of parasites, along with an inability to monitor response to therapy. Also, if the result is negative, microscopy is still mandatory. Therefore RDTs do not eliminate the need to examine thick and thin blood films. The maintenance of expert microscopy remains a diagnostic priority until a new gold standard is developed.
QBC Microhematocrit Centrifugation Method
Microhematocrit centrifugation with use of the QBC malaria tube (glass capillary tube and closely fitting plastic insert; CBC malaria blood tubes [Becton Dickinson, Tropical Disease Diagnostics, Sparks, MD]) has been used for the detection of blood parasites (20, 21). At the end of centrifugation of 50 to 60 µl of capillary or venous blood (5 min in a QBC centrifuge; 14,387 × g), parasites or erythrocytes containing parasites are concentrated into a small, 1- to 2-mm region near the top of the RBC column and are held close to the wall of the tube by the plastic float, thereby making them readily visible by microscopy. Tubes precoated with acridine orange provide a stain which induces fluorescence in the parasites. This method automatically prepares a concentrated smear, which represents the distance between the float and the walls of the tube. Once the tube is placed into the plastic holder (Paraviewer) and immersion oil is applied onto the top of the hematocrit tube (no coverslip is necessary), the tube is examined with a ×40 to ×60 oil immersion objective (there must be a working distance of 0.3 mm or greater) (Fig. 7.10).
Figure 7.10 The QBC Malaria Test components include the ParaLens UV microscope adapter (blue-violet light module providing fiber-optic illumination with AC outlet or bulb illumination) with rechargeable battery for mobile use (attachable to any conventional microscope), the ParaFuge battery-powered centrifuge, and the ParaViewer microscope tube holder, a specially designed QBC tube viewing block that accepts standard microscope oil. doi:10.1128/9781555819002.ch7.f10
Note Although a malaria infection could be detected by this method, appropriate thick and thin blood films must be examined to accurately identify the species of the organism causing the infection.
The range of sensitivities for the QBC test has been as low as 55% to >90% compared with microscopy. While most workers think the method is rapid, some think there are some disadvantages such as the high cost of capillary tubes and equipment, problems in species identification and quantification, technical problems, broken capillaries, and the fact that the capillaries cannot be stored for later reference. This may not be the most appropriate approach for a laboratory that receives a small number of specimens for malaria testing, but it could be helpful in other settings. In a recent study of 745 specimens from an area of low endemicity along the Colombian Pacific coast, the agreement between the QBC method and thick blood smear was reported to be 99.5% (22). However, another study concluded that in spite of the simplicity of the QBC method, it cannot be considered an acceptable alternative to stained thick blood films in routine laboratory situations (23). An excellent review of this and other methods is provided in reference 24.
This method has been reported to have a high degree of sensitivity in the detection of cases of human filariasis (25). In one study evaluating the technique in the detection of canine Dirofilaria immitis as a model for human filariasis, the QBC analysis was more sensitive (55%) than the thick blood smear (39%) and was more efficient (79% versus 72%). However, accurate identification to the species level was impossible using the QBC method.
Diagnosis of tick-borne relapsing fever has also been reported using the QBC system with a high level of sensitivity, more so than the thick blood smear. This method seems to be useful for this purpose and might be considered in the management of fever in travelers returning from tropical regions (26).
A P. falciparum antigen detection system, the ParaSight F test (Becton Dickinson), is very effective in field trials (27). This procedure is based on an antigen capture approach and has been incorporated in a dipstick format; the entire test takes approximately 10 min (Fig. 7.11 and 7.12) (27). The overall performance of the ParaSight F test was reviewed in 1995 at a World Health Organization meeting looking at 15 studies with a total of 7,926 assays for the detection of P. falciparum. Overall, the sensitivity ranged from 84 to 94%, with specificities ranging from 81 to 99% (28). However, the test detects only P. falciparum, and at low parasitemias (100/µl) the sensitivity drops to 11 to 40%. Also, since HRP2 is not present in mature gametocytes, cases where only gametocytes were present could be missed. There is also a possibility that some false-negative results, even at high parasitemias, are due to the lack of the HRP2 gene (29). After successful therapy, many patients continue to have circulating HRP2 antigen for 7 to 14 days after microscopic and clinical cure. Also, as many as 60% of patients positive for rheumatoid factor have a false-positive test (30). The ParaSight F test may be used in situations where no trained microscopists are available or where malaria is strongly suspected and the microscopy results are negative. This kit has proven to be very useful in many areas of the world.
Figure 7.11 Diagram of the ParaSight F test format. (Adapted from reference 24 with permission.) doi:10.1128/9781555819002.ch7.f11
Figure 7.12 ParaSight F test showing (from left to right) the positive test strip with a reagent control mark above the positive test result and a negative test strip with the reagent control mark. doi:10.1128/9781555819002.ch7.f12
In one systematic review and meta-analysis of controlled studies evaluating the diagnostic accuracy of the ParaSight F test in comparison with light microscopy, data sources included 15,359 subjects (4,119 with P. falciparum malaria) in 32 studies reported in 29 publications. Overall, the ParaSight F test demonstrated 90% sensitivity and 94% specificity. Both the sensitivity and specificity were significantly higher in the nonresident population than in the resident population. The posttest probability indicates that in settings of low malaria prevalence a negative test almost absolutely excludes infection, while in settings of high prevalence the same result still gives a substantial chance of infection being present. The authors conclude that the ParaSight F test is a simple and accurate test for the diagnosis of P. falciparum infection. The test could be of particular value in the diagnosis of malaria in travelers returning from areas of endemic infection (31).
The ICT Malaria P.f. test (ICT
