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Alternative Methods Tested for Detection of Soil-Transmitted Helminths

By Labmedica International staff writers
Posted on 12 Jun 2014
Image: The Mini-FLOTAC device for the detection of helminthic eggs (Photo courtesy of Prof. Giuseppe Cringoli).
Image: The Mini-FLOTAC device for the detection of helminthic eggs (Photo courtesy of Prof. Giuseppe Cringoli).
Image: Photomicrograph of Ascaris sp., Trichuris sp., and hookworm eggs in a fecal sample (Photo courtesy of Dr. Mae Melvin).
Image: Photomicrograph of Ascaris sp., Trichuris sp., and hookworm eggs in a fecal sample (Photo courtesy of Dr. Mae Melvin).
The commonly used diagnostic technique for the detection of soil-transmitted helminths (STH) is the Kato-Katz method, but it has low sensitivity arising mainly from the nonrandom distribution of eggs in stool and day-to-day variation in egg output.

An alternative to the Kato-Katz method is a new flotation and translation-based technique, which was compared to the Kato-Katz method and exhibits greater sensitivity for detecting STH species such as Ascaris lumbricoides, Trichuris trichiura, and hookworm.

Scientists at the London School of Hygiene and Tropical Medicine (UK) working with colleagues in East Africa, collected stool samples were from 652 children attending 18 schools in Bungoma County (Kenya). Helminths infections were diagnosed by the Kato-Katz and Mini-FLOTAC coprological methods. The diagnostic performance of the methods was further assessed in terms of positive predictive value (PPV, proportion of true positive results detected), negative predictive value (NPV, proportion of true negative results detected), and accuracy (proportion of readings that have given a valid result) based upon modeled prevalence, sensitivity and specificity.

Overall, 93 of the 657 samples were positive for at least one STH species on both tests (14.2%), 485 samples tested negative for any STH species on both tests (73.8%), 45 samples tested positive with Kato Katz and negative with Mini-FLOTAC (6.8%), and 34 tested negative with Kato Katz and positive with Mini-FLOTAC (5.2%). When the scientists combined the results from consecutive days, children were reclassified as infected if they were positive on either day, and uninfected if testing negative on both days: 23.5% of the 132 children included were classified as positive for any STH species by both tests, 60.6% as negative by both tests, 9.1% as positive by Kato Katz only, and 6.8% by Mini-FLOTAC only. Sensitivity analysis revealed the cost of case detection for any STH decreased nonlinearly as prevalence rates increased and was influenced by the number of samples collected.

The authors concluded that their evaluation shows that the Kato-Katz and Mini-FLOTAC methods were comparable to one another in diagnostic sensitivity, yet Kato-Katz afforded greater cost-effectiveness. They encourage the wider use of simulation, cost-effectiveness and field studies to evaluate the cost-effectiveness of diagnostic and sampling strategies for STH surveillance in a variety of settings and for the wider surveillance of different neglected tropical diseases. The study was published on May 8, 2014, in the journal Public Library of Science Neglected Tropical Diseases.

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