Papers

Rapid diagnosis of falciparum malaria by using the ParaSight F test in travellers returning to the United Kingdom: prospective study

I M Cropley, senior registrar in infection and tropical medicine ^a, Diana N J Lockwood, senior registrar in infection and tropical medicine ^a, D Mack, chief medical laboratory scientific officer ^b, G Pasvol, professor of infection and tropical medicine ^a, R N Davidson, senior lecturer in infection and tropical medicine ^a. 

^a Lister Unit, Northwick Park Hospital, Harrow, Middlesex HA1 3UJ, ^b Department of Haematology, Northwick Park Hospital

Correspondence to: D N J Lockwood, Hospital for Tropical Diseases, London WC1E 6AU diana.lockwood{at}lshtm.ac.uk

A simple diagnostic strip test for Plasmodium falciparum malaria (ParaSight F test, Becton Dickinson Advanced Diagnostics) detects a water soluble antigen, histidine rich protein 2, which is produced by blood stages of P falciparum. High sensitivity and specificity have been reported for the test in areas where malaria is endemic^1-3 and in studies of travellers returning from such areas. ^{4 5} We compared the test with standard blood film microscopy in febrile travellers returning to the United Kingdom from such areas.

	Subjects, methods, and results

Top Subjects, methods, and results Comment References

We studied 160 consecutive patients aged 9-77 years presenting between April 1994 and June 1996 to our unit with a history of fever and travel in the previous year to an area where malaria is endemic. Thin films were stained with Giemsa and read by an experienced microscopist. The ParaSight F test was performed in accordance with the manufacturer's instructions; a pink band indicates a positive result. Each test took less than 10 minutes to perform. Thin films and test strips were read blind to each other.

In 45 patients falciparum malaria was the final diagnosis (table). At presentation 42 cases were detected by microscopy and 42 by the ParaSight F test. Parasitaemias ranged from <0.01% to 15% of erythrocytes parasitised. In one patient, the test was positive at presentation, and scanty (<0.001%) P falciparum trophozoites were detected on blood film only on day 2. In two other patients both the blood film and the test gave negative results at presentation but positive results on subsequent days. One patient had a positive test with a negative blood film; three days previously he had had halofantrine treatment for presumed malaria. One patient with pneumococcal meningitis had positive tests over three days with negative daily blood films. The test was negative in one patient with a P falciparum parasitaemia of <0.01%.

Test results were negative in all 113 other patients who did not have P falciparum infection, including 27 infected with other malarial species (23 with P vivax, 3 with P ovale, 1 with P malariae). Other diagnoses included diarrhoeal disease, dengue fever, typhoid, pneumonia, urinary tract infection, brucellosis, acute myeloid leukaemia, and infectious mononucleosis.

Compared with the final diagnosis, the ParaSight F test used at first presentation had a sensitivity of 93.3%, a specificity of 98.3%, a positive predictive value of 95.6%, and a negative predictive value of 97.4%.

	Comment

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The ParaSight F test is simple, rapid, and has adequate sensitivity and specificity for initial assessment of P falciparum infection in returning travellers. It identified all patients with P falciparum apart from one patient with a low parasitaemia of <0.01% and two patients with parasites not detected on initial microscopy. Positive test results in the patient treated with halofantrine are explained by the established persistence of histidine rich protein 2 in the blood for up to 10 days. Positive results of the patient with pneumococcal meningitis were taken to be a genuine false positives.

The test does not remove the need for blood film examination as it is not 100% sensitive at low parasitaemias, and repeated daily testing may be necessary to establish the diagnosis. Nor does the test give any indication of density of parasites, essential in planning management.

The ParaSight F test has a useful role in the initial screening of febrile returning travellers with suspected falciparum malaria, particularly where laboratory staff are not experienced in diagnosing malaria. The test can be considered a "side room" investigation, as it requires no special training. It may also be used to distinguish between the benign malarias and the potentially lethal falciparum malaria.

	Acknowledgments

   Contributors: DNJL and RND designed the study; DNJL, DM, and IMC carried out the study; IMC wrote the first draft of the paper, with subsequent input from GP, RD, and DNJL. DNJL is the guarantor for the study.

	Footnotes

Funding: ParaSight F test strips were provided by Becton-Dickinson.

Competing interests: None declared.

	References

Top Subjects, methods, and results Comment References

1.	Shiff CJ, Premji Z, Minjas JN. The rapid manual ParaSight-F test. A new diagnostic tool for Plasmodium falciparum infection. Trans R Soc Trop Med Hyg 1993; 87: 646-648[CrossRef][Medline].
2.	Beadle C, Long GW, Weiss WR, McElroy PD, Maret SM, Oloo AJ, et al. Diagnosis of malaria by detection of Plasmodium falciparum HRP-II antigen with a rapid dipstick antigen-capture assay. Lancet 1994; 343: 564-568[CrossRef][Medline].
3.	Genton B, Paget S, Beck HP, Gibson N, Alpers MP, Hi I. Diagnosis of Plasmodium falciparum infection using ParaSight(R)-F test in blood and urine of Papua New Guinean children. J Southeast Asian J Trop Med Public Health 1998; 29: 35-40.
4.	Van den Ende J, Vervoort T, Van Gompel A, Lynen L. Evaluation of two tests based on the detection of histidine rich protein 2 for the diagnosis of imported Plasmodium falciparum malaria. Trans R Soc Trop Med Hyg 1998; 92: 285-288[Medline].
5.	Pieroni P, Mills CD, Ohrt C, Harrington MA, Kain KC. Comparison of the ParaSight-F test and the ICT Malaria Pf test with the polymerase chain reaction for the diagnosis of Plasmodium falciparum malaria in travellers. Trans R Soc Trop Med Hyg 1998; 92: 166-169[CrossRef][Medline].

(Accepted 22 February 2000)