## Diagnostic Methods for Plasmodium — Sensitivity and Limitations **Key Point:** Rapid diagnostic tests (RDTs) do NOT have uniform sensitivity across all Plasmodium species. P. malariae and P. ovale detection by RDT is significantly lower than P. falciparum and P. vivax, especially at low parasitemia levels. ### Comparison of Diagnostic Modalities: | Method | Sensitivity | Specificity | Advantages | Limitations | |--------|-------------|-------------|------------|-------------| | **Thick smear** | Higher (concentrates parasites) | Lower (morphology harder) | Detects low parasitemia | Species ID difficult | | **Thin smear** | Lower | Higher (clear morphology) | Species identification | Misses low parasitemia | | **RDT** | 90–98% (P. falc/vivax) | 95–99% | Rapid, field-friendly | Poor for P. malariae/ovale; false negatives at <100/μL | | **PCR** | >99% | >99% | Gold standard, detects 1/μL | Expensive, not field-applicable | ### Why Option 2 is Incorrect: **High-Yield:** RDTs have **variable sensitivity by species**: - **P. falciparum & P. vivax:** ~95% sensitivity at >100 parasites/μL - **P. malariae & P. ovale:** 50–80% sensitivity (much lower) - **Mixed infections:** RDTs may miss minority species The claim of ">95% sensitivity in ALL species" is false. P. malariae and P. ovale are frequently missed by RDTs, particularly at lower parasitemia levels. **Clinical Pearl:** A negative RDT does NOT exclude malaria, especially in P. malariae/ovale endemic areas. Microscopy or PCR confirmation is mandatory if clinical suspicion is high. **Mnemonic: "RTDP" — RDT Pitfalls:** - **R**are species (P. malariae, P. ovale) have lower sensitivity - **T**iming: early infection may be missed - **D**ensity: low parasitemia (<100/μL) reduces detection - **P**rozone effect: very high parasitemia may give false negatives [cite:Park 26e Ch 3; WHO Malaria Diagnosis Guidelines 2022]
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