The determination of the most appropriate serological criterion related to the blood slide examination in malaria surveys was carried out in western Thailand, an area of low persistent malaria transmission. Six different serological criteria were assessed against the blood slide results among 5001 paired blood samples of one-month interval. The indices used in the analysis were sensitivity, specificity and kappa coefficient and the blood slide examination was used as the reference test. It was apparent that a titre of greater than or equal to 1:1024 was the most appropriate serological criterion related to the blood slide results. This serological criterion can be used to indicate the high or low malaria exposure group, particularly in an area of low transmission.