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

Abstract

BackgroundKato-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.MethodologyThe 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 FindingsThe 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/SignificanceThe 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.

Quantitative Polymerase Chain Reaction for Diagnosis of Soil-Transmitted Helminth Infections: A Comparison with a Flotation-Based Technique and an Investigation of Variability in DNA Detection

“…A further limitation is that samples were stored in formalin for 1-3 months before analysis using SNF. Diagnostic accuracy of flotation techniques for samples stored in formalin may decrease after 15 days for hookworm infections 28 ; therefore, SNF sensitivity for detecting hookworm may have been suboptimal.…”

The Right Tool for the Job: Detection of Soil-Transmitted Helminths in Areas Co-endemic for Other Helminths

“…Following the methods used by [ 14 , 27 – 38 ], the sensitivity and negative predictive value (NPV) were assessed for each diagnostic technique in comparison to a composite reference standard, which was defined as being positive if any of the 3 tests were positive. The specificity of the composite standard was assumed to be 100% given the small number of false positives found when experienced technicians examine the slides microscopically [ 39 ].…”

New diagnostic tools in schistosomiasis

“…Subsequently, FLOTAC has been broadly validated for the diagnosis of human nematodes and trematodes, including S. mansoni [40,[74][75][76]. Mini-FLOTAC is a further development of the initial FLOTAC technique [66,77,78]. Of note, the principle of the FLOTAC family is homogenisation of approximately 1 g of stool in formalin, filtration to remove debris, and addition of a flotation solution with a specific gravity to bring helminth eggs into flotation.…”