Client Script: Validate Field on Item Record

I have two item fields, A & B.
These two fields concatenate, reference saved search results, and compare vs. duplicate values.  The script works well, but it triggers after every every field - not only after editing fields A and B, which results in a lengthy delay after entry for each field. 

Example:
var A= nlapiGetFieldText('FieldA');
var B= nlapiGetFieldValue('FieldB');
var C= (FieldA+FieldB);
if (C == validation){
           alert('Alert!');
}


Any ideas on how to set criteria to trigger only after editing fields A and B?