Ideal 1:1 (Langmuir). Smooth exponential rise to a clear plateau during association; smooth single-exponential decay at the same rate for every curve during dissociation. Curves fan out by concentration, and the highest concentrations flatten at the same height because they have saturated the surface (`Rmax`). Residuals are flat noise. Trust the numbers: this is the case the fit is built for.
Never reaches plateau. Association is still climbing when the injection ends, with no curvature. Either kon is slow or every concentration sits below KD. The fit can still work if dissociation is well-defined, but KD leans on extrapolation; widen the concentration range to tighten it.Suggestion. Add higher concentrations, or extend the association time, so the curves approach saturation and pin down KD.
Flat dissociation (no come-off). The decay barely drops within the dissociation window. The target essentially doesn't let go. koff is slower than the experiment can measure, so it (and therefore KD) is reported as an upper bound: "≤" a value, not "=". Typical of very tight binders. Longer dissociation times can pull it into range.Suggestion. Ask for a longer dissociation window (up to 30–60 min) so koff can be resolved; otherwise KD stays an upper bound.
Fast-on / fast-off (square wave). Rapid rise, low plateau, rapid return to baseline the instant buffer flows. A weak, fast-equilibrating interaction where the kinetics are too fast to resolve. Here steady-state (equilibrium) analysis (response at plateau vs. concentration) gives a more reliable KD than the kinetic fit.Suggestion. Use steady-state (equilibrium) analysis, and widen the concentration range so it brackets KD.
Mass-transport limitation. Association looks oddly linear instead of exponential because target is binding faster than flow can replenish it at the surface. This underestimates kon. Diagnostic: apparent kon changes with flow rate. We mitigate it with lower ligand density and higher flow; if it persists we flag the affected fits.Suggestion. Lower the ligand density and raise the flow rate, then confirm kon no longer changes with flow.
Biphasic / heterogeneous. Two visibly different phases: a fast then a slow component in the association or (more often) the dissociation. Points to avidity, a heterogeneous or partly-misfolded ligand population, a conformational change, or aggregation. A 1:1 fit will show systematic residuals; we report these with caveats and, where it helps, an alternative model.Suggestion. Ask for the flipped assay and HPLC-SEC to find the cause (avidity, heterogeneity, or aggregation); we can also fit a bivalent or conformational model, or report ranking-only.
Non-specific binding / drift. Signal keeps drifting upward during dissociation, or the reference channel itself responds. A "sticky" analyte or surface inflates apparent binding. Reference and blank (buffer) subtraction remove most of it; a curve that survives subtraction is real.Suggestion. Improve reference and blank subtraction, adjust the buffer or blocking, or lower the analyte concentration; a curve that survives subtraction is real.
Super-stoichiometric (aggregation). Response climbs past a sensible Rmax without saturating, because more mass lands than a monolayer of target should allow, usually from analyte self-association at the surface. The KD from a 1:1 fit is unreliable; cleaner material or lower concentrations are the fix.Suggestion. Re-run with cleaner, monomeric material (SEC-purified) at lower concentrations, and use a dilution series to confirm; the 1:1 KD here is not reliable.