Soil-transmitted helminths, or parasitic intestinal worms, are among the most prevalent and geographically widespread parasitic infections in the world. Accurate diagnosis and quantification of helminth infection are critical for informing and assessing deworming interventions. The Kato-Katz thick smear technique, the most widely used laboratory method to quantitatively assess infection prevalence and infection intensity of helminths, has often been compared with other methods. Only a few small-scale studies, however, have considered ways to improve its diagnostic sensitivity. This study, conducted among 4,985 school-age children in an area of rural China with moderate prevalence of helminth infection, examines the effect on diagnostic sensitivity of the Kato-Katz technique when two fecal samples collected over consecutive days are examined and compared with a single sample. A secondary aim was to consider cost-effectiveness by calculating an estimate of the marginal costs of obtaining an additional fecal sample. Our findings show that analysis of an additional fecal sample led to increases of 23%, 26%, and 100% for Ascaris lumbricoides, Trichuris trichiura, and hookworm prevalence, respectively. The cost of collecting a second fecal sample for our study population was approximately USD4.60 per fecal sample. Overall, the findings suggest that investing 31% more capital in fecal sample collection prevents an underestimation of prevalence by about 21%, and hence improves the diagnostic sensitivity of the Kato-Katz method. Especially in areas with light-intensity infections of soil-transmitted helminths and limited public health resources, more accurate epidemiological surveillance using multiple fecal samples will critically inform decisions regarding infection control and prevention.