Introducing the Stool Stomper: A Device Designed to Enable Accelerated and Standardized Stool Sample Preparation Using the Kato-Katz Technique

Abstract

Soil-transmitted helminths (STHs) are parasitic worms that impact over 1.5 billion people globally. The Kato-Katz technique analyzes stool samples for STHs, allowing for individual diagnoses of STH infection and the estimation of community-level prevalence. One challenge that arises with the procedure is that lab technicians often struggle to prepare microscope slides of sufficient quality for analysis after one attempt. As a result, Kato-Katz procedures are repeated, wasting time and resources. To aid technicians during in-field slide preparation, we created the Stool Stomper. The Stool Stomper is a user-friendly, handheld mechanical device that applies constant, uniform pressure to stool samples to ensure standardized sample preparation onto microscope slides to improve egg counts. The Stool Stomper was assessed using artificial eggs during in-country testing in a lab setting in Dodoma, Tanzania, by lab technicians with various experience levels, from beginner to advanced. Compared to the traditional method, we found that the Stool Stomper reduced slide preparation time, reduced artificial egg counting time, and standardized artificial egg counts with more consistent and accurate readings. The current pilot study highlights the potential for future development and integration of the Stool Stomper device into the Kato-Katz technique to improve community-based STH treatment.

Keywords: Kato–Katz technique; global health; human-centered device design; neglected tropical diseases; soil-transmitted helminths.

Figure 1

Schematic of the Kato–Katz protocol with the addition of the Stool Stomper. Lab technicians follow the standard Kato–Katz procedure to prepare stool samples through the step of covering the stool sample with cellophane. At this point, rather than the “hand-pressed” preparation of the sample onto the microscope slide, the “Stool Stomper” is introduced to uniformly, evenly, and consistently distribute the stool sample onto the slide via a plunger mechanism to improve downstream microscopic analysis.

Figure 2

The Stool Stomper device. (A) The Stool Stomper’s first iteration utilized a lever press mechanism. (B) The current version of the Stool Stomper utilizes a plunger push–press mechanism. The plunger is built to permit 20 N of force applied to a slide, causing stool samples to be spread evenly in a 14 mm diameter circle. (C) Photograph of the Stool Stomper in use by a lab technician in Dodoma, Tanzania.

Figure 3

Diagram of the sample and slide preparation study workflow. For each stool sample, four slides were prepared in total—two via the traditional hand-pressed approach and two via the Stool Stomper approach. Excess slides were kept throughout the study to blind lab technicians and create the appearance of a larger reading pool.

Figure 4

Brightfield micrographs of Stool Samples. (A) A 40× view of one grain of shape-cut craft glitter under compound light microscopy. The distinct hexagonal nature of shape-cut glitter allowed technicians to distinguish artificial eggs from background detritus when reading samples, creating a controlled, reproducible test scenario. (B) A 40× view of one Taenia Solium tapeworm egg in an egg-spiked sample.

Figure 5

Comparison of reading acuity for each method of preparation on a per-sample basis for 12 fecal samples. The dashed lines indicate the gold standard egg count, or ground truth, for each sample determined by a weighted average of readings from advanced-level technicians. Daggers denote significance level for a one-tailed t-test comparing the differences in the means of the egg count for each preparation method. The study employed significance thresholds of 0.05, 0.01, and 0.001, denoted with red daggers †, ††, and †††, respectively.

Figure 6

Analysis of differences in egg counts for each preparation method compared to the gold standard. Each data point represents the difference between the average egg count across all reads from technicians with different experience levels compared to the ground truth. Note the y-axis bridge between 25 and 110 due to the large difference in hand-pressed samples observed solely in Sample 1 readings. Hand-pressed preparation is denoted by red triangles. Stool Stomper preparation is denoted by black circles. Error bars represent the 95% confidence interval. Daggers denote significance level for a one-tailed t-test, comparing the differences between the ground truth and each method of preparation. The study employed significance thresholds of 0.05, 0.01, and 0.001, denoted with red daggers †, ††, and †††, respectively.

Figure 7

Analysis of Stool Stomper time savings separated according to lab technician experience levels. (A) Slide preparation time study comparing methods of preparation across technician skill levels. (B) Egg counting time study comparing methods of preparation across technician skill levels.