Accuracy of a fluorescence flow cytometer (Sysmex XN-31) for diagnosing malaria: two years of experience in a tertiary care hospital

Question: Early and accurate diagnosis is essential for effective management of malaria. In non-endemic areas, laboratory staff may lack experience with malaria and fail to detect parasites when examining blood smears microscopically. Non-microscopy based methods like immunochromatographic assays, PCR and more recently fluorescence flow cytometry using Sysmex XN-31 (Sysmex Deutschland GmbH, Norderstedt) has been registered to support malaria diagnosis. XN-31 is fully automated with a limit of detection of 20 parasites/µl. We assessed the diagnostic accuracy of the XN-31 within a prospective study for malaria diagnosis compared to microscopy and PCR. Methods: 203 blood samples were analysed by XN-31, thin blood film after automatic Giemsa staining with Aerospray Pro (Kreienbaum GmbH, Langenfeld) and RealStar Malaria Screen & Type PCR Kit 1.0 (altona Diagnostics GmbH, Hamburg). An immunchromatographic assay was also performed but results were not taken into account in ths study. In order to assess interlaboratory performance of XN-31, a small subset of samples (n=14) was evaluated in a second laboratory. Results: Over two years, 137 negative and 70 malaria positive blood samples were analysed initially and 145 follow-up samples. Compared to microscopy, XN-31 showed high sensitivity (98.4%), specificity (100%) as well as excellent negative predictive value (NPV, 99,3%) and positive predictive value (PPV, 100%) for the initial diagnosis of malaria. Agreement was high with Cohen`s kappa of 0.988. In comparison to PCR, values were similar: sensitivity 92.6%, specificity 100%, NPV 96%, PPV 100% and high agreement (Cohen`s kappa 0.941). In five cases, the XN-31 result was not available. XN-31 separated P. falciparum from non-falciparum successfully and parasitaemia determination was performed within one minute. Interlaboratory comparison showed high reproducibility of XN-31. Conclusions: Malaria diagnosis can be improved in non-endemic areas by use of XN-31 combined with microscopy and PCR. Parasite load is provided in significant shorter time by the device making diagnosis of malaria faster. Especially in follow-up samples microscopy can likely be replaced by XN-31.