Malaria laboratory findings
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]
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Laboratory Findings
Peripheral Blood Smear
- The most economic, preferred, and reliable diagnosis of malaria is microscopic examination of blood films because each of the four major parasite species has distinguishing characteristics.
- Two sorts of blood film are traditionally used.
- Thin films are similar to usual blood films and allow species identification because the parasite's appearance is best preserved in this preparation.
- Thick films allow the microscopist to screen a larger volume of blood and are about eleven times more sensitive than the thin film, so picking up low levels of infection is easier on the thick film, but the appearance of the parasite is much more distorted and therefore distinguishing between the different species can be much more difficult.
- With the pros and cons of both thick and thin smears taken into consideration, it is imperative to utilize both smears while attempting to make a definitive diagnosis.[1]
- From the thick film, an experienced microscopist can detect parasite levels (or parasitemia) down to as low as 0.0000001% of red blood cells. * Microscopic diagnosis can be difficult because the early trophozoites ("ring form") of all four species look identical and it is never possible to diagnose species on the basis of a single ring form; species identification is always based on several trophozoites. Please refer to the articles on each parasite for their microscopic appearances: P. falciparum, P. vivax, P. ovale, P. malariae.
Antigen Detection Tests
- In areas where microscopy is not available, or where laboratory staff are not experienced at malaria diagnosis, there are antigen detection tests that require only a drop of blood.[2]
- OptiMAL-IT® will reliably detect falciparum down to 0.01% parasitemia and non-falciparum down to 0.1%. Paracheck-Pf® will detect parasitemias down to 0.002% but will not distinguish between falciparum and non-falciparum malaria.
Polymerase Chain Reaction
- Parasite nucleic acids are detected using polymerase chain reaction. This technique is more accurate than microscopy. However, it is expensive, and requires a specialized laboratory.
- Moreover, levels of parasitemia are not necessarily correlative with the progression of disease, particularly when the parasite is able to adhere to blood vessel walls.
- Therefore more sensitive, low-tech diagnosis tools need to be developed in order to detect low levels of parasitaemia in the field. Areas that cannot afford even simple laboratory diagnostic tests often use only a history of subjective fever as the indication to treat for malaria.
- Molecular methods are available in some clinical laboratories and rapid real-time assays (for example, QT-NASBA based on the polymerase chain reaction)[3] are being developed with the hope of being able to deploy them in endemic areas.
Geimsa Staining Technique
- Using Giemsa-stained blood smears from children in Malawi, one study showed that unnecessary treatment for malaria was significantly decreased when clinical predictors (rectal temperature, nailbed pallor, and splenomegaly) were used as treatment indications, rather than the current national policy of using only a history of subjective fevers (sensitivity increased from 21% to 41%). [4]
Severe malaria is commonly misdiagnosed in Africa, leading to a failure to treat other life-threatening illnesses. In malaria-endemic areas, parasitemia does not ensure a diagnosis of severe malaria because parasitemia can be incidental to other concurrent disease. Recent investigations suggest that malarial retinopathy is better (collective sensitivity of 95% and specificity of 90%) than any other clinical or laboratory feature in distinguishing malarial from non-malarial coma.[5]
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Blood smear from a P. falciparum culture (K1 strain). Several red blood cells have ring stages inside them. Close to the center there is a schizont and on the left a trophozoite.
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Malaria (organisms in cells)
Electron Microscope
References
- ↑ Warhurst DC, Williams JE (1996). "Laboratory diagnosis of malaria". J Clin Pathol. 49: 533–38. PMID 8813948.
- ↑ Pattanasin S, Proux S, Chompasuk D, Luwiradaj K, Jacquier P, Looareesuwan S, Nosten F (2003). "Evaluation of a new Plasmodium lactate dehydrogenase assay (OptiMAL-IT®) for the detection of malaria". Transact Royal Soc Trop Med. 97: 672–4. PMID 16117960.
- ↑ Mens PF, Schoone GJ, Kager PA, Schallig HDFH. (2006). "Detection and identification of human Plasmodium species with real-time quantitative nucleic acid sequence-based amplification". Malaria Journal. 5 (80). doi:10.1186/1475-2875-5-80.
- ↑ Redd S, Kazembe P, Luby S, Nwanyanwu O, Hightower A, Ziba C, Wirima J, Chitsulo L, Franco C, Olivar M (1996). "Clinical algorithm for treatment of Plasmodium falciparum malaria in children". Lancet. 347 (8996): 223–7. PMID 8551881..
- ↑ Beare NA et al. Am J Trop Med Hyg. 2006 Nov;75(5):790-797.