I did an ELISA, where I obtained following mean values for my standard curve:

OD / concentration(pg/ml)

2,514 /1000,0

1,601 / 333,3

0,590 / 111,1

0,265 / 37,0

0,122 / 12,4

0,066 / 4,1

0,043 / 0,0

How can I find out where my linear range of the curve is? How do I find this out in general, when using 4-pl curve-fit and obtaining a sigmoidal curve? (if there is a way to analyse calculate the upper and lower limit of quanitification by using Graphpad, please let me know how I can do after having made the standard curve)

Many thanks in advance!