Sensitive immunoluminometric assay for the detection of procalcitonin.

When both calcium intake and calcium absorption are measured under controlled conditions, variation in absorption efficiency explains more of the interindividual differences in balance than does actual calcium intake (1 ). Small wonder, therefore, that interest in measuring absorption has remained high for nearly 40 years. True calcium absorption is defined as the quantitative, unidirectional flux of calcium from intestinal lumen into the blood. It is most accurately measured by a dual-tracer method, with one tracer labeling the oral calcium load and the other labeling the miscible calcium pool into which the absorbed calcium is introduced. This approach was first developed into a practicable human test by deGrazia et al. (2 ). As described, it is usually timeconsuming and expensive. To reduce these barriers for widespread use, Heaney and Recker (3, 4) developed a single-tracer variant for women, requiring only a single blood sample, and calibrated it against a simultaneously performed double-tracer method. The single-tracer method has been used efficiently in thousands of women (5 ). However, because the calibration is empirical and based on body-size variables, it is not directly suitable for use in men who, with a typically higher proportion of fat-free mass than women, would be expected to distribute absorbed tracer in a larger mass of calcium. To fill this methodologic gap, we performed a small set of parallel measurements in adult men, using the female-based algorithm together wiketh a modified double-tracer approach. Participants in the study were 30 Caucasian men (age range, 20–60 years; weight range, 63.5–104 kg; height range, 1.67–1.93 m). All participants were free of known diseases affecting bone remodeling or calcium homeostasis, and tests were not performed if the individual had experienced any gastrointestinal disturbance in the preceding 5 days. Each gave informed consent after the procedures of the study were explained. Both the project and the consent were approved by the Creighton Institutional Review Board. Each volunteer was tested twice. We performed the first test for several unrelated projects, using the standard, single-dose protocol, giving a Calabeled oral load and obtaining the usual 5-h serum sample for measurement of serum calcium specific activity. The volunteers abstained from all food after the test breakfast until the 5-h blood sample was drawn. The test calcium load (depending on the individual projects) was 300 mg in 25 individuals and 500 mg in 5. Sources were calcium-fortified orange juice in 20 volunteers, skim milk in 5, and precipitated calcium carbonate in 5. The second test, performed 6.2 ( 3) days later, used an intravenous (i.v.) dose of high-specific activity Ca, given 2 h after an identical test breakfast that contained the same oral calcium source and the same calcium load as on the first test day. With the second test, serum was obtained 3 h after the i.v. dose for measurement of serum calcium specific activity. This timing reproduces accurately the dosing scheme of a simultaneous double-tracer experiment in which, as originally described (2 ), the i.v. tracer is given 2 h after the oral tracer. (The 2-h lag introduces the i.v. tracer at the approximate midpoint of absorption of the oral tracer.) Because the same tracer was used to label both the oral load and then subsequently the miscible calcium pool, a baseline serum sample was obtained at the second test to determine the concentration of residual tracer from the earlier oral test. (Mean correction was 9.6% of the total counts in the 3-h blood sample, with the maximum being 25% for the shortest interval and the minimum being 3.1% for the longest.) Absorption fraction was calculated in two ways. The first consisted of the female-derived algorithm: