NHANES Liver Enzyme Measurement Protocols

In NHANES, liver enzymes, including ALT, AST, ALP, and GGT were measured during different cycles using specific analyzers. Specifically, the Hitachi Model 704 multichannel analyzer was employed during the 1999–2002 cycles, followed by the Beckman Synchron LX20 during the 2003–2006 cycles, and the Beckman UniCel^® DxC800 Synchron during the 2007–2016 cycles, all measured by medical technologists. Rigorous quality control and assurance measures were implemented in compliance with the 1988 Clinical Laboratory Improvement Act. For values of liver enzymes falling below the lower detection limit, an adjustment was made by replacing them with a value equal to the detection limit divided by the square root of the two. In this study, abnormal liver enzymes were defined in line with prior research: ALT > 47.0 IU/L in men or > 30.0 in women, AST > 33.0 IU/L in men and women, ALP > 113.0 IU/L in men and women, GGT > 65.0 IU/L in men and > 36.0 IU/L in women^{27 (link)}. In addition, the AST/ALT ratio, calculated by dividing AST by ALT, is also an important indicator of liver injury.