A new microchip absorption technique has been developed and applied for the clinical determination of calcium ions in urine. An absorbance detection design was developed for hybrid microdevices incorporating optical fibers and a ball lens. A reflective mode was employed to increase the sensitivity of the assay. This method was used in the determination of calcium ions through interaction with arsenazo III to form a complex with an adsorption max at 668 nm. Absorption changes with pH were determined to optimize the assay, with an maximal absorption difference in pH 9.0 buffer. A linear range was determined as 0.125-2.5 mM calcium with a limit of detection of 0.085 mM. Interference from other monovalent and divalent cations normally present in clinical samples was investigated in the expected range. Interference was not observed except at greatly increased levels. This method was applied to urine samples and compared to the current clinical spectrophotometric method with limited success. (c) 2005 Elsevier B.V. All rights reserved.