A New, Non-Invasive Scale for Steatosis Developed Using Real-World Data From Russian Outpatients to Aid in the Diagnosis of Non-Alcoholic Fatty Liver Disease

The present analysis of the retrospective observational DIREG_L_06725 study conducted in Russia showed that a higher proportion of patients (42.0%) had NAFLD compared with the proportions reported in a meta-analysis of previous studies, which ranged from 13.5% in Africa to 30.5% in South America [4]. This can most likely be explained by the baseline characteristics of enrolled individuals, who were outpatients visiting medical practitioners (often gastroenterologists), and many of whom were aged > 50 years, obese/overweight, and had cardiometabolic comorbidities. Thus, this study population was not representative of the general population. Patients with NAFLD included in our analysis also had more pronounced signs of overweight/obesity/central adiposity, a higher prevalence of comorbidities/accompanying pathologies, were older on average, and had a higher female-to-male ratio than the general population. Interestingly, the prevalence of arterial hypertension in patients with NAFLD was considerably higher than that of the pooled estimate from previous studies, while the frequency of other comorbidities was close to that reported previously [4]. The frequency of NAFLD in the current analysis is also higher than that in the original DIREG_L_06725 report [19] because we focused on ultrasound screening for NAFLD; therefore, we further limited our inclusion criteria.

The subgroup analysis indicated that female patients with NAFLD were older, more obese, and more frequently had comorbidities and liver enzyme deviations; thus it is the older age of female patients, rather than sex per se, that was responsible for the other factors. It should also be noted that the same inverse age/sex relationship was observed in the overall cohort, showing that older women make up a higher proportion of outpatients. Moreover, this may be associated with the reduced life expectancy of men in Russia, since according to an official local public report in 2017, the female-to-male ratio in Russia was 1.6–1 for individuals aged 60–65 years and 3–1 for those aged 80 years [21].

Another interesting finding was that, compared with older patients, young patients with NAFLD (aged 12–18 years) showed greater deviations from normal ranges in ALT, AST, fasting insulin, and some other laboratory values, but they less frequently had abnormal levels of GGT; the latter may be related to the lower probability of alcohol intake. In this analysis, nearly half of patients with NAFLD had normal levels of liver enzymes (ALT, AST, and GGT). This observation shows that a surrogate blood marker-based approach to NAFLD identification can lead to underdiagnosis. Subgroups of patients having NASH or simple steatosis were distinguished retrospectively on the basis of ALT, AST, and GGT (based on current Russian NAFLD/NASH guidelines). This approach yielded the 7.5% prevalence of NASH in the overall studied population, while in the cohort of patients with NAFLD, 18% had NASH and 82% had pure steatosis. Further analysis revealed a number of statistically significant differences between these subgroups; however, these differences were of no practical clinical value for distinguishing steatosis from steatohepatitis. Of note, a small subgroup of patients had results of biopsy analysis. Histological steatohepatitis was found in a similar proportion of patients diagnosed with NASH and simple steatosis on the basis of liver enzymes. Thus, biopsy results were not consistent with laboratory tests. This finding is consistent with data showing that aminotransferase levels can fluctuate and are not necessarily reflective of steatosis or fibrosis, or indeed the degree of liver inflammation [22]. Therefore, steatohepatitis is definitively distinguishable by histology only, which is not performed routinely in patients with NAFLD. Increased ALT and AST levels can alert physicians to a potential problem [23], but they cannot provide a definitive diagnosis, nor does NASH constitute merely an asymptomatic elevation of enzymes, itself a common clinical misconception. The challenge is thus to develop a standardized approach to steatohepatitis screening and management in patients with NAFLD with or without elevated liver enzymes, and in the absence of liver ultrasound or biopsy. This may need to include not only acknowledging the potential seriousness of suspected “pure” steatosis but also assuming that NASH is present and managing patients accordingly.

While it is clear that NAFLD is associated with cardiovascular disease, it is not yet known whether this association applies to NASH only, or also to simple steatosis [24]. In a 2011 meta-analysis, Musso et al. found that individuals with NASH had higher liver-related mortality than those with simple steatosis, but not higher cardiovascular disease mortality [25]. On the other hand, a considerable number of studies have shown a link between imaging- or biopsy-proven NAFLD in the absence of liver enzyme data, and the development of cardiovascular disease [5]. It has been shown that simple steatosis has the potential to progress regardless of the presence of NASH, with fibrosis, particularly advanced fibrosis, being the most important prognostic factor [25, 26].

Using multivariate regression analysis, we developed a fully non-invasive diagnostic test for NAFLD, the “St-index”. The resulting predictive model was not cumulatively highly accurate, although the AUROC for the St-index was higher than for the other indices tested in the present dataset (HSI, WC, WCHR). More importantly, we achieved high specificity (91.4%) in ruling in and high sensitivity in ruling out (93.8%) NAFLD, using only non-invasive parameters. For HSI, NAFLD was ruled out at a proposed cutoff of < 30, with higher sensitivity than in the original study (98.6% vs. 93.1%), but was ruled in at a proposed cutoff of > 36.0 with much lower specificity than in the original study cohort (25.7% vs. 92.4%) [12]. Thus, in the present study sample, HSI was effective at excluding NAFLD, but ineffective at diagnosing it.

Lower than expected diagnostic efficacy of the indices is not an uncommon finding [10]. Discrepancies in results can be explained by differences in baseline characteristics of the studied populations. These may also include ethnic differences: WCHR was originally proposed for a Taiwanese population [17] and HSI for a South Korean cohort [12]. Other differences include measurement techniques. In a study that determined the accuracy of WC in quantifying longitudinal change, NAFLD diagnosis was based on proton magnetic resonance spectroscopy [16], while other studies used ultrasound. Thus, in the present population, the tests used were less accurate than in the populations from which the tests were derived.

The St-index may, therefore, not provide a way to definitively diagnose steatosis, but to suspect it and provide physicians with a practical decision-making tool to identify patients requiring ultrasound. Use of such a tool in clinical practice may be more readily reimbursed by health insurers who may be reluctant to reimburse diagnostic tests for asymptomatic conditions. Additionally, we revealed that multivariate regression models in the subgroups of patients aged ≥ 12 and < 18 years and individuals with BMI < 25.0 kg/m² (subgroups in which associated factors displayed the greatest effect sizes) had a more accurate AUROC. This may be because in children, lipid metabolism disturbances, and in particular excess of fat in the liver, are more accurately reflected in an abnormal WCHR. Hence, developing diagnostic tests for these patient categories may be more promising in the future.

Our analysis has some limitations. Firstly, the high prevalence of individuals with gastroenterological/cardiometabolic conditions could potentially hinder identification of factors associated with the risk of NAFLD. The age of patients was ≥ 12 and ≤ 80 years due to the primary dataset inclusion criteria; therefore, our analysis in children is limited and exploratory in nature. NAFLD was diagnosed using ultrasound, which is characterized by high sensitivity and specificity in the diagnosis of grade 2–3 steatosis (84.8% and 93.6%, respectively) [27]. Therefore, it cannot distinguish mild steatosis from severe cases because it lacks sensitivity and is highly operator-dependent. Simple steatosis and NASH were distinguished on the basis of liver enzyme levels, which can likewise only provide an approximate differentiation. In addition, laboratory data were not necessarily presented with unified reference ranges because different ranges were used at the various institutions. Finally, the level of physical activity of the patients was not measured.

Despite these limitations, our large sample size allowed us to detect a variety of factors, with a high level of statistical significance, and the revealed associations support further strong, complex, and multidirectional relationships between NAFLD and excess weight, abdominal obesity, metabolic syndrome and its components, T2DM, and related cardiometabolic disorders. Our findings also support the idea that simple steatosis is almost indistinguishable from NASH in routine clinical practice, thus demanding consistent vigilance in diagnosis. When it comes to diagnosis, our tree of NAFLD risk, where age > 35 years, T2DM presence, and WCHR > 0.54 were predictive factors, could help physicians to distinguish patients with NAFLD more easily. While the newly developed St-index is not accurate enough to diagnose steatosis, it is a fully non-invasive test, unlike BARD, HSI, and ALT/AST measurements. Importantly, it could help physicians to promptly identify potential candidates for further ultrasound testing, which cannot be feasibly performed in all patients presenting to their general practitioners without additional substantiation.