Abstract: The present invention provides an assay for an analyte in solution. The assay relies, in part, on associating two catalytic activities in close proximity to each other (e.g., by conjugating each enzyme to a ligand that binds the analyte) and providing substrates that produce a colored product only when both activities are bound to the same molecule in solution.

Abstract: A method is provided for measuring an analyte in a sample comprising adding substantially transparent particles to a sample in solution or suspension, said particles having an affinity for said analyte; fractionating the particles from the solution or suspension to form a particle-rich fraction and a substantially particle-free fraction; optically reading the particle-rich fraction at a first and a second wavelength; optically reading the substantially particle-free fraction at at least the first wavelength; and correlating the readings through the particle-rich fraction and the substantially particle-free fraction of the sample, with similar measurements in a particle-containing "blank" to obtain a quantitative determination of the analyte originally present in the sample.

Abstract: Methods and apparatus for separating red blood cells by density are described. Older red blood cells are more dense than younger ones. Certain changes in the physiology are recorded in changes in hemoglobin or the red blood cells. By assaying different aged red blood cells, one can determine the historical physiological changes over a period of many weeks. Improved separation of red blood cells is accomplished by using rigid capillary tubes having an inner surface which augments the density equilibrium of red blood cells and/or incrementally increasing the centrifugation forces, as well as by chemically treating red blood cells to improve their deformability.

Abstract: As red blood cells age, their hemoglobin is progressively glycosylated. This invention relates to analyzing the history of blood sugar metabolism over a period of several months by taking a blood sample, separating the cells into age-ordered cohorts, determining glycohemoglobin levels in the cells of each cohort, and correlating glycohemoglobin levels with red blood cell age, while correcting for the reversibility of the Amadori rearrangement.