In one sentence
A score built from your age, two liver enzymes and your platelet count that was designed to say, in a clinic, whether a person with liver disease was unlikely to have advanced scarring; VT shows it as context beside the liver enzymes, not as a verdict.
Why it matters
Scarring of the liver (fibrosis) is silent until it is advanced, and the score exists because a biopsy is not something to do routinely. It was developed in 832 people with HIV and hepatitis C: age times AST, divided by platelets times the square root of ALT, and at a value below 1.45 the score ruled out advanced fibrosis in nine of ten people, while above 3.25 it had a specificity of 97 percent, so that biopsy could have been avoided in 71 percent of the validation group (Sterling 2006). It has since been carried into fatty liver disease, where guidelines use lower cut-points (the clinician specification lists 1.3 and 2.67), and it has an age problem: in 634 European patients with fatty liver disease, the score performed poorly under 35 and its specificity fell to 35 percent at 65 and over, so a new lower cut-point of 2.0 was derived for that age group (McPherson 2017). For a well adult the score is almost always low, and its value is in being low on every draw.
What moves it
Age, which rises every year whatever the liver does, so the score drifts upward through life; AST and ALT, which rise with fat in the liver, alcohol and a hard session (muscle releases both); and platelets, which fall with some liver conditions and rise with inflammation and iron deficiency. A draw after heavy training raises the score for a reason that has nothing to do with the liver.
How to measure it well
From a liver panel and a blood count taken on one draw under the panel's rules, above all no hard training for 48 hours and no alcohol for 48 hours. Compare draws taken the same way.
How Vitality Tracker uses it
Computed only when AST, ALT and platelets come from one collection, shown on the labs dashboard beside the enzymes with the published cut-points named as their authors' (1.45 and 3.25 from the original study; 1.3 and 2.67 as the fatty-liver literature's; 2.0 at 65 and over), never as a VT target, and listed on the liver line of the pre-visit report. No working optimal line is drawn, and the score is not computed from a draw taken within 48 hours of hard training when the Activity log shows one.
How far to trust the number
Calculated from precise inputs; its meaning is the limitation. It was built for people with liver disease, its cut-points are population trade-offs between missing and over-calling, and its specificity collapses with age. For a well adult it is a quiet number to watch stay quiet.
Sources
- Sterling RK, et al. Development of a simple noninvasive index to predict significant fibrosis in patients with HIV/HCV coinfection. Hepatology 2006;43:1317–25. doi:10.1002/hep.21178
- McPherson S, et al. Age as a confounding factor for the accurate non-invasive diagnosis of advanced NAFLD fibrosis. Am J Gastroenterol 2017;112:740–51. doi:10.1038/ajg.2016.453
- Vitality Tracker, Clinician report content specification (derived indices).