Because malaria symptoms are very non-specific and overlap with those of other febrile illnesses, a biological diagnosis is a necessity1; especially in Multi-drugs resistance areas like Southeast Asia.
Microscopy is the most widely tool used to diagnose malaria at peripheral levels. In capable hands it is very sensitive for parasitaemia > 50 / µl (0.001 %)2 and it can give important information to the clinician like species, parasites stages and parasite density.
However good quality microscopy is difficult to implement and maintain: it is labor intensive, it requires highly skilled technicians and regular quality control.
The development in the past 5 years of rapid diagnosis tests (RDT) has open new perspectives in the diagnosis of malaria. These RDT require little or no additional material, they are easy and quick to perform (results obtained after 15 to 20 minutes) and they do not require extensive training.
However, interpretation of the results is not always simple. It is important for medics to understand how RDTs work and what are their limitations in order to be able to interpret the results according to clinic and patient's history.
Principle
These tests are based on the detection of antigens derived from malaria parasites, using immunochromatographic methods3. Immunochromatographic methods are based on the capture of parasite antigen from peripheral blood using antibodies, prepared against these antigens, and conjugated to dye particles in order to get a colored reaction.
There are two antigens detected by currently available tests: HRP-2 (Histidine-rich protein 2) and pLDH (parasite Lactate Dehydrogenase).
Tests detecting HRP-2: Mainly used on the border is the Paracheck-Pf test (Orchid Biomedical Systems, India).
It is the cheapest on the market: 20 to 30 Baths / test.
Since only P.falciparum releases HRP-2, this test will give negative results with patients infected with P.vivax, P.ovale, or P.malariae. Non-falciparum malaria may therefore be misdiagnosed as malaria negative.
Performances & limitations of Paracheck-Pf:
Sensitivity.
Different studies showed a global sensitivity of over 90% in the detection of P.falciparum compared with microscopy: this means that the test will give less than 10 negative results (false negative) for 100 P.falciparum microscopically confirmed.
False negative results were associated with low parasitaemia under 100 parasites / µl: often these patients are asymptomatic.
Important note: False negative results have been reported in patients with severe condition and very high parasitaemia (> 20 % of RBC parasitised)4. This phenomenon (prozone event) is rare but severe malaria should not be excluded on the sole basis of a negative test.
Specificity.
The specificity of Paracheck-Pf is high (> 90%)5: this means that the test will give less than 10 positive results (false positive) for 100 negative patients microscopically confirmed.
False positive results are partly explained by the fact that the body slowly eliminates HRP-2 after parasite clearance: HRP-2 tests can remain positive for up to one month (mean time being about 2 weeks) after parasite clearance.
HRP-2 clearance time is function of many factors and not well-understood yet2. However HRP-2 clearance time is linked to the patient's parasitaemia at Do: the highest the parasitaemia the longer it will take to the body to eliminates HRP-2.
False positive occurs also because microscope cannot detect less than 108 parasites in the body; RDTs can if HRP-2 (or pLDH) is still hanging in the circulation.
For example a person who takes quinine as self-treatment for few days before presenting to OPD may have a malaria smear negative while Paracheck-Pf (or OptiMAL) can be positive.
As a result patient history is important when diagnosis is based on HRP-2: if possible, all patients with a positive test and a history of P.falciparum infection ADEQUATELY TREATED [see note (***)] in the previous month should always have a malaria smear for confirmation. Remark: Pregnant women may remain HRP-2 positive after parasite clearance for a much longer time than non-pregnant patients.
False positive HRP-2 results are also reported in patients with autoimmune diseases like rheumatoid factor.
This RDT can detect P.falciparum and non-P.falciparum (P.vivax, P.malariae and P.ovale) but cannot detect mixed infections or distinguish between P.vivax, P.ovale and P.malariae.
It cost about 70 to 80 Baths / test.
Performances & limitations of OptiMAL:
Sensitivity.
For the detection of P.falciparum OptiMAL has a reported sensitivity of over 90%.
As for Paracheck-Pf false negative results were associated with low parasitaemia under 100 parasites / µl.
For the detection of Non-P.falciparum, OptiMAL has a global sensitivity ranging between 80 to 95 %. Sensitivity for the detection of P.malariae is reportedly lower (about 50%)2.
Specificity.
The specificity of OptiMAL for both P.falciparum and non-P.falciparum is reportedly higher (> 95 %).
There is less false positive with OptiMAL, probably because pLDH levels follow more closely parasites level in the blood6: After treatment, patients become OptiMAL negative quickly (within a week).
In that regard, OptiMAL may be used to monitor therapeutic response (more studies needed).
How to Interpret Paracheck-Pf tests results?
NOTES: (*): Do not forget that you cannot exclude severe malaria on the sole basis of a NEG test. (**): A strong positive test is a test that turns positive as soon as the lysed blood reaches the reactive band and give an intense broad red line. (***): Adequate treatments in this area are: MAS3 or A7T7(D7) or Q7T7(D7) supervised (refer to SMRU malaria handout).
References:
Clinical features cannot predict a diagnosis of malaria or differentiate the infecting species in children living in an area of low transmission by Luxemburger C
Transaction of the royal society of tropical medicine and hygiene (1998) 92, 45-49.
Rapid Diagnostic Tests for Malaria Parasites by Anthony Moody
Clinical Microbiology Reviews, Jan. 2002, p. 66-78 Vol. 15, No. 1
New Perspectives Malaria Diagnosis,
Report of a Joint WHO/USAID Informal Consultation.
WHO/CDS/RBM/2000.14
WHO/MAL/2000.1091
Self-use of rapid tests for malaria diagnosis by L.Rish
The lancet, Vol 355, January 15 2000, Page 237.
Rapid diagnostic techniques for malaria control by Chansuda Wongsrichanalai
Research Update, TRENDS in Parasitology Vol.17 No.7 July 2001
P. falciparum: Evaluation of LDH in monitoring therapeutic responses to standard Antimalarial drugs in Nigeria by A.M.J. Oduola & Al. Experimental Parasitology 87, 283-289 (1997).
