In one sentence
A score from the ratio of your two liver enzymes, your body mass index, your sex and whether you have diabetes, built to say whether fat in the liver is likely or unlikely without an ultrasound; VT pairs it with the scale's visceral-fat figure.
Why it matters
Fat in the liver is the liver's side of the same story the waist tells, and it is common and silent. The index was derived in 10,724 Korean adults attending health check-ups, half with fatty liver on ultrasound and half matched without: eight times the ALT-to-AST ratio, plus BMI, plus 2 for a woman and 2 for diabetes. Below 30 it ruled fatty liver out with a sensitivity of 93 percent, above 36 it ruled it in with a specificity of 92 percent, and 86 percent of people outside the 30-to-36 band were correctly classified (Lee 2010). The score was built where BMI reads differently from a European population, which is one reason VT shows it as context beside a direct measure of where fat sits.
What moves it
Weight and the ALT-to-AST ratio, so it rises with fat in the liver and with weight gain, and falls with weight loss over months; a hard session moves the ratio for days; and the BMI term rises in a muscular person for reasons that have nothing to do with the liver.
How to measure it well
From a liver panel on one draw under the panel's rules, with a current weight and height in the profile, and with the diabetes flag in the profile kept accurate. Compare draws taken the same way.
How Vitality Tracker uses it
Computed when ALT and AST come from one collection and the profile holds a weight within 30 days, shown on the labs dashboard beside the scale's visceral-fat index and the waist-to-height ratio, with the 30 and 36 cut-points named as the Korean study's and never as VT's verdict. No working optimal line is drawn. A score above the upper cut-point does not prompt anything on its own; a liver enzyme outside the laboratory's range does, in the How it works page's words.
How far to trust the number
Calculated from precise inputs; its meaning depends on the population it was built in, and the BMI term reads muscle as fat. The direction across draws is worth more than the value.
Sources
- Lee JH, et al. Hepatic steatosis index: a simple screening tool reflecting nonalcoholic fatty liver disease. Dig Liver Dis 2010;42:503–8. doi:10.1016/j.dld.2009.08.002
- Vitality Tracker, Clinician report content specification (derived indices).