Two years ago a cbc showed absolute NEU of 2.06 (wbc=5.7). Last week cbc shows absolute NEU of 1.51 (wbc=4.6). Caucasian adult. Is this a concern?

Physiologic variatio. The range of normal for absolute neutrophils is fairly broad, and intended to include the changes due to individual variation as well as physiologic variation. Your values suggest the latter, changes over time due to the natural course of life encountering new and different stimuli. Not to worry.