Materials and Methods
The present study represents an analysis of data from the 2017 to 2018 cycle of the National Health and Nutrition Examination Survey (NHANES), a cross-sectional survey aimed at including a representative sample of the general non-institutionalized US population of all ages. Briefly, it consists of a structured interview conducted in the home, followed by a standardized health examination performed at a mobile examination center, which includes a physical examination as well as laboratory tests. The original survey was approved by the Centers for Disease Control and Prevention Research Ethics Review Board and written informed consent was obtained from all adult participants. The present analysis was deemed exempt by the Institutional Review Board at our institution, as the dataset used in the analysis was completely de-identified.
Participants ≥ 18 years with a reliable vibration-controlled transient elastography (VCTE) exam were included in the analysis. To evaluate the degree of concordance of the two definitions, we excluded subjects with missing data on key variables for the diagnosis of NAFLD and MAFLD. Details on the exclusion process are shown in Figure S1.
Presence of liver steatosis was defined by a median Controlled Attenuation Parameter (CAP) ≥ 274 dB/m, a cut-off that yielded 90% sensitivity in distinguishing S0 from S1-S3 in a recent study by Eddowes et al. Sensitivity analyses were also conducted using a cutoff of 290 dB/m (showing 90% sensitivity in differentiating between S1 and S2-S3) and 302 dB/m, a cut-off based on the highest Youden index. Advanced liver fibrosis was defined as a median Liver Stiffness Measurement (LSM) ≥9.7 KPa; this cut-off was derived from the same study. The exam was deemed reliable if at least 10 valid measurements were obtained with an interquartile range/median LSM < 30% after a fasting time of at least 3 hours. Hepatitis C virus infection was indicated by the presence of viral RNA and/or a confirmed antibody test and hepatitis B virus infection as a positive surface antigen test.
Laboratory methods for measurements of haemoglobin A1c (HbA1c), glucose, lipid profile, alanine aminotransferase (ALT), aspartate aminotransferase (AST), γ-glutamyltranspeptidase (GGT), platelet count and albumin are reported in detail elsewhere. The fibrosis-4 (FIB-4) and NAFLD fibrosis score (NFS) were calculated as originally described.[7,8]
Alcohol consumption was estimated based on self-reported data on the amount and frequency of alcohol use within the previous year. The amount of alcohol consumed was reported in standard drinks and converted to grams using a multiplication factor of 14. It was considered significant if > 30 g/day for men and > 20 g/day for women. NAFLD was defined as the presence of liver steatosis in the absence of other forms of chronic liver disease (viral hepatitis, significant alcohol consumption or use of steatogenic medications), whereas MAFLD was diagnosed according to recently proposed criteria (consisting in the presence of steatosis and at least one among overweight/obesity, type 2 diabetes and metabolic alterations).
Appropriate sampling weights were applied to all analyses to account for the complex survey design of NHANES. Data are expressed as weighted proportions (± Standard Error (SE)) for categorical variables and as weighted means ± SE for continuous variables. Prevalence rates are expressed as percentages with corresponding confidence intervals. Analyses were performed using Stata version 13.0 (StataCorp, College Station, TX).
Liver International. 2021;41(6):1290-1293. © 2021 Blackwell Publishing