Detection of pathogenic protozoa in the diagnostic laboratory: result reproducibility, specimen pooling, and competency assessment

Abstract

Stool microscopy as performed in clinical parasitology laboratories is a complex procedure with subjective interpretation. Quality assurance (QA) programs often emphasize proficiency testing as an assessment tool. We describe a result reproducibility assessment tool, which can form part of a broader QA program, and which is based on the blinded resubmission of selected clinical samples, using concordance between the reports of the initial and resubmitted specimen as an indicator. Specimens preserved in sodium acetate-acetic acid-formalin can be stored for several months for use in such a program. The presence of multiple protozoa in one specimen does not affect concordance. Some dilution of specimens occurs in this process, and this may explain poor concordance when specimens with low protozoal concentrations are resubmitted. Evaluation of this tool in a large parasitology laboratory revealed concordance rates for pathogenic protozoa (Entamoeba histolytica/Entamoeba dispar, Giardia lamblia, and Dientamoeba fragilis) of about 80%, which may be considered for use as a benchmark value. We also used this tool to demonstrate that when pairs of specimens from one patient are pooled to create a single specimen, concordance between the results of the individual and pooled specimens is high.

FIG. 1.

Distribution of protozoal concentrations among study specimens compared to that among all specimens received in the lab over a similar time period. Results for each protozoan are shown separately. (a) E. histolytica/E. dispar; (b) Giardia; (c) D. fragilis.

FIG. 2.

Effect of protozoal concentration on concordance between the resubmitted paired subset and the pooled subset. EH, E. histolytica; GL, G. lamblia; DF, D. fragilis.

FIG. 3.

Percentage of submissions positive in both the resubmitted paired subset and the pooled subset, or positive only in one or the other, for each parasite.