Diagnostic sensitivity of formalin-fixed faecal microscopy for the detection of soil-transmitted helminths.

Background: 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 the 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 Trichuris trichiura. The FECT (72.70%, credible interval [CrI]: 68.92-76.56%) and McMaster2 method (67.93%, 95% CrIs: 62.41-73.31%) had the highest sensitivities for A. lumbricoides. For hookworm, the McMaster2 method (70.56%, 95% CrIs: 64.10-76.96%) was more sensitive than all other tests. For T. trichiura, the McMaster (90.10%, 95% CrIs: 83.29-94.67%) and McMaster2 (89.3%, 95% CrIs: 82.28-94.52%) methods were the most sensitive.

Conclusions: 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.