ELISA- Limit of Detection(LOD) - just need an idea PLEASE - Easy for you, hard for me...just need an idea PLEASE (May/14/2011 )

Ok, I know limit of detection (LOD) is the lowest concentration at which any substance can be detected....blah blah BUT when it comes to determining the LOD from a calibration curve, I always get it wrong...i think I'm missing a way to determine the LOD here..

Eg. in the picture below, I'd say LOD is 0.001% for goat and for buffalo and sheep i'd say it's 0.001% but this is wrong....as the right answer is 0.01% in goat and -0.1% in buffalo and sheep



(http://img641.imageshack.us/img641/3180/elisao.jpg)

Could you kindly suggest what I need to look at...anything in x-values?....currently i look at the lowest y-value and read the corresponding x-value where the line touches!

PLEASE HELP!

The LOD is not only the lowest value on the chart. If that were so you could dilute the sample on and on and thereby lower the limit of detection infinitely. The LOD in biological samples is the value that still produces a signal higher than a certain cut-off. The cut-off value can be arbitrary (e.g. OD>0.2 or 3xOD of background) or derived from testing negative samples.