Sentence:  The lowest response region in the spectrum is that which has the <WSD>LOD</WSD> as the upper bound. It is clear that if a response that is smaller than the response associated with the <WSD>LOD</WSD> is observed, the analytical result should be reported as "less than the <WSD>LOD</WSD>" or "not detected." 
Meaning of the word marked with <WSD> tags:  Limit of detection; the smallest concentration that can be detected using a specific method.