Logical test for integer or non- integer

I am trying to calculate the relative percent difference (rpd) between the values in two cells where one or both the values may be input as "<0.1" (as an example). The rpd formula is:

=ABS(x-y/((x+y)/2))

Where the data values could be any of the following four formats:

x , y
1 , 2
1 , <0.2
<0.2 , 2
<0.1 , <0.1

If both values are in the "<0.1" format I need "BBL" to be displayed, If one is "<0.1" I need the formula to see that cell as "0.1" (I can have another cell with "0.1" that it can reference).

I think all I need is a nested IF statement where the logical test decides if each cell is an integer or not.

Closest I've got (which doesn't work) is:

=IF((AND(A1>0,B1>0)),ABS((A1-B1)/((A1+B1)),IF(A1<0,ABS((C1-B1)/((C1+B1)/2)),IF(B1>0,ABS((A1-C1)/((A1+C1)/2)),(IF((AND(A1<0,B1<0)),BBL,"Error")))))

Where C1 is "0.1"

Any help would be awesome!