A three-hour measurement to evaluate bone calcium turnover

Abstract

It is well established that short-term clearance of an intravenous calcium load in vivo reflects bone uptake. Using results from isotope-dilution experiments with 42Ca, a 3-h test has been developed to measure a quantity, γ, related to bone accretion. This test is proposed as a useful, clinically applicable measure of bone status. For early times, t, after a bolus of 42Ca, plasma tracer dilution was well approximated by t −γ, where γ is related to the fractional rate of loss of tracer, q, from blood into bone ((1/ q)( dq/dt) = −γ/ t). Gamma was evaluated from kinetic measurements on 91 normal female children, adolescents, and adult women in the age range 4–50 years. For t ⩽ 3 h, all clearance curves were well fit by a power function. Gamma was found to vary from 0.244 ± 0.031 for adult premenopausal women (N = 22) to 0.392 ± 0.056 for prepubertal children (N = 29). Using the Spearman rank-order correlation test, γ was correlated with bone accretion measured from classic calcium kinetic studies with a correlation coefficient of 0.721, significant at p < 0.005. In those cases in which accretion and resorption remain tightly linked, γ also provides information on the state of calcium loss from bone. Gamma was evaluated in 14 subjects with bone disease characterised by increased resorption (osteoporosis, Paget's disease) and in 27 subjects with decreased accretion (osteogenesis imperfecta, types I, III, IV; steroid-treated juvenile dermatomyositis). All subjects with Paget's disease and with osteoporosis showed increased γ, consistent with high bone turnover. The osteoporotic patients furthermore exhibited γ increasing monotonically by approximately 1% per year after age 55. The osteogenesis imperfecta patients consistently showed either normal or decreased γ, depending upon the severity of the disease. The steroid-treated dermatomyositis patients had consistently decreased γ, in concordance with previous studies demonstrating decreased accretion in disease states requiring steroid medication.