Short communication: Comparison of the OptiMAL rapid test and microscopy for detection of malaria in pregnant women in Nigeria
T. A. VanderJagt, J. Belmonte, R. H. Glew, D. J. VanderJagt (corresponding author), Department of Biochemistry and Molecular Biology, MSC08 4670, University of New Mexico School of Medicine, Albuquerque, NM, 87131-0001, USA. E-mail: email@example.com; firstname.lastname@example.org; email@example.com
E. I. Ikeh, Department of Medical Microbiology, Faculty of Medical Sciences, University of Jos, PMB 2084, Jos, Nigeria. E-mail: firstname.lastname@example.org
I. O. A. Ujah, Department of Obstetrics and Gynaecology, Jos University Teaching Hospital, PMB 2076, Jos, Nigeria. E-mail: email@example.com
Malaria during pregnancy causes anaemia in pregnant women and low birthweight in infants. The purpose of this study was to evaluate the OptiMAL rapid malaria test in comparison with standard microscopy for malaria diagnosis in 268 pregnant women attending antenatal clinics at the Jos University Teaching Hospital and the Federal Medical Centre-Gombe in Nigeria. Positive results by either method were confirmed using a polymerase chain reaction assay for malaria. Although the OptiMAL assay did not detect malaria in the blood of pregnant women with low levels of parasitaemia, it may be useful for the detection of placental malaria which predisposes to low birthweight infants.
Approximately 30 million women in malaria-endemic regions of Africa become pregnant every year. Infection with malaria during pregnancy poses health risks for both mother and foetus and it is estimated that between 75 000 and 200 000 infant deaths each year are attributable to maternal malarial infection (Steketee et al. 2001).
The ‘gold standard’ method for diagnosis of malaria is microscopy. Although microscopy has a high level of sensitivity for malaria detection, it is time-consuming and requires highly trained personnel. In developing countries with limited health care systems and resources, these requirements are usually difficult to fulfill, particularly in rural areas. Although methods based on the polymerase chain reaction (PCR) have the greatest sensitivity for detection of malaria, these methods require reagents and instrumentation that are not widely available in developing countries. Alternative rapid diagnostic tests for malaria have been developed which utilize immunochromatographic-capture procedures with monoclonal antibodies that detect parasite-specific antigens. The OptiMAL Rapid Malaria Test uses monoclonal antibody-coated strips to detect parasite-specific lactate dehydrogenase (pLDH) in blood (Piper et al. 1999). This test can be performed in 20 min and does not require highly trained personnel. An advantage of the OptiMAL test is that it detects only live parasites, thereby providing a sensitive means of monitoring treatment.
In areas where women may have acquired some immunity to malaria, infected women may be asymptomatic with anaemia and placental infection being the only manifestations. Sequestration of malaria parasites in the intervillous spaces of the placenta leads to an inflammatory response and decreased blood flow to the foetus. A major consequence of placental malaria infection is the delivery of low birthweight infants. Therefore, a method that is capable of detecting placental infection prior to delivery is of importance.
All women attending the antenatal clinics at the University of Jos Teaching Hospital and the Federal Medical Centre-Gombe between June and August 2002, were invited to participate in the study. Informed consent was obtained from each participant and the study was approved by the Ethics Review Committee of each institution. Age, weight, and height and information regarding gestational age and parity were obtained for each subject.
Blood samples were collected by venipuncture into vacutainers containing EDTA. After mixing, the blood samples were used to determine the haematocrit, for the preparation of thick and thin slides for microscopy, and for the OptiMAL assay. The microscopy slides were prepared and stained as described previously (Ujah et al. 2003) and were examined for the presence of malaria parasites by an experienced microbiologist (E.I.I.). The OptiMAL test was carried out according to the manufacturer's instructions. The OptiMAL test uses monoclonal antibodies to malaria-specific pLDH in whole blood and detects the presence of pLDH when parasite densities are >100 parasites/μl blood. There is no cross-reaction with human LDH. Results of the OptiMAL assay were not made known to the microscopist until all blood smears had been examined. Those samples that were identified as positive either by microscopy or by the OptiMAL assay were subjected to PCR analysis according to the procedure of Snounou et al. (1993).
Blood samples from a total of 268 pregnant women were tested for malaria using microscopy and the OptiMAL assay. There were 15 first-trimester, 99 second-trimester, and 154 third-trimester women (Table 1). The thick-smear microscopy detected malaria in 22 (8%) of the total women tested. The parasitic loads for the thick-smear positives ranged from 40 to 10 000 parasites/μl. Fourteen of the women who were positive for malaria by microscopy reported taking antimalarial drugs, whereas seven of the women were first-time visitors to the antenatal clinic and had not yet received a prescription for malaria prophylaxis.
Table 1. Summary of the characteristics of the study population
|Age (years)||26.6 ± 6.2||28.2 ± 5.0||27.4 ± 5.3|
|Height (cm)||160 ± 7||160 ± 6||160 ± 5|
|Weight (kg)||62.9 ± 11.3||66.6 ± 11.5||69.8 ± 12.6|
|Haematocrit (%)||28.1 ± 6.5||27.7 ± 6.3||27.2 ± 5.6|
The OptiMAL assay detected less than half as many malaria cases (e.g. 20 vs. 7) as microscopy. Of the 20 samples that tested positive by microscopy, only four were positive by the OptiMAL assay. Those four specimens had a parasite count of 400–10 000 parasites/μl (Table 2). Specimens that were positive by microscopy but negative by the OptiMAL assay had parasite counts between 40 and 200 parasites/μl. Four specimens (Table 2) that were positive by the OptiMAL assay were negative by microscopy.
Table 2. Comparison of the number of malaria-positive subjects detected by three diagnostic methods
Polymerase chain reaction analysis confirmed the presence of malaria parasites in 16 of the 18 blood samples that were positive by microscopy but negative by OptiMAL assay. The four blood samples that were positive by OptiMAL but negative by microscopy were positive by PCR. These four specimens probably represent instances of placental parasitaemia in the absence of peripheral parasitaemia.
The major finding of our study of malaria infection in pregnant women in Nigeria was that the OptiMAL monoclonal assay detected less than half as many malaria-positive subjects as standard microscopy. The discrepancy between microscopy and the OptiMAL assay results may have either been due to the low parasite counts in the blood of the subjects or to the fact that the majority of the women were taking antimalarial drugs. Microscopy may have recognized dead parasites. The positive OptiMAL results that were confirmed by PCR but that were not detected by microscopy warrant further study to confirm whether these results represent placental parasitaemia in the absence of peripheral parasitaemia.
In a study by Monkhambo et al. (2002), pregnant women with a positive OptiMAL result had significantly more low birthweight babies. The OptiMAL test may detect parasite loads that are associated with poor infant outcomes.
In principle, the simplicity of the OptiMAL assay ought to make it suitable for use in developing countries, particularly in rural areas, as it eliminates the need for a highly trained microscopist. If the sensitivity of the OptiMAL test could be increased to that of microscopy (40 parasites/μl blood) it would provide a simple and sensitive method for monitoring malaria in pregnant women. The major drawback to its widespread use presently is its cost.
This study was supported by a minority international research training (MIRT) grant from the Fogarty International Center of the National Institutes of Health.