Human hydatid disease: evaluation of an ELISA for diagnosis, population screening and monitoring of control programmes.

The routine use of ELISA and complement fixation tests in the diagnosis of suspected clinical cases of hydatid disease was evaluated. In the ELISA test, dialysed and filtered sheep cyst fluid was used as antigen and two positive cut-off points--+3SD and +2SD of the mean absorbance values of the control sera--were evaluated. The predictive values of ELISA tests were 82% and 90% for positive tests, and 86% and 82% for negative tests, respectively with the two cut-off points. In a population survey of blood donors and veterinary workers in Powys, 4% and 8%, respectively, had ELISA values above the lower cut-off point. However, it would not be appropriate to use the same test for diagnostic population screening in Wales since the predictive value of the test is likely to be very low in this setting. Serological surveys with the ELISA may be of use in monitoring the progress of the South Powys Hydatid Control Programme. The use of cumulative percentages was found to be a useful method of comparing whole distributions of results in different populations.