Does the standard curve encompass the Lower Limit of Detection (LLD) for each cytokine?
The LLD is the lowest concentration of an analyte where its signal is distinguishable from the background (2 times the standard deviation of the mean of the background). This is different than the lower limit of quantification (LLOQ) which is the lowest concentration of an analyte that can be accurately measured. In many cases, the typical standard curve does not reach the LLD nor the LLOQ for a given protein.
The user has the option of using additional dilutions of the calibrator mix to achieve the LLOQ for a given protein. A quick determination of LLOQ is to examine the backfit of your regression analysis and determine where the model no longer predicts within 80-120% of the actual standard value. I.E. the LLOQ is the lowest point where the backfit is within 80-120% of its corresponding value on the standard curve.