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Using a KinExA assay for making K_d measurements is ideal because measurements are made without perturbing the solution equilibrium and with unmodified molecules, unfettered, in solution. For an accurate K_d determination the concentration of receptor in the samples should be near or below the K_d. With the tight binding antibodies (low or sub pM) being developed now, the measurements need to be made at these very low sample concentrations. The KinExA's ability to make quantitative measurements at these low concentrations allow for accurate K_d measurements for those very tight binders.

Binding curves are dependent on K_d and receptor concentration. A binding curve can be generated by making a series of samples with constant receptor concentration and a titration of ligand. After equilibrium is reached, a KinExA measurement is made of the free receptor concentration in each sample of the titration series. Therefore the free receptor directly represents the binding curve. The binding curve generated is then analyzed to find the K_d (for low ratio curves) or receptor active concentration (for high ratio curves). Multiple curves with different receptor concentrations may be analyzed together to get both K_d and active receptor concentration.