Simultaneous detection of multiple analytes using Copalis technology: a reduction to practice.

of developing osteoporosis (2) and has been approved by the Food and Drug Administration for use in monitoring antiresorptive therapies. A more complete assessment of a patient's osteoporosis status requires, among other things, both bone mineral densitometry (a measure of current bone status) and measurement of a biochemical marker of bone turnover (a measure of the rate of change in bone status). Because NTx and other bone markers are measures of the rate of turnover (as opposed to extent), they can reflect changes in bone turnover that occur within a relatively short period of time, often as little as 4 weeks. Bone densitometry measurements, although quite precise, are a static measure. Often 1–2 years pass before marked bone loss can be detected by densitometry. This makes NTx useful in assessing response to therapeutic interventions (3, 4) such as estrogen replacement and bisphospho-nates. The reflectance of specific test zones on each strip is measured by a 4-channel reflectometer, and the clinical results are displayed as a ratio of NTx nmol BCE (bone collagen equivalent) per mmol creatinine (as calculated by an on-board microprocessor). The microprocessor also performs corrections for lot-specific reagent characteristics and several forms of optical variation, in addition to tests for proper electrical functioning and adequate sample volume. In measurements of standard gray-scale materials (Munsell Color, GretagMacbeth), the miniaturized reflec-tometer had very good stability (Ͻ0.1% CV over 270 s, n ϭ 73 for each channel), high reflectance reproducibility (Յ0.5% CV, n ϭ 10 for each channel), and excellent linearity (signal vs gray scale r 2 Ն0.999, slope ϭ 1.00 Ϯ 0.01, intercept Ͻ0.01 in units of reflectance). The range of NTx that can currently be measured extends from 30 to Ͼ1000 nmol/L BCE and from ϳ1 to 25 mmol/L for creatinine. Clinical precision (CV of the analyte concentration test result) for NTx and creatinine is in the range of 5–9% (NTx assay precision measured at 300 nmol/L; creatinine assay precision measured at 4 mmol/L; n ϭ 20). The precision (CV) of ratioed results (NTx/creatinine) from 30 prototype DRx tests performed over a 3-month period with a buffered aqueous calibrator solution was 10.9% (38.3 Ϯ 4.19 nmol BCE/mmol creatinine). Comparison of clinical sample test results (n ϭ 20, performed in duplicate) to an NTx microtiter plate ELISA (Osteomark ௢ , Ostex International, Inc.) and to a Boehringer-Mannheim creatinine assay yielded correlation coefficient (r) values of 0.946 and 0.967, respectively. Correlation …