|Year : 2000 | Volume
| Issue : 10 | Page : 421-424
Evaluation & usefulness of a immunochromatographic test for rapid detection of plasmodium falciparum infection
Harjeet Kaur1, A Mani2
1 Department of Microbiology, Christian Medical College, Ludhiana, India
2 Department of Medicine, Christian Medical College, Ludhiana, India
Department of Microbiology, Christian Medical College, Ludhiana
|How to cite this article:|
Kaur H, Mani A. Evaluation & usefulness of a immunochromatographic test for rapid detection of plasmodium falciparum infection. Indian J Med Sci 2000;54:421-4
|How to cite this URL:|
Kaur H, Mani A. Evaluation & usefulness of a immunochromatographic test for rapid detection of plasmodium falciparum infection. Indian J Med Sci [serial online] 2000 [cited 2013 Jun 19];54:421-4. Available from: http://www.indianjmedsci.org/text.asp?2000/54/10/421/12124
Psasmodium falciparum is the most pathogenic of malarial species and is frequently fatal, if untreated. Hence early and rapid diagnosis of falciparum malaria is required for effective management of patients. The problem of drug resistance and substitution of newer, costlier drugs brings with it the need for rapid, accurae and in-expensive diagnostic procedures so that the directed therapy can be used.
Plasmodium falciparum parasite synthesises several proteins containing large amounts of the amino acid histidine, which is reffered to as histidine rich proteins (PfHRP). One of these PfHRP 2 has been sequenced and shown to contain 34% histidine and 37 alanine as deduced from cloned genomic DNA.  PfHRP-2 is synthesized by the parasite and released from infected erythrocytes as a water soluble protein.  It can be detected in in-vitro culture supernatant of synchronised parasites as early as 2 to 8 hours, after ring development, indicating it is actively secreted from infected cells. The amount of PfHRP-2 released in vitro, continues to increase throughout the erythrocytic cycle, with a large amount being released during schizont rupture. All parasites isolated from Africa, Asia and South America express similar PfHRP-2.  Thus, this extracellular protein PfHRP-2 is being used for rapid detection of this infection. PfHRP-2 is not detectable in mature circulating gametocytes which tend to remain in the blood stream, weeks to months after clearance of trophozoites. Thus infection with only gametocytes are not responsible for false positive results.  'Parachek Pf' (Orchid Biomedical systems, Goa) is a rapid test based on he principle of immunochromatography and detects Pf HRP-2. This report evaluates and high lights the usefulness of this technique.
| ¤ Material and Methods|| |
This prospective study was carried out over a period of one year (January to December 1999) in the department of Microbiology at Christian Medical College & Hospital, Ludhiana. All patients (adults and paediatric) suspected of cerebral malarial fever were tested. Whole blood samples from 257 patients were received in the laboratory and these were examined for HRP-2 as per manufacturer's instructions. The test took 5 to 6 minutes to complete. Thick and thin blood smears were also axamined for malarial parasites by two experienced microscopists. The number of parasites was determined by counting asexual forms in 1000 RBC (10 oil Immersion Fields) and density was calculated based on the assumption that 5000,000 RBC are present per µl of blood.  Hundred oil immersion fields of thick smear were examined before declaring the slide negative.
| ¤ Results|| |
By light microscopy, 15 smears had P.falciparum, 15 P.vivax and 7 mixed infection (P.falciparum & P. vivax) Prevalence of P.falciparum infection & P.vivax was 8.5% each and that of mixed infection was 2.7;% (7/257). P.falciparum parasitaemia ranged between 0.2% (10,000/µl) to 11% (560,000/µl), with geometric mean of 43651 /µl, both in single and mixed infections. Details are given in [Table 1].
The test Paracheck Pf had a high sensitivtiy and specificity when compared with microscopy. Of the 22 slides positive for infection, all tested positive by Parachek Pf (sensitivity 100%). One patient's blood was negative by microscopy but was positive for HRP - 2 (specicity 99.5%). This could be due to low parasitaemia because sensitivity of test is reported to be 85% in parasiaemias between 50 and 500 parasies/µl and 100%, for parasitaemias over 501 parasites/µl. 
The patients who tested positive for P.falciparum antigen were treated with quinine, Doxycyeline and pyremethamine and these had remarkable recovery within 1 to 2 days. Post recovery HPR-2 levels were not determined.
| ¤ Discussion|| |
The reliable diagnosis of malaria, whether in a hospital or in a rural health clinic or patient's home at peripheral level is pre-requisite for selecting the correct treatment and consequently reducng malaria morbidity and mortality especially so in case of Plasmodium falciparum infection. Though light microscopy, still is the best method for accurate diagnosis, it is tedious and labour intensive. Thus the need for rapid techniques. The first report using rapid immunochromatographic strip test that detects presence of P. falciparum specific antigen HRP2 appeared in 1996.  Field trials conducted in the Solomon Islands found the test to have a sensitivity of 100% and specificity of 96.2% when compared with light microscopy. Our experience was similar. It has been reported, that HRP-2 assays are sensitive at parasitaemias of 0.08% and above, though there is no clear correlation between the amount of Antigen and low parasitaemia.  One of the reasons being that the antigen persists in the serum for several days (12 weeks) after parasitic clearance. The ability to accurately diagnose low parasitaemia may depend not only on the number of parasites but also on the length of infection.  In the current study, the 'Parachek Pf' performed well in diagnosing falciparum malaria in patient's with fever. The test is rapid, easy to perform and helps the physician to make a 'on the spot' diagnosis. Since the test remains positive for 1-2 weeks even if the patient has taken inadequate antimalarials and microdensity is lowered, the test can come positive. But for follow up, the test should be repeated after 2 weeks, as HRP2 persists for about 15 days. This does not indicate failed therapeutic response. Rapid detection of P.falciparum is a life saving measure. The test is highly recommended and will be very useful in endemic areas and rural set ups and in emergencies.
| ¤ Summary|| |
Rapid test (Parachek Pf) based on detection of HRP-2 protein specific to Plasmodium falciparum by immunochromatographic technique was evaluated. Prevalenc of infection was 8.5%. The test was 100% sensitive & 99.5% specific on comparison with light microscopy. The test is useful for making 'on the spot' diagnosis.
| ¤ References|| |
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