Serum N-telopeptide (Osteomark, Ostex, Seattle, WA) is measured using a competitive-inhibition enzyme-linked immunosorbent assay that utilizes microwells as the solid phase onto which NTx has been adsorbed. Assay values are reported in nmoles Bone Collagen Equivalents (BCE) per liter. The normal range is 6.2 - 19.0 in adult women and 5.4 to 24.2 in adult men. The intra-assay and inter-assay variability are 4.6% and 6.9% respectively.