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The QBC Test, developed by Becton and Dickenson Inc., is a new
method for identifying the malarial parasite in the peripheral blood. It involves staining
of the centrifuged and compressed red cell layer with acridine orange and its examination
under UV light source. It is fast, easy and claimed to be more sensitive than the
traditional thick smear examination. Method: The QBC tube is a high-precision glass hematocrit
tube, pre-coated internally with acridine orange stain and potassium oxalate. It is filled
with 55-65 microliters of blood from a finger, ear or heel puncture. A clear plastic
closure is then attached. A precisely made cylindrical float, designed to be suspended in
the packed red blood cells, is inserted. The tube is centrifuged at 12,000 rpm for 5
minutes. The components of the buffy coat separate according to their densities, forming
discrete bands. Because the float occupies 90% of the internal lumen of the tube, the
leukocyte and the thrombocyte cell band widths and the top-most area of red cells are
enlarged to 10 times normal. The QBC tube is placed on the tube holder and examined using
a standard white light microscope equipped with the UV microscope adapter, an
epi-illuminated microscope objective. Fluorescing parasites are then observed at the red
blood cell/white blood cell interface. The key feature of the
method is centrifugation and thereby concentration of the red blood cells in a predictable
area of the QBC tube, making detection easy and fast. Red cells containing Plasmodia are
less dense than normal ones and concentrate just below the leukocytes, at the top of the
erythrocyte column. The float forces all the surrounding red cells into the 40 micron space
between its outside circumference and the inside of the tube. Since the parasites contain
DNA which takes up the acridine orange stain, they appear as bright specks of light among
the non-fluorescing red cells. Virtually all of the parasites found in the 60 microliter
of blood can be visualized by rotating the tube under the microscope. A negative test can
be reported within one minute and positive result within minutes.
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Studies that have
compared the QBC with the peripheral smear report that the test is
as sensitive as the smear; however, identification of the species
and quantification of parasitemia are difficult with the QBC
technique. [See references below] Therefore, in spite of the
speed and simplicity of QBC technique, it cannot be considered an
acceptable alternative to GTF under routine clinical laboratory
situation.
Comparative
Analysis of QBC and Thick Film Microscopy:
Study done at Kasturba Medical College Diagnostic Centre, Mangalore (Urmila Shenoi et al)
Total number of samples examined |
18,845 |
Total number positive by QBC |
4,824 |
Total number positive by thick film |
3,490 |
Positivity rate of QBC |
25% |
Positivity rate of thick film |
18 % |
Comparison
between peripheral smear and QBC test for detecting malaria
| |
Peripheral smear |
QBC |
| Method |
Cumbersome |
Easy |
| Time |
Longer, 60 - 120 minutes |
Faster, 15 - 30 minutes |
| Sensitivity |
5 parasites/µl in thick film and 200 / µl in thin film |
Claimed to be more sensitive, at least as good as a thick film |
| Specificity |
Gold standard |
? False positives, artifacts may be reported as positive by
not-so-well-trained technicians |
| Species identification |
Accurate, gold standard |
Difficult to impossible |
| Cost |
Inexpensive |
Costly equipment and consumables |
| Acceptability |
100% |
Not so |
| Availability |
Everywhere |
Limited |
| Other |
-- |
Accidentally can detect filarial worms |
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Therefore, whenever in
doubt, ask for a peripheral smear study, particularly for species identification. There
are instances of cases diagnosed as vivax malaria on the QBC, but soon after developed
fatal complications of falciparum malaria.
A 65 year old man came to
the hospital at 8 P.M. with history of fever and chills of one day's duration. His QBC
test was reported positive for many trophozoites of P. vivax. He was started on
chloroquine and primaquine. Next morning at 9.30 A.M. he developed generalised tonic
clonic convulsions and lost consciousness. He was immediately brought to the hospital and
found to be in deep unarousable coma. His peripheral smear examination showed P.
falciparum and the parasite count was 35%. He was started on Inj. Quinine, but he died
after 3 days.
See this article cited in US FDA CBEA Presentation
More from the Company Web site
References:
-
Adeoye GO, Nga IC.
Comparison of Quantitative Buffy Coat technique (QBC) with Giemsa-stained
thick film (GTF) for diagnosis of malaria.
Parasitology International 2007;56(4):308-312 At
http://www.sciencedirect.com/science
- Cabezos J, Bada JL. The diagnosis of malaria by the thick film and the QBC: a comparative study of both technics
Med Clin (Barc) 1993;101:91-4. At
http://www.anopheles.org/showabstract.php?pmid=8315991
- Pinto MJ,
Rodrigues SR, Desouza R, Verenkar MP. Usefulness of quantitative
buffy coat blood parasite detection system in diagnosis of
malaria. Indian J Med Microbiol 2001;19:219-21. At
http://www.ijmm.org/article.asp
- Estacio RH,
Edwin RE, Cresswell S, Coronel RF, Alora AT. The Quantitative
Buffy Coat Technique (QBC) in Early Diagnosis of Malaria: The
Santo Tomas University Hospital Experience At
http://psmid.org.ph/vol22/vol22num2topic3.pdf
- Wang X,
Zhu S, Liu Q, Hu A, Zan Z, Yu Q, Yin Q. Field evaluation of the
QBC technique for rapid diagnosis of vivax malaria. Bull World
Health Organ. 1996;74(6):599-603. At
http://www.ncbi.nlm.nih.gov/sites/entrez
- Clendennen
TE, Long GW, Baird JK. QBC® and Giemsa-stained thick blood films:
diagnostic performance of laboratory technologists.
Transactions of the Royal Society of Tropical Medicine and Hygiene
1995;89(2):183-184
At
http://www.sciencedirect.com/science
Other Tests:
1. Peripheral Smear
2. Para Sight F test
3. OptiMal Assay
4. The immuno chromatographic test (ICT Malaria P. f. test)
5. Polymerase Chain Reaction
6. Detection of antibodies by Radio immuno assay,
immunofluorescence or enzyme immuno assay
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