QBC (Quantitative Buffy Coat)

• A special glass capillary tube is filled with 20 µl blood (a droplet via a finger prick).
  
• The inner side of this tube is coated with anticoagulants and the dye acridine orange.
  
• Acridine orange will bind to the DNA in the nuclei of the malaria parasites, and also to ribosomal RNA.
  
• Afterwards the tube is centrifuged (10,000 g x 5 minutes). The blood cells are thus separated according to density. The buffy coat is the part of the centrifuged blood which contains platelets and white blood cells (buff = pale yellow). In the tube is a longitudinal plastic float with the same density as the buffy coat.
  
• The float serves to spread the buffy coat and adjacent cells and press them in a thin layer against the wall.
  
• Since parasitised red blood cells are lighter than non-parasitised ones and heavier than white blood cells, infected red blood cells will be found on top of the red cell column, just below the white blood cells, right against the buffy coat.
  
• The parasites in this layer can be observed using a fluorescence microscope. However, be aware that Howell-Jolly bodies (nuclear residues) may look similar to parasites.
  
• This technique is much quicker than reading thick or thin smears but requires training and appropriate apparatus.
  
• Nevertheless, its use can considerably reduce the workload of the laboratory staff especially in larger hospitals where many samples are processed every day.
  
• No species identification can be obtained using QBC.
  
• With QBC there is quite wide inter-observer variability.
  
• The specially prepared disposable tubes need to be available as well as a microhaematocrit centrifuge and a microscope with a UV-lens.
  
• The thin tubes sometimes break.

Category: Medicine Notes