Diagnosis is based on microscopic identification of eggs in stool specimens. However, the eggs of Opisthorchis are practically indistinguishable from those of Clonorchis.

For medical diagnosis there is need to find eggs of Opisthorchis viverrini in feces using Kato technique.

An antigen 89 kDa of Opisthorchis viverrini can be detected by ELISA test.

A PCR test capable of amplifying a segment of the internal transcribed spacer region of ribosomal DNA for the opisthorchiid and heterophyid flukes eggs taken directly from faeces was developed and evaluated in a rural community in central Thailand. The lowest quantity of DNA that could be amplified from individual adults of Opisthorchis viverrini was estimated to 0.6 pg.