Rapid detection of bacteria in the ascitic fluid and early diagnosis of ascitic infection is the key to improve the survival of cirrhotic patients with ascitic fluid .
The current study investigated the diagnostic ability of PCT, CRP, dCHC, sNFI and WBC, and showed that when threshold values of PCT, dCHC and sNFI were taken into consideration by calculation of a bioscore value, this could be considered a statistically significant diagnostic score for ascitic fluid. The bioscore demonstrated impressive accuracy for early diagnosis of ascitic fluid infection. These biomarkers are all tested in blood. Compared to simply measuring WBC in ascitic fluid, the biomarkers will provide a new and feasible choice for clinical practice.
In the present study, 42.1% of patients with cirrhosis had ascitic fluid infection, without infection elsewhere; this prevalence is comparable with previous reports  . It is noted that no patient had bacterascites in the current study. Actually, we believed that the study of biomarkers for diagnostic accuracy of bacterascites was more complex than that of culture-positive SBP and culture-negative SBP, because it was still controversial whether bacterascites require a prompt initiation of antibiotherapy [3, 28]. The clinical diagnosis and antibiotherapy of bacterascites may divide into different groups according to the further ascites neutrophils count and ascites culture. And in order to get more powerful judgments, we suggested that the study of biomarkers for diagnostic accuracy of bacterascites needs further results based on clinical diagnosis and also a large number of cases.
Culture was positive in 51/259 (19.7%) of the sample; this is lower than the incidences previous reports [29,30,31,32]. E. coli, K. pneumoniae, and other Enterobacteriaceae are most likely to cause SBP by bacterial translocation . Diverse organisms were isolated in our culture positive patients but, consistent with earlier reports, the most frequently isolated were E. coli (29.4%), S. epidermidis (21.6%), and K. pneumonia (13.7%).
Various biomarkers have been tested for their potential for use in rapid screening tests for infection [33,34,35,36,37]. The discriminative capabilities of PCT and CRP for ascites infection in patients with cirrhosis in this study are in line with but a little lower than previous reports, where the AUCs of PCT have ranged from 0.89 to 0.94 (sensitivity 30–95%; specificity 70–98%) and that of CRP from 0.75 to 0.78 (sensitivity 64–75%; specificity 61.2–95%) [24, 27, 38, 39]. This difference between the studies is mainly due to differences in the study population. The AUC of CRP was lower than that of PCT in all groups, which finding is also consistent with previous studies . Additionally, previous research has suggested that while the basal level of CRP in cirrhotic patients is higher than in non cirrhotic patients, the degree of increase in CRP is less when liver function is impaired during infection. Thus, it appears that CRP is relatively less diagnostic value of infection in patients with advanced cirrhosis [40, 41]. Accordingly, it may explain why the significant lower AUCs for CRP (0.645–0.676) in the current study for diagnosis of ascitic fluid infection. WBC is a traditional marker for infection. Our data support the view of some authors that peripheral blood WBC has little value for diagnosing SBP .
Unlike WBC count, which relies on increased cells numbers to respond to infection, new biomarkers of sNFI directly reflects the inflammatory activity of both existing and increased WBC. dCHC involved in the early innate immune response and the reflected bone marrow production of innate immune cells. Hematologic indices (HI) were frequently abnormal in patients with cirrhosis especially with decompensated cirrhosis . The baseline levels of sNFI and dCHC may have been affected by abnormal HI and varies in degree. However, a previous study found that only 3% of cirrhosis patients had abnormal bone marrow biopsy results . And we speculated that in ascites infection, the levels of sNFI and dCHC in patients with cirrhosis were unaffected or only partially affected by cirrhosis. In the current study, both dCHC and sNFI showed good diagnostic performance in diagnosing ascitic fluid infection in patients with cirrhosis. The AUCs of dCHC and sNFI compared with that of PCT, 0.837 and 0.838, respectively. Of note, using the best cut-offs, dCHC showed high sensitivity /low specificity of 92.5%/70.0% and sNFI showed low sensitivity/high specificity of 77.5%/90.1%.
The complementary sensitivity/specificity profiles of each marker allowed the construction of a new bioscore which more discriminating and highly specific than each single component. Multivariate analysis found statistical significance of PCT, dCHC and sNFI, and the AUC of the total bioscore using those three markers was 0.937. The sensitivity and specificity of the bioscore were improved to 92.6% and 95.3%, respectively (Additional file 4: Table S2). The present study addressed that multi-marker approach may be an aid for the diagnosis of ascitic fluid infection.
It is known that complications are more frequent in culture-positive SBP than in culture-negative SBP and it may be involved in process and severity of ascites infection . Therefore, the ability of biomarker for diagnosing ascitic fluid infection was studied separately in culture-positive and culture-negative groups. In the current study, 51 patients had culture-positive SPB and 58 had culture-negative SBP. The same as PCT [4, 45], we found a lower diagnostic value of dCHC, sNFI and bioscore in culture-negative SBP patients than those in culture-positive SBP patients. Our population was similar to our latest finding that dCHC and sNFI may associate with different mechanisms of ascitic infection.
Some relatively new biomarkers (lipopolysaccharide-binding protein, ascites leukocyte esterase activity, lactoferrin, and bacterial DNA) were useful for diagnosing infection [33,34,35,36,37]. Compared with them, PCT, dCHC and sNFI are available in most of the hospital laboratories, and their combination in an easily computable score could improve the accuracy of ascitic infection diagnosis in patients with cirrhosis. In each group of patients, including culture-positive SBP and culture-negative SBP group, this method provided a more reliable diagnostic score for ascitic infection patients with cirrhosis.
The bioscore could easily be incorporated into clinical practice, as the bioscore has the following advantages. Firstly, the biomarkers are all tested in blood. It means that compared with diagnostic paracentesis, haematological examination is less traumatic and patient compliance is better. Accordingly, haematological indicators analysis compared to ascitic fluid test is conducive to the routine monitoring of ascites infection. Secondly, the diagnostic value of this bioscore in ascites infection is higher than that of PCT and CRP, and these two commonly used ascites infection indicators. In addition, dCHC and sNFI can be available directly from the WBC detection. This means that the score does not increase any instrument compared with PCT. In particular, the bioscore is of great value for early diagnosis of ascites infection and avoiding unnecessary diagnostic paracentesis. If the bioscore of patient was showed positive, the patient is likely to have infection and requires paracentesis. The bioscore was especially clinical significant for the early diagnosis in culture-negative SBP group. Totally, 94.7% of cirrhotic patients with ascites can be directly identified as ascites infection by this score. More valuable is that, if negative by this bioscore, although 9.5% of cirrhotic patients with ascites used by the bioscore as false-negatives would have been observed, more than 90% of cirrhotic patients with ascites can exclude ascites infection, thus can be spared paracentesis for diagnosis. As a score that can be calculated only by PCT and WBC detection, it is very feasible for screening ascites infection that does not need diagnostic paracentesis. Furthermore, in the current study, the CV of each of those markers showed a lower value ranged between 2.0 and 2.6%, which mean that the assay has very small variability and high reproducibility.
This study has some limitations. This is a single-center study and should be considered as a pilot study defining the new bioscore and its cutoff values for diagnosis of ascites infection in patients with cirrhosis. As the development of a diagnostic scale is only the first step in research; the next steps would be external validation using the same cut-off points. Accordingly, another independent cohort of cirrhotic patients with ascites should validated the bioscore. Furthermore, external validation also should contain multicenter studies with larger samples to confirm our findings. As we know, different laboratories may have different study populations, ascitic fluid culture conditions and culture methods, even the transportation and storage of specimen. The validation of multicenter studies will bring the bioscore more power and make the results more generalizable.