Screening for intestinal parasites. Is a single specimen valid?

To assess the validity of a single stool or perianal swab specimen when screening for potentially pathogenic parasites, 88 residents of a centre for developmentally handicapped adults were screened by collecting on alternate days three stool samples and pinworm swabs. Both sensitivity and negative predictive values were so low that three samples were required for some parasites. The species being screened for and its underlying prevalence must be considered before collecting only one specimen to reduce laboratory costs.