Here are a couple:
The Diagnostic Challenges of Diagnosing Giardia in Cats and Dogs | petMD
quote from the above link regarding diagnosis:
If the fecal examination is negative, but I’m still suspicious that Giardia is the cause of a pet’s diarrhea, I will run a Fecal ELISA (enzyme-linked immunosorbent assay). These are now available as bench-top snap tests (or samples can be sent out to a laboratory) and have a much lower incidence of false negative results in comparison to microscopic fecal exams. Portland Veterinary Specialists > Library > Gastrointestinal > Giardia in Dogs