In this study, we investigated whether NFI, one of the indices for liver fibrosis, might be associated with the cross-sectional activity of AAV and furthermore could predict the poor outcomes, particularly all-cause mortality in immunosuppressive drug-naïve patients with AAV. We found four valuable results as follows: firstly, NFI could reflect the cross-sectional neither activity of AAV based on BVAS nor acute-phase reactants including ESR and CRP. Secondly, when the cut-off of NFI at diagnosis was defined as the lower limit of the higher quartile, AAV patients with NFI at diagnosis ≥ 1.24 exhibited a significantly lower cumulative patients’ survival rate than those with NFI at diagnosis < 1.24. Thirdly, in the multivariable Cox hazards model analysis, NFI at diagnosis ≥ 1.24 could independently predict all cause-mortality during follow-up together with age, BVAS and interstitial lung disease at diagnosis. Lastly, AAV patients with NFI at diagnosis ≥ 1.24 had a significantly higher risk for all-cause mortality than those with NFI at diagnosis < 1.24 (RR 3.735).
When we assessed the frequency of affected organs based on the nine items of BVAS in our study population, the most common affected item was renal manifestation (60.0%), followed by pulmonary manifestation (57.1%) and ear nose throat (ENT) manifestation (44.8%). We wondered which specific organ involvement was associated with NFI, and found that only the frequency of ENT manifestation among nine manifestations exhibited a significant difference in NFI. Ninety-four AAV patients with ENT manifestation exhibited a significantly higher median NFI than 116 AAV patients without (0.74 vs. 0.50, P = 0.024). So, what mechanism made NFI higher in patients with ENT manifestation compared to those without? Although it has been controversial whether sinusitis might be a typical surrogate marker for GPA or not, it is clear that sinusitis is the most common symptom of ENT involvement of AAV [13]. We conducted the literature review on the association of sinusitis with the four variables that compose a formula of NFI and found a previous study reporting that serum albumin was reported to be statistically lower in patients with chronic rhinosinusitis compared to those without [14]. However, in this study serum albumin did not differ between AAV patients with ENT manifestation and those without. Also, since serum albumin is more clinically linked to renal manifestation, it is difficult to explain the difference in NFI between AAV patients with and those without ENT manifestation by the results of this previous study alone.
The cur-off of NFI or FCI that predict the all-cause mortality was not obtained by using the ROC curve that does not include the period in the analysis. For this reason, the cut-offs of NFI and FCI at diagnosis for all-cause mortality were inevitably set as the lower limit of the highest quartile for each. In the Kaplan Meier survival analysis, in addition to all-cause mortality which showed a statistically significant difference based on NFI at diagnosis ≥ 1.24, ESRD also showed a trend of significance with a P value of 0.1 or less (P = 0.072). Therefore, although statistical significance was not reached, the areas of NFI and FCI at diagnosis for all-cause mortality and ESRD using the ROC curve were qualitatively compared. First of all, in the ROC curve analysis based on all-cause mortality, NFI at diagnosis showed a wider area under the curve than FCI at diagnosis. Moreover, in the ROC curve analysis based on ESRD, NFI at diagnosis also showed a wider area under the curve than FCI at diagnosis (Supplementary Fig. 1). Therefore, although the statistical significance was low and the follow-up period was not taken into account, NFI at diagnosis tended to better predict the occurrence of all-cause mortality and ESRD during follow-up than FCI at diagnosis.
The exact mechanism of how NFI at diagnosis independently predict all-cause mortality during follow-up in AAV patients is unclear, however, we made some assumptions. Firstly, with regard to serum albumin, hypoalbuminemia is well-known major risk factor for poor prognosis including all-cause mortality and ESRD occurrence in AAV patients [15]. Since serum albumin is located in the denominator in the formula of NFI, NFI and serum albumin theoretically show an inverse correlation. Therefore, it can be inferred that the smaller the albumin, the higher the NFI, so the highest quartile of NFI at diagnosis, which indicates the lower level of serum albumin, could predict all-cause mortality in AAV patients. The Pearson correlation analysis performed in our study data did not reach statistical significance, but showed a tendency of a negative correlation (r = -0.119, P = 0.084). In fact, using the Spearman correlation analysis, serum albumin was significantly and inversely correlated with NFI (r2 = -0.322, P < 0.001). These results support our assumption that NFI can predict all-cause mortality through serum albumin.
Secondly, with regard to platelet count, there have been studies demonstrating that platelet counts reflect the inflammatory burden of AAV, correlate with BVAS, and may predict the poor outcomes in AAV patients [10, 16]. Since platelet count is also located in the denominator of the formula of NFI, theoretically, the larger the platelet counts, the smaller the NFI. However, the results from our data were not. When we focus on two variables located in the denominator, serum albumin can be considered to have a direct effect on the increase in NFI more than platelet count. Therefore, we concluded that the highest quartile of NFI at diagnosis made a more direct contribution to predicting all-cause mortality through reduced serum albumin which could reflect the high degree of inflammation and malnutrition.
In the Kaplan Meier survival analysis, the cumulative ESRD-free survival rate in AAV patients with NFI at diagnosis ≥ 1.24 was lower than that in AAV patients with NFI at diagnosis < 1.24 (P = 0.072), although it was not statistically significant. This result may suggest that analysing a larger number of patients can yield statistically significant results.
How can NFI at diagnosis, which belongs to the highest quartile, show a tendency to predict ESRD during follow-up in AAV patients? Firstly, with regard to comorbidity of chronic kidney disease, AAV patients with chronic kidney disease at diagnosis showed higher NFI than those without (0.86 vs. 0.55, P = 0.060). Therefore, it can be assumed that NFI reflected the accompanying chronic kidney disease at diagnosis, which could have the potential to predict ESRD occurrence in AAV patients [17].
Secondly, with regard to serum albumin, high NFI means reduced serum albumin at diagnosis as described above. Reduced serum albumin, in turn, is associated with renal manifestation and increased BVAS, and furthermore is another important risk factor for ESRD [3, 18]. Using the univariable Cox hazards model analysis, serum albumin at diagnosis was significantly associated with ESRD during the follow-up period based on ESRD occurrence (HR 0.585, 95% CI 0384, 0.890). These results support our assumption that NFI can predict ESRD through serum albumin.
Thirdly, NFI is originally an index for liver fibrosis. Nevertheless, how can it predict ESRD? Although the cells involved in liver fibrosis and renal fibrosis are different, biological factors such as cytokine and chemokine that ultimately induce and promote collagen deposition in organs may share common signal pathways in the two difference fibrosis-process [9, 20]. Therefore, although it is difficult to explain the exact mechanism, it is expected that the index of liver fibrosis can predict the process of renal fibrosis to some extent.
To our best knowledge, this is the first study to demonstrate that NFI at diagnosis could independently predict all-cause mortality in immunosuppressive drug-naïve AAV patients without chronic liver diseases. The discovery of a new predictor of all-cause mortality in AAV patients is an important advantage of our research. In addition, our study was conducted in a single institution, which may be a drawback, but it is also an advantage in that chronic liver diseases at the time of diagnosis and during the follow-up period were strictly excluded from this study. However, our study has several limitations. First, although chronic liver diseases were excluded from this study, there was no information on liver fibrosis examined by transient elastography or ultrasonography in patients with high NFI. In particular, the disadvantage is that there was no serial information on liver fibrosis in deceased patients with AAV. Second, due to the limitations of a retrospective single institutional study, there is a possibility of selection bias, and missing data among clinical data could not be completely excluded.