Malaria Site: Quantitative Buffy Coat (QBC) Test

The QBC Test, developed by Becton and Dickenson Inc., (now marketed by QBC Diagnostics, 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.

The QBC Test

	Peripheral smear	QBC
Method	Cumbersome	Easy
Time	Longer, 60 - 120 minutes	Faster, 15 - 30 minutes
Sensitivity	5 parasites/�l in thick film and 200 / �l 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

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 %

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.