Comparative evaluation of Midi Parasep® Solvent Free and the Ritchie concentration technique for helminth and protozoa visualisation in clinical stool samples

Abstract Background This study aimed to evaluate and compare the diagnostic performance of the Midi Parasep ® Solvent Free (SF) system and the Ritchie method for detecting helminths and protozoa in clinical stool samples. It also assessed the most suitable concentration technique for different laboratory contexts on the basis of parasitic burden and epidemiological factors. Methods A retrospective comparative study was performed with 100 helminth-positive samples from the Drassanes Vall d’Hebron Microbiology Laboratory (Barcelona, Spain). Samples were previously identified using the Ritchie technique and were reprocessed using the Midi Parasep ® SF system. All samples were examined by expert microscopists and in accordance with World Health Organization protocols and quality standards. Bivariate analysis was performed using the Z-test or Fisher’s exact test, as appropriate, and differences were considered statistically significant at P < 0.05. Results The Ritchie method detected 139 parasitic aetiologies, whereas Midi Parasep ® SF identified 85, yielding an overall concordance of 61.15%. While protozoan detection showed 100% concordance between both methods, the correlation for helminths was significantly lower (54.6%; P < 0.001). Midi Parasep ® SF exhibited reduced sensitivity, particularly for larger helminths (e.g., Strongyloides stercoralis, Schistosoma intercalatum ) and samples with low parasitic burden. In addition, the Alcorfix TM fixative agent caused morphological alterations in some helminth eggs and larvae. Midi Parasep ® SF offers operational advantages and hazard reduction; however, the Ritchie method is more sensitive for helminth detection. Conclusions The choice of concentration technique should be guided by the clinical context. Midi Parasep ® SF is efficient for protozoa and suitable for routine use in high-throughput settings. Conversely, the Ritchie method is preferable when helminth infection is highly suspected, particularly in migrant or travel medicine populations. Moreover, other diagnostic techniques, such as serological assays, could contribute to a more accurate diagnosis, thereby guiding the selection of the most appropriate concentration technique. Graphical Abstract