3.1 General characteristics of the patients for 1HNMR analysis
The information regarding patients with liver failure is provided in Table 1. The data regarding the healthy control group are provided in Table S1. The group treated by ALSSs comprised 54 males and three females, and the control group comprised 46 males and four females. All the subjects in the ALSSs group satisfied the diagnostic criteria for liver failure as described in 2012 in the Clinical Practice Guideline for Liver Failure promulgated by the Liver Failure and Artificial Liver Group, Chinese Society of Infectious Diseases, Chinese Medical Association. The exclusion criteria comprised: presence of mental or neurological disorders; allergic constitution; inability to tolerate blood products during ALSSs; or an incomplete record of liver failure due to various factors. The ALSSs-treated and control groups were comparable as they exhibited no statistically significant differences in age or gender (P>0. 05).
The PTA of the serum samples taken after ALSSs treatment were observed to have significantly increased (28.87 ± 0.98 before treatment vs. 38.21± 2.44% after treatment, P<0.001). In contrast, the MELD score and levels of total bilirubin (TBIL), ALT and AST in serum after ALSSs treatment were markedly decreased. No significant differences were observed in serum creatinine levels or INRs between samples taken before and after ALSSs treatment.
3.2 PCA and PLS-DA analyses of 1H NMR data
In the research described in this paper, 1HNMR data were processed by total normalization and creatinine normalization before multivariate analysis. The scores plots of PCA and OPLS-DA are shown in Figure 1. The results show that the total normalization data regarding the healthy group and that of the ALSSs-treated group both pre- and post-treatment overlaps in the PCA scores plot (Figure 1a). However, the scores plot of OPLS-DA generated from the creatinine-normalized data show partial separation according to group (Figure 1d). The accuracy and predictability of the established OPLS-DA model were evaluated by use of three parameters (R2X, R2Y and Q2Y). The values of R2Xcum, R2Ycum, and Q2 in the OPLS-DA model based on total normalization were 0.302, 0.408, and 0.378, respectively, whereas those in the model based on creatinine calibration were 0.512, 0.428, and 0.414. Thus, the OPLS-DA model based on creatinine normalization was found to show superior predictive power compared with that based on total normalization. The results of the sevenfold cross-validation and permutation test (200 times) show a value of R2 of 0.488 and a value of Q2 of -0.13, indicating that the model is not over-fitted and is effective (see Figure S1) [27].
3.3 Changes of metabolites in liver-failure patients after ALSSs treatment
A total of 21 serum metabolites in 1HNMR spectra were identified and confirmed (Table 2) by comparing their chemical shifts and coupling patterns with the corresponding values according to the method described in a previous paper [26]. Our results show significant differences between samples taken before and after ALSSs treatment in the ratios of serum leucine to creatinine, isoleucine to creatinine, acetate to creatinine, alanine to creatinine, creatine to creatinine, taurine to creatinine, and lactate to creatinine (Independent-sample t-test, P < 0.05). Figure 2a shows that variables 15 (taurine:creatinine), 16 (creatine:creatinine), and 21 (lactate:creatinine) were closely correlated with ALSSs treatment. In addition, it can be clearly seen that these three ratios returned to normal levels after ALSSs treatment (Figure 3).
3.4 ROC analysis of metabolites in patients before and after ALSSs treatment
Analysis of receiver-operating characteristics (ROC) was conducted before and after ALSSs treatment, and the identified metabolites were used as variables. The results are provided in Table 2. The AUROC values of the creatine:creatinine ratio, taurine: creatinine ratio, and lactate:creatinine ratio were 0.633, 0.644, and 0.650, which were higher than the MELD scores (AUROC=0.593) and prothrombin activity (PTA, AUROC=0.562, Table 1). Therefore the lactate:creatinine ratio was found to show the highest diagnostic efficiency regarding ALSSs treatment.
3.5 Correlation of lactate:creatinine ratio with survival of liver-failure patients
Of 105 inpatients with liver failure, 48 survived for more than six months after ALSSs treatment (the survival group), 24 died within three months of treatment, and 33 died within one month of treatment. These data were employed for retrospective analysis. The clinical data are presented in Table 3. The results (Figure 4a) show that the survival group exhibited a lactate:creatinine ratio before treatment of 0.038±0.002 (n=48), the group who died within three months had a lactate:creatinine ratio of 0.048±0.005 (n=24) and the group who died within one month had a corresponding ratio of 0.052±0.005 (n=33). These data indicate that the groups of patients who died showed higher lactate:creatinine ratios than the group of patients who survived for more than six months. The correlation analysis shows that the lactate:creatinine ratio is negatively correlated with survival period of liver-failure patients (r = - 0.26, p = 0.001).
In addition, the result shows that the correlation coefficient between the serum lactate:creatinine ratio and PTA was - 0.186 (p = 0.025, Figure 4b).
3.6 Use of lactate:creatinine ratio and PTA to predict one-month survival of liver-failure patients after ALSSs treatment
A total of 81 inpatients with liver failure (the early to middle stage before ALSSs treatment) who received ALSSs treatment, were included for retrospective cohort analysis in this study. Of these, 49 survived for more than six months, while 32 died within one month after treatment. The results show that the lactate:creatinine ratio before treatment for those who died within one month (0.057±0.004, n=49) was markedly higher than that of those who survived for more than six months (0.035±0.002,n=32). The AUROC (95% confidence intervals) of the lactate:creatinine ratios for potential diagnosis of those who could be expected to survive more than six months against those who would be expected to die within one month was 0.827 (0.735-0.918); the sensitivity was 81.3% and the specificity was 73.9% with the cut-off value of 0.041.
In comparison, the AUROCs (Figure 5) of the PTA and lactate data for potential diagnosis of the survival group were 0.628 (0.503-0.754) and 0.765 (0.661-0.869), respectively. The sensitivities of the PTA and lactate were 52.1% and 87.1%, and the specificities of the PTA and lactate were 75.0% and 61.7% with the cut-off values of 30.84% and 2.45 repectively. These results indicate that use of the lactate:creatinine ratio may be more reliable than measures of PTA and lactate to evaluate the prognosis of ALSSs treatment in liver-failure patients.