Ketosis. Happens during starvation and also by restricting carbohydrates (and protein, to a lesser degree)… might be important for epilepsy and bipolar disorder, too.
Ketostix measure urinary acetoacetate (AcAc) and reflect the degree of ketosis in the blood probably about 2-4 hours ago. Blood ketone meters measure beta-hydroxybutyrate (bHB) right now. bHB fluctuates to a greater degree, eg, it plummets after a meal whereas AcAc takes longer to decline. AcAc/bHB is usually around 1, but increases after a meal (Mori et al., 1990):
Conversely, when glucose levels decline and fatty acid oxidation increases, liver redox potential drops which reduces AcAc/bHB.