Rapid assessment of Schistosoma mansoni: the validity, applicability and cost-effectiveness of the Lot Quality Assurance Sampling method in Uganda

Abstract

Rapid and accurate identification of communities at highest risk of morbidity from schistosomiasis is key for sustainable control. Although school questionnaires can effectively and inexpensively identify communities with a high prevalence of Schistosoma haematobium, parasitological screening remains the preferred option for S. mansoni. To help reduce screening costs, we investigated the validity of Lot Quality Assurance Sampling (LQAS) in classifying schools according to categories of S. mansoni prevalence in Uganda, and explored its applicability and cost-effectiveness. First, we evaluated several sampling plans using computer simulation and then field tested one sampling plan in 34 schools in Uganda. Finally, cost-effectiveness of different screening and control strategies (including mass treatment without prior screening) was determined, and sensitivity analysis undertaken to assess the effect of infection levels and treatment costs. In identifying schools with prevalences > or =50%, computer simulations showed that LQAS had high levels of sensitivity and specificity (>90%) at sample sizes <20. The method also provides an ability to classify communities into three prevalence categories. Field testing showed that LQAS where 15 children were sampled had excellent diagnostic performance (sensitivity: 100%, specificity: 96.4%, positive predictive value: 85.7% and negative predictive value: 92.3%). Screening using LQAS was more cost-effective than mass treating all schools (US$218 vs. US$482/high prevalence school treated). Threshold analysis indicated that parasitological screening and mass treatment would become equivalent for settings where prevalence > or =50% in 75% of schools and for treatment costs of US$0.19 per schoolchild. We conclude that, in Uganda, LQAS provides a rapid, valid and cost-effective method for guiding decision makers in allocating finite resources for the control of schistosomiasis.

Figure 1

(a) Probability of classifying school as having prevalence 50% or greater by prevalence, and (b) sample size required to make classifications in each simulation by prevalence for the sampling plan of n=15, d₁=7 based on 202,000 simulated surveys.

Figure 2

Box plot of true proportion infected by classification scheme (low, moderate and high) for a sampling plan of n=15, d₁=7, d₂=2 based on 202,000 simulated surveys. The horizontal line within each box represents the median; the lower and upper bounds of the box correspond to the 25- and 75 percentiles; the whiskers correspond to the range of non-outlying data. Width of boxes is proportional to the number in each category. Upper dashed line represents the 50% prevalence threshold and lower dashed line the 20% prevalence threshold.

Figure 3

Cost-effectiveness of screening using LQAS relative to mass treatment of all schools without prior screening in a hypothetical population of 23,188 schoolchildren in 34 schools on a yearly basis as a function of the proportion of schools having a prevalence of 50% or greater.

Figure 4

The effect of different costs per child treated on the cost-effectiveness of the LQAS method to screen schools and provide treatment according to low and high prevalence categories relative to a mass treatment approach in a hypothetical population of 23,188 schoolchildren in 34 schools on a yearly basis, where 21.4% of schools are assumed to have prevalence ≥50%.