Co-agglutination (Co-A) test for circulating antigen in hydatid disease

A co-agglutination (Co-A) test with Staphylococcus aureus (Cowan's strain I) bearing protein A coated with specific hydatid antibodies, was used to demonstrate circulating hydatid antigen in serum for the diagnosis of hydatid disease. The test had a sensitivity of 95% and specificity of 89% in detecting hydatid antigen in serum. A false positive reaction was observed with 18.5% of control sera from patients with various parasitic diseases. However, these all gave negative results when tested after further dilution. The test is reliable and rapid; the result could be obtained within 30-45 min of receipt of a serum sample. Since the test is simple and the reagents are inexpensive and easily available, it can be used in less well equipped laboratories in developing countries.