How Long Can Stool Samples Be Fixed for an Accurate Diagnosis of Soil-Transmitted Helminth Infection Using Mini-FLOTAC?

Background: Kato-Katz is a widely used method for the diagnosis of soil-transmitted helminth infection. Fecal samples cannot be preserved, and hence, should be processed on the day of collection and examined under a microscope within 60 min of slide preparation. Mini-FLOTAC is a technique that allows examining fixed fecal samples. We assessed the performance of Mini-FLOTAC using formalin-fixed stool samples compared to Kato-Katz and determined the dynamics of prevalence and intensity estimates of soil-transmitted helminth infection over a 31-day time period. Methodology: The study was carried out in late 2013 on Pemba Island, Tanzania. Forty-one children were enrolled and stool samples were subjected on the day of collection to a single Kato-Katz thick smear and Mini-FLOTAC examination; 12 aliquots of stool were fixed in 5% formalin and subsequently examined by Mini-FLOTAC up to 31 days after collection. Principal Findings: The combined results from Kato-Katz and Mini-FLOTAC revealed that 100% of children were positive for Trichuris trichiura, 85% for Ascaris lumbricoides, and 54% for hookworm. Kato-Katz and Mini-FLOTAC techniques found similar prevalence estimates for A. lumbricoides (85% versus 76%), T. trichiura (98% versus 100%), and hookworm (42% versus 51%). The mean eggs per gram of stool (EPG) according to Kato-Katz and Mini-FLOTAC was 12,075 and 11,679 for A. lumbricoides, 1,074 and 1,592 for T. trichiura, and 255 and 220 for hookworm, respectively. The mean EPG from day 1 to 31 of fixation was stable for A. lumbricoides and T. trichiura, but gradually declined for hookworm, starting at day 15. Conclusions/Significance: The findings of our study suggest that for a qualitative diagnosis of soil-transmitted helminth infection, stool samples can be fixed in 5% formalin for at least 30 days. However, for an accurate quantitative diagnosis of hookworm, we suggest a limit of 15 days of preservation. Our results have direct implication for integrating soil-transmitted helminthiasis into transmission assessment surveys for lymphatic filariasis. Author Summary: Soil-transmitted helminths are parasitic worms (hookworm, roundworm, and whipworm) that affect hundreds of millions of people. Kato-Katz is the most widely used technique for the diagnosis of soil-transmitted helminth infection. It requires the collection, processing, and microscopic examination of stool samples within the same day. In remote areas, laboratories are often far away from where stool samples are being collected, which makes it difficult to examine the samples on the same day. Mini-FLOTAC is an alternative to Kato-Katz technique that allows analysis of fixed stool samples several days after collection. We assessed the accuracy of the Mini-FLOTAC with fixed stool samples. The study was carried out in late 2013 on Pemba Island and 41 children participated. Fresh stool samples were first examined by Kato-Katz and then fixed in formalin and examined by Mini-FLOTAC (12 examinations within one month). We found that for a qualitative diagnosis (presence or absence of infection) stool samples can be fixed for 31 days. However, for an accurate quantitative diagnosis of hookworm eggs, stool samples should not be stored for more than 15 days, as egg counts decline. Our results have implications for integrating soil-transmitted helminth surveys with other neglected tropical diseases. [ABSTRACT FROM AUTHOR]