Quality control in the diagnosis of Trichuris trichiura and Ascaris lumbricoides using the Kato-Katz technique: experience from three randomised controlled trials.

Benjamin Speich,Jürg Utzinger,Shaali M Ame,Said M Ali,Marco Albonico,Jennifer Keiser

doi:10.1186/s13071-015-0702-z

Abstract

BackgroundAn accurate diagnosis of soil-transmitted helminthiasis is important for individual patient management, for drug efficacy evaluation and for monitoring control programmes. The Kato-Katz technique is the most widely used method detecting soil-transmitted helminth eggs in faecal samples. However, detailed analyses of quality control, including false-positive and faecal egg count (FEC) estimates, have received little attention.MethodsOver a 3-year period, within the frame of a series of randomised controlled trials conducted in Pemba, United Republic of Tanzania, 10% of randomly selected Kato-Katz thick smears were re-read for Trichuris trichiura and Ascaris lumbricoides eggs. In case of discordant result (i.e. positive versus negative) the slides were re-examined a third time. A result was assumed to be false-positive or false-negative if the result from the initial reading did not agree with the quality control as well as the third reading. We also evaluated the general agreement in FECs between the first and second reading, according to internal and World Health Organization (WHO) guidelines.ResultsFrom the 1,445 Kato-Katz thick smears subjected to quality control, 1,181 (81.7%) were positive for T. trichiura and 290 (20.1%) were positive for A. lumbricoides. During quality control, very low rates of false-positive results were observed; 0.35% (n = 5) for T. trichiura and 0.28% (n = 4) for A. lumbricoides. False-negative readings of Kato-Katz thick smears were obtained in 28 (1.94%) and 6 (0.42%) instances for T. trichiura and A. lumbricoides, respectively. A high frequency of discordant results in FECs was observed (i.e. 10.0-23.9% for T. trichiura, and 9.0-11.4% for A. lumbricoides).ConclusionsOur analyses show that the rate of false-positive diagnoses of soil-transmitted helminths is low. As the probability of false-positive results increases after examination of multiple stool samples from a single individual, the potential influence of false-positive results on epidemiological studies and anthelminthic drug efficacy studies should be determined. Existing WHO guidelines for quality control might be overambitious and might have to be revised, specifically with regard to handling disagreements in FECs.

Highlights

An accurate diagnosis of soil-transmitted helminthiasis is important for individual patient management, for drug efficacy evaluation and for monitoring control programmes
The most common diagnostic approach for soil-transmitted helminth infections in epidemiological studies is the copro-microscopic detection of helminth eggs using the Kato-Katz technique [5,6]
We report new insights from detailed analysis of quality control data obtained over a 3-year period in a series of randomised controlled trials conducted in Pemba, United Republic of Tanzania

Summary

Introduction

An accurate diagnosis of soil-transmitted helminthiasis is important for individual patient management, for drug efficacy evaluation and for monitoring control programmes. The Kato-Katz technique is the most widely used method detecting soil-transmitted helminth eggs in faecal samples. Detailed analyses of quality control, including false-positive and faecal egg count (FEC) estimates, have received little attention. An accurate diagnosis is important for the identification of infected individuals, for assessing anthelminthic drug efficacy, and for monitoring the control and elimination of soiltransmitted helminthiasis [3,4]. Tarafder et al (2010) and Knopp et al (2014) used Bayesian statistical methods to assess the specificity and sensitivity of the Kato-Katz technique for the diagnosis of soil-transmitted helminth infections [16,17]

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