The time requirements and costs for the diagnosis of soil-transmitted helminth infections in a cross-sectional epidemiological survey, using either the Kato-Katz technique performed on fresh stool samples or the FLOTAC technique using preserved stool samples, are considerable. Indeed, in our study, the costs for a single or duplicate Kato-Katz thick smears were US$ 1.73 and US$ 2.06, respectively. As expected, the costs for the FLOTAC technique were even higher: US$ 2.35 for the FLOTAC double technique and US$ 2.83 for the FLOTAC dual technique. The higher costs of the FLOTAC technique are mainly caused by the longer preparation time for stool samples and microscopic examinations in the laboratory. Previous studies have shown that the FLOTAC technique is more sensitive than the Kato-Katz method for the diagnosis of soil-transmitted helminth infections [15–18], and hence higher costs might be justified. In the present study, the FLOTAC outperformed the Kato-Katz method only for A. lumbricoides and T. trichiura diagnosis, but was seemingly less sensitive for hookworm diagnosis, for reasons reported elsewhere (Knopp S, Speich B, Rinaldi L, Mohammed KA, Khamis IS, Mohammed AZ, Albonico M, Rollinson D, Marti H, Cringoli G, Utzinger J: "Diagnostic accuracy of the Kato-Katz and FLOTAC techniques when used for assessing anthelmintic drug efficacy", submitted).
In view of the high costs to collect a single stool sample in the field and subsequent examination in the laboratory, preventive chemotherapy without prior diagnosis, as advocated by WHO for high-risk groups in endemic settings is, at first sight, justified [3, 26]. In endemic settings, where morbidity control is shifting to infection and transmission control, an accurate assessment of the epidemiological situation is required and renders diagnosis necessary [5, 7]. For evaluating the efficacy of routinely applied and newly developed drugs, as well as for individual patient management, accurate diagnostic tools are undoubtedly needed [5, 7]. Whenever diagnosis for soil-transmitted helminthiasis is warranted, our results can be of value to decision makers and scientists in budget-planning for epidemiological surveys and to heads of diagnostic laboratories for patient management.
It is important to note that in our study, the Kato-Katz method was performed by rigorously adhering to the WHO bench aids . Indeed, all Kato-Katz thick smears were read twice, first 20-40 min after preparation for hookworm and 3-6 h later a second time for A. lumbricoides and T. trichiura. While this procedure increases costs, it results in higher sensitivity of the Kato-Katz method and might explain the differences in the diagnostic performance of the Kato-Katz and FLOTAC techniques compared to previous comparative investigations [15–18]. Routinely, however, Kato-Katz thick smears are read only once for the three common soil-transmitted helminth species, which saves costs, but also decreases sensitivity.
Moreover, our data are derived from a large survey, where many stool samples were collected and processed simultaneously. One-way sensitivity analyses showed that costs decrease by more than 50% if, for example, individual patients are managed at hospital or public health centre laboratories, without a need for a field team and a 4-WD car. However, in those laboratories, stool samples are not necessarily examined in large numbers, and hence cost and time are likely higher than presented here, due to economy of scale issues . Of note, additional expenses for the patient will then arise from travel costs and loss of income due to missed working hours .
Furthermore, in our setting, the prevalences of A. lumbricoides and hookworm were low and infection intensities of all soil-transmitted helminth species were light. Hence, the counting of helminth eggs by microscopists was relatively quick. In settings with higher prevalence and infection intensities, quantitative microscopic examination will be more time consuming and costs will rise.
Of additional cost relevance is that salaries usually constitute the largest part of the total service costs . Our sensitivity analysis indicates that an alteration of salaries is impacting most on the total costs of both the Kato-Katz and FLOTAC method. In contrast to the Kato-Katz, the total costs of FLOTAC are also strongly influenced by alterations in material costs. It should be noted that the costs for material maintenance (e.g. centrifuge and microscope), the sourcing of chemicals and the adequate disposal of hazardous chemicals were not included in our cost calculations. Moreover, transfer of large equipment (e.g. centrifuge) and costs that might be incurred at customs were not considered in our analyses. Highly setting-specific and dependant on economy are infrastructural costs including rent, electricity and petrol.
In our study, the time to prepare and microscopically examine a stool sample using the FLOTAC double technique was 17:19 min. This result is in line with the time of 21 min reported in a recent study, where the FLOTAC technique was applied for the diagnosis of Fasciola hepatica in rats . However, another study applying FLOTAC for the diagnosis of non-human Trichuris infections revealed a preparation and reading time of only 09:48 min . Of note, our laboratory workers were newly trained to use the FLOTAC method and it is likely that they might have performed faster if the method was being applied routinely. Hence, costs would have been saved. On the other side, the costs of more extensive training necessary to learn the FLOTAC technique in comparison with the simple training for the Kato-Katz method were not considered in our analyses.
Another potential bias in our study is that that the laboratory workers were constantly supervised by an external researcher and that their working speed was recorded. It is hence possible that the staff's working speed differed from normal activity . It is also widely acknowledged that every employee has unproductive phases during a normal working day . To account for unproductive working time, we added 30% of the total measured working time for all working steps in the laboratory in one of the sensitivity analyses. This increased the total costs of the diagnostic methods by up to 18%. The working steps in the field (Figure 2, S1-S3) were excluded from the calculations since unproductive time was already included in the respective time measurements (time record of departure from, and arrival at, the laboratory). Since it is well known that employees make conscious and unconscious breaks during the working day , we consider the cost estimations including 30% of unproductive working time closer to reality. For the reason that in our study workers were observed and because unproductive time was not included in our baseline calculation, our cost estimates are rather conservative.
Despite that the cost estimates for the Kato-Katz and FLOTAC method from our study are not readily transferable to other epidemiological settings where additional costs might occur, or costs might be saved, we believe that they give a reasonable idea of the expenses related to soil-transmitted helminth diagnosis and can guide scientists and decision makers in budget planning for epidemiological surveys. The real costs of diagnosis are of considerable relevance to large-scale control programmes, which often need to balance the costs of treatments against the costs of diagnosis. As global funds to control soil-transmitted helminth infections become more widely available this will be an increasingly important issue to ensure that control programmes are cost-effective and sustainable.