Parasitology: diagnostic yield of stool examination

To assess the need for routinely submitting three stool samples per patient for recovery of enteric parasites, we reviewed the records of our parasitology laboratory for 1985-87 to determine the number of parasites that would not have been detected if only one or two samples had been submitted. A total of 16% of all stool samples were positive. For each sample that was positive for a parasite (index sample) a search was done for other stool samples, positive or negative, received from the same patient within 6 days of reception of the index sample. We identified 676 sets of two (276) or three (400) samples of which at least 1 was positive. A total of 93% of the enteric parasites were detected in the first sample in the two-sample sets. Among the three-sample sets 90% of the parasites were detected in the first sample, 8% in the second and 2% in the third. We recommend waiting for the result from the first stool sample rather than routinely submitting three samples for recovery of enteric parasites.