| Summary: | <p>Abstract</p> <p>Background</p> <p>In areas co-endemic for multiple <it>Plasmodium</it> species, correct diagnosis is crucial for appropriate treatment and surveillance. Species misidentification by microscopy has been reported in areas co-endemic for vivax and falciparum malaria, and may be more frequent in regions where <it>Plasmodium knowlesi</it> also commonly occurs.</p> <p>Methods</p> <p>This prospective study in Sabah, Malaysia, evaluated the accuracy of routine district and referral hospital-based microscopy, and microscopy performed by an experienced research microscopist, for the diagnosis of PCR-confirmed <it>Plasmodium falciparum</it>, <it>P</it>. <it>knowlesi</it>, and <it>Plasmodium vivax</it> malaria.</p> <p>Results</p> <p>A total of 304 patients with PCR-confirmed <it>Plasmodium</it> infection were enrolled, including 130 with <it>P</it>. <it>knowlesi</it>, 122 with <it>P</it>. <it>falciparum</it>, 43 with <it>P</it>. <it>vivax</it>, one with <it>Plasmodium malariae</it> and eight with mixed species infections. Among patients with <it>P</it>. <it>knowlesi</it> mono-infection, routine and cross-check microscopy both identified 94 (72%) patients as “<it>P</it>. <it>malariae</it>/<it>P</it>. <it>knowlesi</it>”; 17 (13%) and 28 (22%) respectively were identified as <it>P</it>. <it>falciparum</it>, and 13 (10%) and two (1.5%) as <it>P</it>. <it>vivax</it>. Among patients with PCR-confirmed <it>P</it>. <it>falciparum</it>, routine and cross-check microscopy identified 110/122 (90%) and 112/118 (95%) patients respectively as <it>P</it>. <it>falciparum</it>, and 8/122 (6.6%) and 5/118 (4.2%) as “<it>P</it>. <it>malariae</it>/<it>P</it>. <it>knowlesi</it>”. Among those with <it>P</it>. <it>vivax</it>, 23/43 (53%) and 34/40 (85%) were correctly diagnosed by routine and cross-check microscopy respectively, while 13/43 (30%) and 3/40 (7.5%) patients were diagnosed as “<it>P</it>. <it>malariae</it>/<it>P</it>. <it>knowlesi</it>”. Four of 13 patients with PCR-confirmed <it>P</it>. <it>vivax</it> and misdiagnosed by routine microscopy as “<it>P</it>. <it>malariae</it>/<it>P</it>. <it>knowlesi</it>” were subsequently re-admitted with <it>P</it>. <it>vivax</it> malaria.</p> <p>Conclusions</p> <p>Microscopy does not reliably distinguish between <it>P</it>. <it>falciparum</it>, <it>P</it>. <it>vivax</it> and <it>P</it>. <it>knowlesi</it> in a region where all three species frequently occur. Misdiagnosis of <it>P</it>. <it>knowlesi</it> as both <it>P</it>. <it>vivax</it> and <it>P</it>. <it>falciparum</it>, and <it>vice versa</it>, is common, potentially leading to inappropriate treatment, including chloroquine therapy for <it>P</it>. <it>falciparum</it> and a lack of anti-relapse therapy for <it>P</it>. <it>vivax</it>. The limitations of microscopy in <it>P</it>. <it>knowlesi</it>-endemic areas supports the use of unified blood-stage treatment strategies for all <it>Plasmodium</it> species, the development of accurate rapid diagnostic tests suitable for all species, and the use of PCR-confirmation for accurate surveillance.</p>
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