Diagnostic sensitivity of formalin-fixed faecal microscopy for the detection of soil-transmitted helminths
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Background and objectives: Faecal microscopy is the mainstay of soil-transmitted helminth diagnosis and commonly completed on formalin-fixed samples when resources are insufficient to analyse fresh samples. This study assessed the diagnostic sensitivity of microscopic techniques using formalin-fixed samples.Methods: Formalin-fixed faecal samples from 574 individuals were tested by formalin-ethyl acetate concentration technique (FECT), Malachite smear, McMaster and McMaster2 methods. Agreement between tests was assessed by Kappa. Bayesian latent class models and a composite reference standard estimated the diagnostic sensitivity of each test.Results: Moderate-to-good agreement between tests was observed for A. lumbricoides. Agreement was poorer for hookworm and T. trichiura. The FECT (72.70%, CrI: 68.92–76.56%) and McMaster2 method (67.93%, CrI: 62.41–73.31%) had the highest sensitivities for A. lumbricoides. For hookworm, the McMaster2 method (70.56%, CrI: 64.10–76.96%) was more sensitive than all other tests. For T. trichiura, the McMaster (90.10%, CrI: 83.29–94.67%) and McMaster2 (89.3%, CrI: 82.28–94.52%) methods were the most sensitive.Discussion: The McMaster2 method is a viable alternative to FECT and provides important information on the intensity of infection. The effect of formalin-fixation on test performance may not be as great as previously assumed. This study reports formalin-fixed sensitivities similar to previous estimates using fresh samples.