Besides Hb A, albumin and various globulins may undergo nonenzymatic glycosylation. In contrast to hemoglobin, which has a serum half-life of about 60 days, albumin has a half-life of about 17-20 days, and total protein (roughly one half albumin and one half globulins) has a half-life of about 30 days. Either glycosylated albumin or glycosylated total protein can be assayed, but most laboratories assay total protein using the fructosamine procedure. This does not involve the sugar fructose and is based on biochemical reaction with glucose bound to protein with a ketoamine linkage, most often using nitro blue tetrazolium as the reagent. Serum fructosamine assay results indicate average glycosylation within the preceding 2-week time period (range, 1-3 weeks). This time period is considerably shorter than that of glycoHb but substantially longer than that for labile hemoglobin glycosylation. Drawbacks of fructosamine assay include changes in serum level due to changes in albumin rather than blood glucose. According to one report, changes in albumin affect fructosamine levels significantly only if decreased albumin levels are due to increased catabolism (decreased half-life) or increased albumin loss, but not when there is decreased metabolism of protein. Reducing substances in serum may interfere with the assay in some methods.