult CBDS lesions, and clinical diagnosis and evaluation of occult CBDS and its prognostic effect can be conducted by detecting inflammatory factors  . CRP in serum is an acute protein synthesized by the body liver, and its level is obviously increased when the body is infected or injured, effectively reflecting the inflammatory activity state in the body. Although CEA in serum is often applied in the early diagnosis and assessment of colon and rectal cancer and other cancers, its pathological up-regulation can also be seen in the inflammatory diseases of hepatobiliary pancreas such as colitis, pancreatitis, cirrhosis and hepatitis, effectively suggesting the level of inflammatory activity in patients with such diseases. Usually secreted by fibroblasts and T cells, IL-6 is a multifunctional inflammatory factor affecting the growth of all kinds of cells in the body, and can produce a large number of inflammatory cells in the active state to accelerate the process of inflammatory response [27-30]. In this study, CRP, CEA and IL-6 detection in serum were combined with ERCP to observe their combined value in the diagnosis of occult CBDS.
The research results manifested that, versus ERCP alone, an elevated diagnosis rate exerted in ERCP combined with CRP, CEA, and IL-6 in serum. ERCP may be influenced by operation or patients' personal factors, producing certain errors in diagnosis. However, as the main inflammatory indicators in the body, CRP, CEA and IL-6 in serum can effectively reflect the inflammatory activity in patients with occult CBDS, especially in the biliary tract. Therefore, in this study, the diagnosis rate of a combined method was obviously elevated versus ECRP alone. In addition, in the study, CRP, CEA and IL-6 levels in serum were further compared between patients with CBDS or without, revealing that CRP, CEA and IL-6 in serum were distinctly upregulated in patients with CBDS. In addition, there were high specificity and sensitivity of CRP, CEA and IL-6 in the diagnosis of CBDS in patients with occult CBDS, applied as their reference indicators. The enhance of the inflammatory activity via the presence of CBDS in biliary tract in patients with occult CBDS causes apparently unregulated CRP, CEA and IL-6 in serum. Clinically, the diagnosis rate of patients with occult CBDS could be alleviated via the combination of CRP, CEA and IL-6 in serum with ERCP or CT, MRCP, EUS and ultrasound, helping patients get timely diagnosis and treatment.
All in all, ERCP combined with CRP, CEA and IL-6 examination in serum manifest a high value in the diagnosis of occult CBDS, and is supposed to be widely promoted and applied in clinical practice. There are still some limitations in this study. For example, owing to the limited research conditions and the limited sample size of the research object, more reliable conclusions cannot be gained from large-scale data. In future studies, the time period can be extended or more hospitals can be joined to expand the sample size and obtain more reliable results.