Coefficient of variation (CV) among replicates is excessively high at the lowest standard concentrations (highest OD in competitive format), reducing precision at the sensitive end of the curve.
Common Causes
1Higher OD values inherently demonstrate greater variance in competitive format
2Pipetting errors more pronounced at small volume differences
3Standard concentrations approaching detection limit of the assay
4Samples falling outside optimal measurement range of curve
Solutions
1Accept that higher ODs (lower concentrations) naturally show more variance in replicates
2Increase the concentration range of the standard curve to shift measurement window
3Dilute samples less so they fall in the middle of the standard curve
4Use technical triplicates or quadruplicates at low concentrations to improve precision